Paleontology, taphonomy, and sedimentology of the Mygatt-Moore Quarry, a large dinosaur bonebed in the Morrison Formation, western Colorado—Implications for Upper Jurassic dinosaur preservation modes

© 2018 Utah Geological Association. All rights reserved. For permission to copy and distribute, see the following page or visit the UGA website at www.utahgeology.org for information. Email inquiries to GIW@utahgeology.org. PALEONTOLOGY, TAPHONOMY, AND SEDIMENTOLOGY OF THE MYGATT-MOORE QUARRY, A LARGE DINOSAUR BONEBED IN THE MORRISON FORMATION, WESTERN COLORADO—IMPLICATIONS FOR UPPER JURASSIC DINOSAUR PRESERVATION MODES


INTRODUCTION
The Morrison Formation is an Upper Jurassic alluvial deposit that originally covered at least ~1,000,000 km² of what is now the western United States (Dodson and others, 1980;Turner and Peterson, 2004). The formation is well known for having produced some of the largest and most iconic dinosaurs from the Late Jurassic, and it has yielded one of the most diverse and abundant Mesozoic paleofaunas from anywhere in the world. Although forms such as Brachiosaurus, Stegosaurus, and Allosaurus are recognized even by non-paleontologists worldwide, the Morrison Formation's real significance lies in its preserved biota, including more than 90 vertebrate forms (more than 60 of which are not dinosaurs), plus plants, bivalves, gastropods, crustaceans, and others (Chure and others, 2006).
Dinosaur quarries in the Morrison Formation number in the hundreds and include those producing a range from a few bones of one individual up to more than 10,000 bones of multiple individuals and species (Dodson and others, 1980;Foster, 2003). Most quarries are dominated by dinosaurian remains; rare others produce almost exclusively non-dinosaur material. The diversity of preservational modes is great, particularly between those preserving mostly dinosaurs versus mostly non-dinosaurians, but several taphofacies are consistently encountered in widely scattered areas and levels. Among sites producing primarily dinosaurs, a number are considered "large, " defined here as having produced multiple individuals and 500 or more mappable bones (i.e., excluding fragments unmappable at approximately 10 cm = 1 m). Among these are sites such as the Carnegie Quarry at Dinosaur National Monument, Howe Quarry, Dry Mesa Quarry, and Cleveland-Lloyd Dinosaur Quarry (Curtice and Wilhite, 1996;Miller and others, 1996;Carpenter, 2013). The greatest number of bones, individuals, and species among these sites appears to be preserved in relatively coarse sand deposits of channel sandstones. These localities also preserve relatively even numbers of each species, often reflective of the relative abundances of the species in the formation as a whole. In contrast, "large" dinosaur quarries (as defined above) preserved in drab mudstone may produce very large numbers of bones, but generally contain fewer species than channel sandstone sites and are often dominated by one or a couple of species (e.g., Cleveland-Lloyd Dinosaur Quarry, Gates, 2005 andothers, 2017;Howe Quarry, Bird, 1985;Mygatt-Moore Quarry, Foster and others, 2016). Among the latter mudstone sites, with relatively low diversity and a predominance of just a few species, is the Mygatt-Moore Quarry in western Colorado. This site is unusual among Morrison localities in preserving a moderate diversity of plant macrofossils and palynomorphs, abundant dinosaur bones but of relatively low taxonomic diversity, and rare microvertebrate fossils, all in the same productive layer.
This study examines the general paleontology, vertebrate taphonomy, and sedimentology of the Mygatt-Moore Quarry as a large dinosaur bonebed and their implications for the origin of the deposit.

LOCALITY AND PREVIOUS WORK
The Mygatt-Moore Quarry was discovered on March 14, 1981, by J.D. and Vanetta Moore and Pete and Marilyn Mygatt (Armstrong and Perry, 1985;Armstrong and others, 1987;Mygatt, 1991;P. Mygatt, verbal communication, 2001). It is approximately 2.5 km from the Utah-Colorado state line in far western Mesa County, Colorado ( The quarry has been worked every summer since 1985 and has produced more than 2300 mapped bones. The quarry has developed into a significant excavation over the years (figures 2C to 2F) but has shown no signs of playing out. In fact, results presented here suggest the bone deposit is much larger than previously suspected.
Compared to its fossil diversity, abundance, and the number of years it has been worked, relatively little has been published about the Mygatt-Moore Quarry. Plant

GEOLOGIC SETTING
Rabbit Valley (and the Mygatt-Moore Quarry) is on the northwestern edge of the Uncompahgre Plateau uplift and northwest of the local uplift of Ruby Canyon along the Colorado River; on its western edge the Uncompahgre borders the laccolithic uplift of the La Sal Mountains, which are to the southwest of Rabbit Valley. This region is on the northeastern edge of the Colorado Plateau. In Rabbit Valley, the Salt Wash Member of the Morrison Formation is about 96 m thick (Armstrong and McReynolds, 1987), and the Brushy Basin Member is about 100 to 140 m thick. The Morrison Formation in the area is underlain by what is called either the Upper Jurassic Summerville Formation (Carter and Gualtieri, 1965;Lohman, 1965;Lucas and others, 2006) or the Wanakah Formation (Scott and others, 2001) and the Entrada Sandstone (both Middle Jurassic), and is overlain by the Lower Cretaceous Cedar Mountain Formation and then the Naturita Formation (Stokes, 1952;Young, 1960;Kirkland and others, 1997;Currie and others, 2008;Sprinkel and others, 2012;Carpenter, 2014). The Cedar Mountain Formation is equivalent to the Burro Canyon Formation of the south and east side of the Colorado River in Colorado and Utah (Young, 1960); the formation is traditionally considered Cedar Mountain in Rabbit Valley only because of the valley's position north and west of the Colorado River. The Mygatt-Moore Quarry is in the upper part of the G o r e R a n g e P a rk R a n g e Figure 1. Location of the Mygatt-Moore Quarry in western Colorado (red star). Major outcrops in Colorado of the Morrison Formation in blue-green.

METHODS
The Mygatt-Moore Quarry has been excavated for approximately three to four months per year since around 1990. Before that time, most seasons were approximately two weeks. Mapping of the bones in the quarry has always been from a main datum established near the original point of discovery, consisting of a survey marker-type cap, and a second marker at 40 m due north; these two markers establish the north-south coordinate axis. The grid is measured along meters approximately east-west and north-south of the datum. (It was recognized several years ago that the grid system used year to year is aligned on a bearing of approximately 350 o and not true north, though the northern survey marker up slope from the quarry is correctly aligned.) Most bones have been found in an arc approximately southwest to northeast of the main quarry datum; the main quarry layer (may also be referred to as quarry layer) has already been eroded away to the south and east, and it is under the hillside to the north and west. Each bone removed was also assessed for stratigraphic level within the quarry. The main bone layer is approximately 1 m thick in most areas; this interval is divided into lower, middle, and upper thirds (each approximately 33 cm thick), and as each bone was collected its stratigraphic interval was noted. In areas where the full bone layer is somewhat thinner, each stratigraphic level is correspondingly thinner so that the full layer was still divided visually into thirds. This system was developed a number of years ago after abandonment of a measurement of a Z-coordinate in the mapping. The Z-coordinate had been mapped off the top of the so-called "pebble layer" (see below). It was eventually discovered, however, that the "pebble layer" was indurated (and thus usable) in some areas of the quarry but not others, that where it was present its upper (and lower) boundaries undulated laterally, and that it is almost non-existent in other parts of the quarry. Thus, the simple three-part lower, middle, and upper bone layer subdivision was adopted.
Stratigraphic sections were measured in ten locations along approximately 1.62 km of strike (southwest-northeast) on the north end of Rabbit Valley. West to east, these are (1) Far West Nose, (2) West Nose, (3) Quarry, (4) Cam Stop, (5) Caudal Draw, (6) Channel Lake Traverse 1, (7) Channel Lake Traverse 2, (8) Gravel Hill, (9) East Trail Through Time, and (10) Far East Ridge (figures 3 and 4). Two sections (Quarry and Far East Ridge; figures 3 and 4) covered the complete Brushy Basin Member from the top of the Salt Wash Member of the Morrison Formation up through the Cedar Mountain Formation and into the Naturita Formation. The other stratigraphic sections were connected to at least one laterally adjacent section by traceable beds (generally the "fish layer" or various sandstone beds), so that all ten sections could be correlated. Sections were measured with a Jacob staff and Brunton compass set to the local dip of 5° northwest.
Five trenches were excavated with a backhoe (figure 5). Three sites were within the quarry excavation and two sites were excavated adjacent to the quarry on either side. These sections were also measured with a Jacob staff. Two drill holes-MM-1, which was about 87 m to the northwest of the quarry, and MM-2, which was about 95 m to the southwest-were bored with a 28 Paleontology, Taphonomy, Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 track-mounted rig (figure 5). The 2.5-inch-diameter core was logged at the surface. Only 3 to 5 m of core below the "fish layer" (see below) was collected and is preserved in the Museums of Western Colorado paleontology collections. Positions of the trenches and drill holes were recorded by marking with 40 cm diameter circular orange markers and digitally photographing the whole area with a fixed-wing drone aircraft; the camera had a resolution of approximately 10 cm. Five hand samples were collected from standard layers in each trench; the "fish layer" was sampled from all five. Two samples from the main quarry layer were collected from each trench; other samples varied between trenches. Thin sections were made from these samples. Lithologic descriptions of the trenches were recorded in the field and from the hand samples and thin sections. Thin sections were studied under a petrographic microscope and digitally photographed. In addition to the trench hand samples, numerous calcareous nodules were collected from the base of the quarry, and the five of these were thin sectioned (PB-1 through PB-5). Four bones were also thin sectioned to compare their microstructures; two rounded, pebble-sized bone fragments (with only trabecular bone left) and two fragments containing cortical bone. All thin sections are preserved at the Museums of Western Colorado.
In addition to the thin sectioning of the calcium carbonate nodules, two of them plus three other representatives of these nodules (PB-1, PB-2, and PB-7 through PB-9) were analyzed for rare earth element ratios at the Department of Geosciences, Boise State University. Samples were crushed in a mortar and pestle to a fine powder and 125 mg of that powder was added into Teflon microwave digestion vessels. The samples were then soaked in 30% trace grade H2O2 for an hour to extract any trace elements from any remaining organic matter. After an hour, a 4.5:1.5:5.5 ml solution of concentrated trace grade HNO3:HCl:DDI H2O was added to the microwave digestion vessels. Samples were digested on a MARS microwave digester using the pre-programmed 3052XP-15min Xpress method that uses a 15 minute ramp-up time, a 15 minute hold at 190ºC, and 29 Paleontology, Taphonomy, Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology     Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H. (Gromet and others, 1984). Matrix samples for X-ray flourescence (XRF) analysis were collected from within the quarry's main bone layer, from the same stratigraphic level (about 2 to 3 m below the "fish layer") but outside the quarry about 250 m to the east (Caudal Draw), and a bone sample from within the quarry. XRF of sediment and bone samples carried out at Indiana University of Pennsylvania using an Innov X Delta Professional handheld XRF analyzer in soil mode. Concentrations of selected elements are given in ppm (parts per million). Sediment samples were ground prior to analysis in a glass mortar and pes-tle. Bone samples were cleaned of matrix with de-ionized water, but analyzed whole.
The U-Pb (zircon) radiometric age for the Mygatt-Moore Quarry was obtained from an ash-fall bed isolated from the quarry mudstone of the main bone layer, collected from the northern part of the quarry (quarry map coordinates 1021N, 998E) on June 6, 2011. The sample was analyzed by the University of Wyoming Geochronology Lab and reported by Trujillo and others (2014). A variety of zircon morphologies were separated from the mudstone sample; of more than 125 zircons, at least 30 demonstrated characteristics (elongate tips, longitudinal bubble trails, transverse channels) consistent with ash grains. Selected zircons were annealed at 850°C for 48 hours and then dissolved in two steps, modified from the chemical abrasion method of Mattinson (2005). The first step used concentrated HF and HNO 3 for 12 hours at 180°C. This removed the most metamict domains and surficial Pb. After rinsing and discarding the leachate, individual grains were completely dissolved in HF and HNO 3 at 240°C for 30 hours; the solutions were converted to chlorides and evaporated with 0.05N H 3 PO 4 in preparation for thermal ionization mass spectrometry.
Nearly every piece of matrix from the main bone layer at the Mygatt-Moore Quarry contains carbonized plant fragments. The only plant specimens saved are those that are complete enough to be identified to some taxonomic level. Some but not all of the wood was collected as well. Plants were identified based on comparisons with publications such as Miller (1987), Tidwell (1990), Tidwell and Medlyn (1992), Ash (1994), Ash and Tidwell (1998) and Tidwell andothers (1998, 2006), as well as by consultation with some paleobotanists. Mollusks were identified using Yen and Reeside (1952) and Evanoff and others (1998).
Identifications of fossil vertebrates were compared to museum collections around the country as well as many references including, but not limited to, the following: Hatcher (1901), Osborn and Mook (1921), Gilmore (1925Gilmore ( , 1936, Madsen (1976), Galton (1980Galton ( , 1981Galton ( , 1983Galton ( , 2007, McIntosh (1990McIntosh ( , 2005, Carpenter and McIntosh (1994), Kirkland and Carpenter (1994), Kirkland (1998), Kirkland and others (1998) Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 and Welles (2000), Upchurch and others (2004), and Tschopp and others (2015). Taphonomic scoring for bones is based on (1) the 0 to 3 scales of Fiorillo (1988) for abrasion, (2) a 0 (none) to 3 (extensive) scale for corrosion (e.g., Eberth and others, 2007), (3) Behrensmeyer's (1978) 0 to 3 scale for weathering, and (4) a 0 to 3 scale for breakage (0 = none, 3 = broken on most sides; Ryan and others, 2001). Abrasion was scored based on rounding of otherwise sharp edges, as well as on any surficial scratching. Corrosion relates to chemical or biochemical erosion of bone surfaces and was scored by the rotting of bone ends or edges that was not clearly due to abrasion. That most end and edge loss of bone surface in Mygatt-Moore material is due to corrosion rather than abrasion is evidenced by the occurrence of outer bone-surface spalling, with sediment preserved between the flaking outer surface and the spongy inner surface that is then so often found (the spalling outer surface would be long eroded away had the material been abraded). Weathering relates to degradation and cracking of bone surfaces due to surface exposure to the elements. Breakage involves pre-burial breaks in the bone that, in most cases, fragment the element. Breakage and weathering are fairly straight forward to assess, but abrasion and corrosion can be difficult to distinguish, as in either case the result is rounding of edges. These taphonomic data were collected on a randomly selected subsample of the Mygatt-Moore Quarry collections, representing approximately 23.5% of the censused collection. The randomly selected subsample of the Cleveland-Lloyd Dinosaur Quarry collection at the Natural History Museum of Utah consisted of about 289 bones including those excavated by Gates (2005).
Number of identified elements is based on a census of the collections of the Museums of Western Colorado, consisting of material prepared through 2013. Minimum number of individuals (MNI) is based on the census data sorted for element but is not based on any single element. In the case of Apatosaurus, for example, the MNI count is based on the presence of a juvenile, a sub-adult and, based on femora, at least two adults. In the case of Allosaurus, on the other hand, the number of individuals is based on femora and metatarsals of adults, plus one juvenile represented by dentaries.
A significant percentage of the bone material in the Mygatt-Moore Quarry is fragmented and mostly unidentifiable to element or taxon. Therefore, most bones used for MNI calculations were taken from a relatively small overall sample of more complete elements. A true calculation of the number of identified specimens to minimum number of skeletal elements (ratio NI-SP:MNE, e.g., Lyman, 1994) would be difficult, and, if knowable, the ratio would likely be rather high (although we did not attempt calculating either for this study).
Shed teeth of theropods were censused and measured for maximum basal crown length with a digital caliper. Mean and standard deviation were calculated in a spreadsheet. Allosaurus and Ceratosaurus teeth were distinguished by the more laterally compressed maxillary and dentary teeth, and smaller serrations, in Ceratosaurus, plus the lingual ridges of anterior dentary and premaxillary teeth in Ceratosaurus, as demonstrated by Madsen (1976), Madsen and Welles (2000), and Bakker and Bir (2004). Two rose diagrams for orientations of bones were constructed, one bidirectional for bones with no clearly more massive end, and a unidirectional rose diagram for those with a clearly more massive end. Bidirectional bones were measured toward the orientation between 0° and 180° and made bidirectional in representation in the computer program. Unidirectional bones were measured toward the less massive end. The rose diagrams were constructed using 30° intervals in the program Rose.Net (https://en.freedownloadmanager.org/Windows-PC/Rose-Net-FREE.html). Compass readings for the rose diagram data were taken off the quarry map.
Voorhies group analysis was based on the group I-III bone categories in Carpenter (2013, table 6), and the work of Frison and Todd (1986) on transportability of elephant bones. It is recognized that the original sheep material studied (Voorhies, 1969) may not be fully comparable to bones of large dinosaurs, as noted by Gates (2005) and Carpenter (2013); however, the analysis is carried out here to facilitate comparisons with other similar mudstone sites, such as the Cleveland-Lloyd Dinosaur Quarry, for which such analyses have also been 32 Paleontology, Taphonomy, Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 conducted previously. There is currently no other system available, based on flume or river studies of more appropriate (large) analogs, with which to conduct these analyses. A randomly selected, previously unexcavated single grid square (1010 to 1011N, 1009 to 1010E) was selected to be excavated and every bone, regardless of size or preservation, was collected. This was done as a possible representation of what an unbiased square meter of the main quarry layer was like, because in the past, excavation of the quarry had been biased towards larger and/ or scientifically significant specimens. The test square meter was excavated in the 2013 season and each bone, tooth, and bone fragment was collected and mapped on X, Y, and Z coordinates. The Z coordinate was taken off a datum at the top of the bone layer (below the soil layer). The bones were measured for maximum length, width, and height and identified if possible. Volume was calculated for each bone incorporating the formula for the volume of an ellipsoid cylinder, using the three dimensions measured. These volumes (as a rough approximation of overall size) were then graphed in a histogram and the mean volume calculated in a spreadsheet.
Articulation ratios for Mygatt-Moore, and other quarries used for comparison, were calculated by dividing the number of bones in articulation with at least one other bone by the total number of bones mapped from the site. These ratios were calculated by counting the bones on available quarry maps (published and unpublished). These ratios were calculated for the Mygatt-Moore, Cleveland-Lloyd, Carnegie, Howe, Howe-Stephens, Poison Creek, Little Houston, and Dry Mesa Quarries.

Measured Sections
The top of the Salt Wash Member of the Morrison Formation in Rabbit Valley is at the top of one or several light-brown, laterally continuous channel sandstones that can be traced across the valley. The overlying Brushy Basin Member consists of approximately 100 to 140 m of gray, maroon, and greenish-gray claystone with numerous channel sandstone and thin splay sandstone beds and only a few thin limestone beds (figure 4). The largest laterally traceable channel sandstone beds appear to be at least 250 to 500 m wide. Most channel sandstone beds are medium-to very coarse grained, contain many tabular and trough cross-beds, and basal lag deposits. Mudstone beds of the Brushy Basin appear to be smectitic and contain at least some silt-sized grains (Trujillo, 2006).
The top of the Morrison Formation was observed in four of the measured sections. In most sections, the top is recognized as a deep maroon-red paleosol overlain by light green, sometimes silty or pebbly claystone of the Cedar Mountain Formation. Approximately 6 m up into the Cedar Mountain is a 0.5-to 2.0-m-thick nodular limestone to paleosol carbonate layer similar to one often found low in the formation farther west in eastern Utah (figure 2B); this unit is consistently at this level low in the Cedar Mountain and is overlain by a thicker unit of silty light green mudstone. The top of the Cedar Mountain is formed by light green claystone or a lightbrown sandstone overlain by a basal conglomerate and/ or a soft white sandstone marking the base of the Naturita Formation. Immediately above this white sandstone are several meters of dark gray to black carbonaceous, and in some places coaly, mudstone characteristic of the Naturita as well. Above this first dark-gray carbonaceous mudstone is the first of two thick, yellow-brown sandstone beds that cap the ridges of Rabbit Valley to the west, north, and east. In one measured section (West Nose), the top of the Morrison Formation is at the top of a thick, pebbly and cross-bedded channel sandstone; this unit is overlain by typical light green claystone of the lower Cedar Mountain, with the caliche/nodular limestone layer approximately 5 to 6 m up section. Interestingly, this channel sandstone pinches out to the west within about 100 m from the West Nose section, so that the Morrison-Cedar Mountain contact is a red claystone and paleosol directly below the light green claystone of the Lower Cretaceous at the next section (Far West Nose) to the west (figure 2B).
The Mygatt-Moore Quarry occurs approximately 64 m above the base of the Brushy Basin Member of the  Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Morrison Formation, in the Quarry section (figure 4) in which the member is 135 m thick. This puts the quarry near the top of the lower half of the Brushy Basin Member (47% of the way up in the section). Galli (2014) measured a composite section of part of the Brushy Basin Member but had the quarry in a slightly lower position; Turner and Peterson (1999) had Mygatt-Moore Quarry at a slightly lower relative stratigraphic position on their master composite section. In both, however, the quarry was in the upper part of the lower half of the member.
Other vertebrate bone sites in the northern part of Rabbit Valley were tied in to the measured sections based on correlations of the channel sandstones in which they occur and were measured and correlated with the network of sections (figure 4). The stratigraphically lowest of these is a thin sandstone that contains a partial skeleton of Camptosaurus (Averett locality), a partial skeleton of Allosaurus (Under the Cedar Tree site), and a goniopholidid crocodylomorph osteoderm. These three sites are on the east end of the Trail Through Time (a BLM trail in the Rabbit Valley Research Natural Area that begins at the Mygatt-Moore Quarry). Next higher site in the Brushy Basin Member section is a sandstone that contains a proximal caudal series and several other elements of a diplodocid sauropod (probably Diplodocus); this is also a stop on the east end of the Trail Through Time. The stratigraphically youngest site contains a partial diplodocid skull (and elements of a skeleton still in the sandstone) collected several years ago from the Far East Ridge section east of and above the Trail Through Time. Between these sites and the Mygatt-Moore Quarry there is a channel sandstone approximately 340 m wide east to west. This channel is stratigraphically a few meters above the Mygatt-Moore Quarry. It contains the partial skeleton of a Camarasaurus (a stop on the Trail Through Time) and a single caudal centrum of a theropod dinosaur at the Caudal Draw section a little to the east. These latter sites are stratigraphically younger than the partial Diplodocus from the eastern Trail Through Time, but whether they are similar in age to the diplodocid skull, or are slightly older, is difficult to determine (see figure 4).
The "fish layer" above the quarry is a distinct marker bed that can be traced more than 500 m to the east of the quarry. Tracing the relationships of beds stratigraphically immediately adjacent to this marker bed indicates that (1) the Camarasaurus channel sandstone pinches out laterally to the west of the sauropod stop along the trail, before it gets to the quarry outcrop, (2) the Camarasaurus channel sandstone scours out the "fish layer" at the Channel-Lake Traverse 2 section, (3) the Camarasaurus channel sandstone pinches out on its east end, west of the Gravel Hill section, and (4) from at least the Caudal Draw section east, the stratigraphic level of the Mygatt-Moore Quarry (about 2 m below the fish layer) no longer consists of medium light-gray claystone with abundant plant debris. The Caudal Draw and sections east that level are an often dark-red mudstone devoid of any fossil content (see figure 4).

Detailed Quarry Sections
Five trenches excavated by backhoe were dug in the quarry area; three were in the back wall of the quarry itself, and one each was just outside the excavated quarry to the south and east (figures 5 and 6). These trenches were excavated from the "fish layer" down to below the main quarry layer. The base of the main quarry layer to the base of the "fish layer" is 2 to 3 m thick. The main quarry layer unit (defined as a nodule-lag based, medium light gray claystone with abundant carbonized plant debris) was approximately 70 cm (in trench 1 south of the main quarry) to 1.5 m (in trench 3 on the northwest edge of the quarry) thick. In trenches 2 and 4, also in the excavated quarry, the main quarry layer was a more typical 1 to 1.2 m thick. The main quarry layer is directly underlain by a soft, light-green silty claystone that sometimes has some white mottling and is generally devoid of fossil vertebrate material, although some specimens have been found at the very top of this layer at its contact with the overlying bone layer. Most notable, the soft, light-green claystone layer contains no carbonized plant material at all, unlike the mudstone immediately above it. The main quarry layer is a greenish-gray (5GY 6/1) to medium light-gray (N6) claystone with silt-sized clasts of quartz and minor plagioclase feldspar (in thin section), abundant carbonized plant fragments, and common greenish-colored or white clay-ball clasts that are rounded and about 2 to 5 mm in diameter (figure 6; Paleontology, Taphonomy, Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 appendix A). Trenches 2 to 4 have dinosaur bone and were in areas from which dinosaur material had been collected nearby in years past. Some of the trenches have carbonate nodules near the base of the main quarry layer (trenches 1 to 3). Above the main quarry layer is greenish-gray, light-green, and medium-green claystone with no carbonized plant fragments except in trench 2. A thin bed with minor amounts of plant debris is about 1.75 m above the base of the main quarry layer in trench 2. Other trenches were devoid of plant material above about 1 m above the base of the main quarry layer. To the east, some reddish-colored claystone was present about 1.5 m from the base of the trench 5, unlike any of the other trenches. This and the fact that the main quarry layers are thinner in trench 5 compared to the other trenches, and that the base of the main layer in trench 5 lacked clay balls, bones, or nodules, and contained fewer plant fragments, all suggest that trench 5 may be closer to the eastern edge of the bone deposit than the other trenches. The indurated, laminated silty claystone at the top of each trench (the "fish layer") was consistent except in trench 4 where the layer was mottled and laminations were less obvious.

Thin Section Micrographs
Comparison of thin section micrographs from the different layers across the five trenches indicates subtle differences in the claystone content that are not obvious in hand sample (figure 7; appendix A). The soft, lightgreen claystone underlying the main quarry layer appears to contain a higher percentage of larger silt-sized clasts than any of the other layers. The main quarry layer also contains a significant percentage of silt-sized grains but also has a high abundance of carbonized plant debris, clay balls (some containing silt clasts themselves), and wood fragments; these elements are missing from almost all samples from other layers (figure 7). The layer above the main quarry layer is similar in clay and silt content but is devoid of fossils and clay balls (except one sample with a single small wood fragment). The thin sections of the "fish layer" demonstrate the overall finer grained nature of the silt-sized clasts. None of the silt clasts in the "fish layer" from trenches 1 to 3 approach the size of those from underlying units. Laminations are apparent in some of the thin section, particularly trench 1. The very different non-laminated texture of the "fish layer" thin section from trench 4 is reflected in the unusual mottled appearance in outcrop. The coarser size of the silt grains in this thin section matches that of many of the underlying layers. The "fish layer" thin section from trench 5 also has slightly coarser maximum silt grain size. The unusual texture for the "fish layer" thin sections of trench 4 (and to a lesser extent, trench 5) is somewhat unexpected because this laminated bed is traceable in outcrop for more than a half kilometer to the east. Despite the consistent appearance of the "fish layer" for hundreds of meters in outcrop, the apparent differences in the thin sections may partly explain why, to date, fish skeletons have only been found in the quarry area. In any case, the mottled, non-laminated appearance of the "fish layer" at the top of trench 4 reflected in the thin section micrograph is clearly different from the other "fish layer" thin sections. Also important to note from the thin sections (figure 7) is the somewhat different appearance of some of the trench 5 micrographs. This again may suggest that the eastern edge of the quarry deposit was near trench 5. Previous exploration excavations in this area turned up little in the way of bone (B. Britt, Brigham Young University, verbal communication, 2014), which is consistent with this interpretation.

"Pebble Bed" Nodules
A concentration of calcium carbonate nodules commonly found at the base of the main quarry layer is sometimes referred to informally as the "pebble bed. " The "pebble bed" was present in trenches 1 through 3. The nodules are typically smaller and fewer in the southwestern part of the quarry and larger and more numerous in the northern part. The "pebble bed" commonly contains fragmented and heavily abraded dinosaur bone. This layer is well indurated only in some places, especially under larger sauropod bones such as a pubis and scapulae. Based on anecdotal evidence from the earlier years of the excavation, this "pebble bed" was more extensively indurated toward the southern part of the current quarry area, but in the past decade or so of excavation, only isolated spots have proved to be strongly cemented. In most areas of the northern part of the quarry, the basal layer of the main quarry deposit contains abundant large dinosaur bones and bone fragments in relatively soft, plant fragment-rich claystone with a few to abundant, hard calcium carbonate nodules of irregular shape that range from 1 to 10 cm in greatest dimension. Most nodules are approximately 2 to 5 cm across and are devoid of plant material. The southwestern part of the quarry that was opened in 2005 and worked most years since, has no well-indurated areas, and, as mentioned above, has fewer and smaller calcium carbonate nodules; however, the southwest area has a greater number of clay balls than the northern part of the quarry.
The exact source of the calcium carbonate nodules is uncertain. The calcium carbonate nodules are mostly composed of claystone with some angular to subrounded, silty to fine-grained sand grains. In hand sample   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 and when seen in cross section, some of the nodules appear to be laminated and some also have manganese dendrites growing in from their outer edges. Several of these nodules were thin sectioned and were found to contain silt-and fine-sand-sized clasts in a higher concentration than many of the layers sampled from the quarry (figure 8). The silt and sand clasts are not as large as the largest grains in the underlying light-green claystone. The thin sectioned nodules are perhaps most similar in texture to the main quarry layer level from trench 5 or trench 4 and possibly to the lowermost gray claystone in trench 1. The nodules are least similar in texture to the underlying light-green claystone found in the main quarry layer of trenches 1 through 3, and the "fish layer. " The nodules are similar to those in the mudstone layers below, above, and lateral to the quarry, but differ from the mudstone layers in the main quarry layer. This, along with the clay balls in the main quarry layer, and very rare clasts of finely bedded, fine-grained sandstones (and sometimes a basal lag of abraded bone fragments), suggest that the calcium carbonate nodules were washed into the quarry deposits from elsewhere. Taken together, the evidence suggests that the nodules were likely formed in place as incipient soil structures outside of the quarry area but were reworked and redeposited in the Mygatt-Moore Quarry topographic low by hydraulic influence. This reworking of soil-related calcium carbonate nodules was also suggested by Kirkland and Armstrong (1992). Total rare REE concentrations for the nodules in the "pebble bed" layer range from 74.8 to 112.5 ppm. There are two REE patterns when normalized to the North American Shale Composite (NASC) (Gromet and others, 1984). Samples PB-1, 7, and 9 exhibit a light REE pattern typical of shale, which concentrates light REE over middle and heavy REE. Samples PB-2 and 8 are middle REE depleted. This may be due to scavenging of the middle REE by Mn oxides; however, additional analysis needs to be conducted. All samples have a negative Europium (Eu) anomaly typical of a volcaniclastic source rock, consistent with the volcaniclastic nature of the sediment from the Brushy Basin Member of the Morrison Formation. The preliminary REE analysis of several of the calcium carbonate nodules also indicates that the nodules may have mixed together from two separate sources based on the two geochemical signatures (figure 9). More data on these REE signatures should be forthcoming as more nodules are analyzed in the future.

Matrix XRF Analysis
The Brushy Basin section in the Mygatt-Moore Quarry represents a mudflat deposit, which was ephemerally wetted (Foster and Hunt-Foster, 2011). The metal profile detected in the Mygatt-Moore Quarry matrix using XRF analysis is likely the result of apatite diagenesis in an ephemerally subaqueous environment. Samples analyzed from within Mygatt-Moore Quarry differ from Morrison Formation samples collected at the same stratigraphic horizon outside the bonebed. Specifically, the matrix at Mygatt-Moore Quarry contains 38 Paleontology, Taphonomy, Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 elevated K, Ti, V, Ni, Zn, and Pb at levels at least twice those detected outside the quarry (table 1). From within this set, Ni, Zn, and Pb have been reported in obsidians, the chemical composition of which were used as proxies for volcanic ash deposited during the Jurassic (Hubert and others, 1996). Ash would have been incorporated into the sediment of Mygatt-Moore Quarry during ashfall events, resulting in these elements being found in elevated levels in the matrix and bone. Some elements detected via XRF are enriched within Mygatt-Moore Quarry bone samples relative to Mygatt-Moore Quarry matrix. Elevated levels of metals such as As, Sr, and Pb within apatite are known from other Jurassic deposits, such as the Lower Jurassic Kayenta Formation, as well as the Upper Cretaceous Prince Creek, Hell Creek, and Dinosaur Park Formations (e.g., Goodwin and others, 2007). Di-and trivalent ions substitute for Ca 2+ in apatite during early diagenesis within bones in contact with water (Trueman and Tuross, 2002). This is one likely source of the metals, which are elevated in Mygatt-Moore Quarry bone. A second possibility is organic content within the deposit. Mygatt-Moore Quarry contains abundant plant fragments (REF), and organic-rich soil horizons have been shown to be enriched in metals, e.g., Cu, Pb, Mn, Zn, and Hg (Martí and others, 2003). It is possible that decaying dinosaur tissue contributed to the elemental metals found within Mygatt-Moore Quarry, as has been suggested at the Cleveland-Lloyd Dinosaur Quarry (Peterson and others, 2017). However, the elemental metal profiles detected here contrast with those reported from the Cleveland-Lloyd Dinosaur Quarry in regard to the relative concentrations in bone versus matrix from Cleveland-Lloyd Dinosaur Quarry (Peterson and others, 2017). In contrast, the elemental metal profiles seen here resemble those of other Jurassic sites whose elemental metal content was determined to be diagenetic (e.g., Hubert and others, 1996;Goodwin and others, 2007). Whereas abundant sulfide minerals, such as pyrite and chalcopyrite, were detected at Cleveland-Lloyd Dinosaur Quarry, the Mygatt-Moore Quarry sediment and bone samples contain considerably lower levels of sulfides, implying a sometimes more oxygen-rich setting at Mygatt-Moore than Cleveland-Lloyd. Despite evidence from carbonized skin at Mygatt-Moore Quarry, the lower levels of sulfides imply that the levels of decay at Mygatt-Moore Quarry were not high enough to utilize the available oxygen, making it unlikely that enough dinosaur tissue was decaying to provide the observed metals. Furthermore, herbivore tissues, which dominate the Mygatt-Moore Quarry assemblage (by number of identified species and minimum number of individuals [MNI]), generally contain lower concentrations of elemental metals compared to carnivore tissues due to trophic focusing (Vijver and others, 2004). Therefore, the large amounts of dinosaur bone apatite are the most likely source of elevated el- Normalized NASC Element Figure 9. Rare earth element geochemistry plot of five calcium carbonate nodules (e.g., PB-1), showing two apparent signature groups and suggesting multiple sources of these "pebbles. " Log-scale Y-axis.

Drill Hole Cores
The cores from the two drill holes (figures 5 and 10; appendix B) demonstrated several things. First, the "fish layer, " which is traceable in outcrop for more than 500 m to the east along the Trail Through Time and which is mostly covered beyond trench 1 to the southwest, was present in both drill holes. The drill holes are ~100 m apart indicating that the lake deposit extends a significant distance to the west in general. Second, the distinctive main quarry layer is also present in both cores (figures 10A and 10B); in MM-1 at 17.4 to 17.8 m and MM-2 at 15.7 to 16.6 m. More importantly, the core from MM-2 preserves an approximately 4-cmlong fragment of dinosaur bone from the 16.5 m level in the medium light gray claystone with abundant plant fragments (figure 10C).
These results indicate that the main quarry layer is geographically much larger than previously believed, especially to the southwest. The original intent of the drilling was to bracket the western and southwestern edges of the quarry in the directions the main quarry layer goes in the subsurface, but the discovery that the quarry claystone continues at least about 87 m to the northwest (drill hole MM-1) and about 95 m to the southwest (drill hole MM-2) suggests that the main quarry layer may be up to eight times larger than the area excavated so far. The area formed by a triangle linking drill holes MM-1 and MM-2 to the quarry and to each other is estimated to be approximately 4130 m 2 . In contrast, the area of the quarry excavated so far is approximately 500 m 2 .

Age
The age of the Mygatt-Moore Quarry has been estimated by U-Pb dating of ash-fall zircon crystals obtained from smectitic mudstone sampled from near  Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 several dinosaur bones in the main quarry layer. The calculated radiometric age for the quarry was 152.18 ± 0.29 Ma (Trujillo and others, 2014; figure 11), a date that puts the quarry at the Kimmeridgian-Tithonian boundary. This date is very close to that obtained for the famous Reed's Quarry 9 at Como Bluff, Wyoming (Trujillo and others, 2015), a site that yielded many hundreds of microvertebrates from the Morrison Formation (Carrano and Velez-Juarbe, 2006).

SYSTEMATIC PALEONTOLOGY Preserved Taxa-Main Quarry Layer
The macrofossil record of plants from the Mygatt-Moore Quarry is unusual for the Morrison Formation in that it is found among abundant bones of sauropods and other dinosaurs. Unidentifiable plant fragments are abundant in every piece of matrix out of the quarry; in fact, as noted above, this plant-debris layer is one way to identify the quarry level even before bones have been located. Among all this preserved plant material, a small percentage is more than fragmentary and can be identified in some cases. The preserved plants from the Mygatt-Moore Quarry include at least 18 species (Tidwell and others,1998; this report), including horsetails (Equisetum figures 12B, 12E, and 12F) cycadophytes    Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 mostly on wood, cones, and short shoots (Steinerocaulis, Behuninia, Conites, Protocupressinoxylon, Mesembrioxylon, Xenoxylon), but some based on small branch sections (Pagiophyllum, figure 12M; Brachyphyllum, figure 12J). Macrofossil plants reported here for the first time from the quarry include the ferns Coniopteris (figures 12G and 12K) and Sphenopteris? (figure 12L), plus a possible indeterminate Isoetaceae (figure 12A) and the conifer Pagiophyllum ( figure 12M). These identifications are tentative, but the specimens are illustrated here to show the specimens found since the Tidwell and others (1998) publication; only Figure 12I (indet. conifer) was illustrated previously (Tidwell and others, 1998). A palynological sample from the Mygatt-Moore Quarry, collected and studied by Hotton and Baghai-Riding (2010) along with seven other samples from the Morrison Formation from New Mexico to Wyoming, is unusual in having an abundance of lycopodiophytes (relatives of modern quillworts and clubmosses), ferns, and conifers. The conifers are mostly podocarpaceans, with a number of indeterminate bisaccate forms. It is interesting that at the nearby Fruita Paleontological Area, the conifers identified by pollen are mostly Cu-pressaceae. The abundance of fossilized wood in the Mygatt-Moore Quarry (figure 13) was studied by Tidwell and others (1998) and includes podocarpaceous wood species, which parallels the abundance of podocarpaceous pollen found in the palynological sample (Hotton and Baghai-Riding, 2010), suggesting that the wood and pollen are of local origin and that podocarpaceous conifers grew rather close to the quarry deposit.
Several samples of abundant plant fragments in the Mygatt-Moore Quarry (figures 12P and 12Q) show a preferred alignment of long axes of elongate fragments. These have been noted particularly in the southwest part of the excavated quarry. This preferred alignment would seem to suggest orientation by gentle water action, perhaps on the edge of a pond; the plant material is small and light enough that its being aligned in such a way by unidirectional hydraulic transport seems unlikely. More likely, the water action may have been oscillatory. Such alignment seems unlikely to result from trampling by large vertebrates, which would seem more likely to produce random orientations. Trampling undoubtedly occurred at the site, however, and this plant fragment alignment simply indicates that a possibly hydrological influence was overprinting fragmentation   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 and possible in situ churning of the plant material. Taxa marked with an asterisk (*) are those reported from the Mygatt-Moore Quarry for the first time here. Brief descriptions of each of the specimens are provided using the terms and architectural characteristics of Ellis and others (2009). Specimens that were previously described in alternate papers have been appropriately referenced but not formally described here.

PLANTAE Division LYCOPODIOPHYTA
Class LYCOPODIOPSIDA Order ISOETALES Family ISOETACEAE INDET.?* Single, partial, lanceolate leaf lamina specimen. Greater than 4.5 cm in length, 0.5 cm in width at the base, and less than 0.2 cm approaching the apex. The base of the specimen is lobate with a wide, obtuse angle.
One fragmentary plant specimen is believed to represent a quillwort (figure 12A), a group also identified from the site by spores. Although rare at the site as macrofossils, if correctly identified, isoetaceaeans appear to be relatively abundant in the palyonologial record (Hotton and Baghai-Riding, 2010).

Division SPHENOPHYTA
Order EQUISETALES Family EQUISETACEAE EQUISETUM SP. CF. LATERALE* Complete specimens are unknown. Internodes are long and cylindrical. The stem bulges at each node by approximately a millimeter. Leaf organization is free and whorled or occurring in a sheath. Leaves overlap nodes; course parallel; apex acute and convex.
Horsetails are represented by several macrofossil fragments (figures 12B, 12E, and 12F); as with most macrofossil plants at Mygatt-Moore, they are not particularly complete. The less typical specimens are similar to some assigned to Equisetum from Montezuma Creek and Dinosaur National Monument and illustrated by Ash and Tidwell (1998) and Foster (2007).

Division PTERIDOPHYTA
Order FILICALES Family DICKSONIACEAE CONIOPTERIS SP.* Complete fronds are unknown and specimens at this site are fragmentary and represented by individual pinnae. The pinna is highly dissected into pinnules and appears to be odd-pinnately compound. The pinnules occur opposite along each pinna rachilla with a decurrent attachment and are obovate in shape.
This fern is represented by four fragments (figures 12G and 12K). This is the first identification of fern macrofossils from the Mygatt-Moore Quarry, although Hotton and Baghai-Riding (2010) identified fern spores in their sample from the site. Why ferns are rare as macrofossils may reflect their rarity in the original flora, but this is more likely a taphonomic artifact.

Order INCERTAE SEDIS SPHENOPTERIS(?) cf. METZGERIOIDES*
Complete fronds are unknown. It is unknown whether the frond is imparipinnate or paripinnate. The overall appearance is feathery and delicate with a robust and linear imparipinnate costa. Shape and course are elongate, feathery and highly dissected. The costa attachment is decurrent and alternate. The pinnules are asymmetrical and deeply dissected. The apex of the pinnules are rounded. The base of the pinnule is indistinguishable from the costa and has a decurrent attachment. This may be a new fern taxon for the Brushy Basin Member, identified based on this small piece from the Mygatt-Moore Quarry (figure 12L) and a recently discovered, more complete specimen from southeastern Utah (S. Ash, University of New Mexico, verbal communication, 2017).

CYCADOLEPIS(?) SP.
Cycadolepis is represented by a single scale (Tidwell and others, 1998), but since then several more possible scale specimens of this cycad have been found.

JENSENSISPERMUM REDMONDII
This cycad was represented by a single seed in Tidwell and others (1998), and since then several more have been found at the site.
A single leaf with similarities to a specimen from the Montezuma Creek locality, illustrated by Ash and Tidwell (1998), may represent Ginkgo (figure 12D). It is possible that this specimen instead represents the conifer Steinerocaulis radiatus (see below) or a single leaflet from the Bennettitalean, Anomozamites sp.

Division CONIFEROPHYTA BRACHYPHYLLUM RECHTENII
This species was identified at Mygatt-Moore by Tidwell and others (1998) based on individual leaves; the specimen illustrated here (figure 12J) is a partial branch that appears to belong to this species of conifer.

UNIDENTIFIED CONIFER
The counterpart of the specimen illustrated here (figure 12I) was also illustrated and discussed by Tidwell and others (1998), who also noted the broad base of the leaves. This specimen remains unidentified although it was compared to Elatides and Geinitzia by Tidwell and others (1998).

BEHUNINIA PROVOENSIS
This species is represented by a short-shoot from Mygatt-Moore (Tidwell and others, 1998), and is known from several other sites in Utah and Wyoming.

STEINEROCAULIS RADIATUS
Already described from Mygatt-Moore by Tidwell and others (1998), this conifer is represented by a number of similar specimens now.

CONITES SP.
Two seed cones are known from the quarry, and they represent the largest known cones from the Morrison Formation (Tidwell and others, 1998).

PAGIOPHYLLUM SP.*
Only fragmentary specimens are known. The rachis is very thick and robust. The pinna can be arranged opposite to spirally around the rachis. The pinna is diamond shaped and comes to an acute, needle-like point at the apex. The base attachment is sessile and the pinna show a rounded attachment to the rachis. Both the base and the apex are rounded.
This new conifer identification for the Mygatt-Moore Quarry (figure 12M) was previously known only from the Belt area sites in Montana (Tidwell and others, 1998) and the Temple Canyon Locality (Gorman and others, 2008).

PROTOCUPRESSINOXYLON MEDLYNII MESEMBRIOXYLON CARTERII XENOXYLON MOOREI
These three types of fossil wood from the Morrison Formation were described as new species by Tidwell and others (1998). All three types of wood exhibited evidence of wood-rotting fungi that, in modern woods, most commonly function in moderately warm and wet conditions (either humidity or soil); in some cases Mygatt-Moore wood exhibited evidence of collapse, which happens in some modern woods during drying of previously soaked heartwood -this is most common in modern wetland species (Tidwell and others, 1998). Paleontology, Taphonomy, Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 METAZOA Phylum MOLLUSCA Class GASTROPODA Family VIVIDPARIDAE VIVIPARUS REESIDEI Gastropods have been found in the "fish layer" and just below it, and well above the main bone layer. However, in 2013, several specimens of Viviparus reesidei were identified at the bottom of the main quarry bone layer at its contact with the underlying light green claystone. The sample included approximately eight internal shell casts and fragments of shell casts (figure 14), and all were in nodular green claystone devoid of plant material.
Amphibians are uncommon fossils in Jurassic sedi-ments and include only two groups: (1) the modern lissamphibians (anurans, caudatans and caecilians), and (2) the larger, brachyopoid temnospondyls. Of these groups, only anurans and caudatans are found in the Morrison Formation. Prior to the 2017 season, amphibians were unknown from the Mygatt-Moore Quarry, but a new specimen of a caudatan has since been recovered at the site. The fragile limb bone (MWC 8649) was found isolated, high in the bone-bearing horizon, just a few centimeters below the level where a rhynchocephalian was found the same season (see below). It is complete, densely mineralized (no primary bone tissue remains visible in the specimen) and is preserved as a part ( figure 15A) and counterpart. There is no ossification of the epiphyses, as is typical in living caudatans, and the diaphysis is asymmetrical with a deeply concave inflection directed toward the midline of the limb. The element is 8.75 mm in length and is much longer than the 2.2 mm radius of Iridotriton from Dinosaur National Monument, which also has a straighter shaft than this specimen. MWC 8649 thus likely represents a different, potentially undescribed caudatan taxon. This element is strikingly similar to the morphology of the ulna in extant Necturus and likely represents either a forelimb (radius or ulna) or hindlimb (tibia or fibula) element in this extinct taxon. Unfortunately, as distal limb elements are not diagnostic at lower taxonomic levels, it is impossible to identify the genus to which this limb belongs or to erect a new genus based solely on this element. MWC 8649 is not referable to Anura, based on its gross morphology. In anurans, the fore-and hindlimb elements are much straighter and there is notable fusion between the radius and ulna, and likewise between the tibia and fibula. No fusion is evident in this specimen. Additionally, the elongate tarsals (tibiale and fibulare) also tend to be fairly straight elements, and are inconsistent with the morphology seen in MWC 8649.
Class REPTILIA Order RHYNCHOCEPHALIA RHYNCHOCEPHALIA INDET.* In 2017, the first rhynchocephalian fossil was discovered at the Mygatt-Moore Quarry after more than three Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 decades of excavations ( figure 15B). The microfossil was found high in the bone-bearing horizon where the mudstone has a higher sand content than lower in the section. At this level, large complete vertebrate remains are not as abundant as they are lower. The element is a partial right dentary (MWC 8671) that is preserved in lingual view and contains five spade-shaped acrodont teeth, each bearing small denticles along the margins. The dentary measures 6.9 mm in length and is broken on both ends, suggesting that taphonomic factors such as trampling or predation/scavenging were at play prior to burial. Although the dentary is too fragmentary to be diagnostic at lower taxonomic levels, MWC 8671 resembles a juvenile Eilenodon with unworn teeth or possibly could be referred to the genus Opisthias (M. Jones, University of Adelaide, written communication, 2017).

REPTILIA INDET.
A small reptilian vertebral centrum from the Mygatt-Moore Quarry (figured 15E to 15G) may belong to either a small ornithopod dinosaur or perhaps a goniopholidid neosuchian crocodylomorph, the latter of which do not have procoelous vertebrae (Steel, 1973), making identification of small reptilian vertebral centra from the Jurassic particularly difficult. This verte-bra also has unfused neurocentral sutures so the neural arch and spine are missing.
A fragment of a very hollow limb bone (figure 15J) was found in the quarry but is too incomplete to identify. Based on the degree of hollowness, however, King and others (2006) suggested that this specimen may represent a pterosaur limb element.
A third, very small, reptilian is represented by a single, tiny conical tooth, approximately 2 mm tall ( figure  15H). This specimen was found by screen washing the matrix in a jacket containing a juvenile sauropod femur. The tooth has slightly wrinkled enamel on the lingual surface and is slightly laterally compressed just at the tip, though it is mostly conical. The tooth is larger than those of most lizard specimens from the Morrison Formation, and it is not of certain crocodylomorph affinity. It is generally similar to some teeth assigned to embryonic sauropods from the Lower Cretaceous of France (Barrett and others, 2016), although even this assignment for the specimen would be tentative.
Small reptilian elements were found in 2005 in the soft green mudstone immediately below the main bone layer in the southwestern part of the quarry. These associated elements include three vertebrae, a fragmentary limb bone, a girdle bone, and several small fragments (figures 15I and 15K to 15L). The vertebrae measure 7 to 9 mm long and 4 to 7 mm in diameter; the centra have wide neural canals and laterally widely spaced pedicals with detached neural arches and exposed sutures. The girdle bone, possibly a scapula, is 30 mm long and has a somewhat blade-like shaft and a preserved "glenoid. " The limb bone is preserved in several elements, approximately 42 mm long total, and appears hollow to some degree. The material suggests either a relatively large rhynchocephalian or possibly a turtle; in the latter case, the girdle bone would be an ilium, not a scapula.

Clade CROCODYLIFORMES Family GONIOPHOLIDIDAE GONIOPHOLIDIDAE INDET.
Goniopholidids are represented by a single vertebra at the Mygatt-Moore Quarry (MWC 1902). This vertebra is small but is very slightly procoelous and has the ventrally projecting hypapophysis typical of neosuchian   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 posterior cervical to anterior dorsal vertebrae (figures 15C and 15D). At least one small tooth of a goniopholidid crocodylomorph was found at the quarry as well, but no other such material has been found in nearly 30 years of excavations.

CERATOSAURUS NASICORNIS
Ceratosaurus is represented in the fauna by nine isolated shed teeth characterized by either ridged lingual enamel surfaces (in the case of mesial teeth of the dentary and premaxilla) or by large strongly laterally compressed, finely serrated distal teeth of the dentary and maxilla (figure 16A; Gilmore, 1920;Madsen, 1976;Madsen and Welles, 2000). The holotype of Ceratosau-rus (USNM 4735) is from the Marsh-Felch Quarry in Garden Park, near Cañon City, Colorado (Gilmore, 1920). Ceratosaurus magnicornis was named from a partial skull and skeleton from the Fruita Paleontological Area, and C. dentisulcatus was named from another partial skull and skeleton from the Cleveland-Lloyd Dinosaur Quarry (Madsen and Welles, 2000;Sanders and Smith, 2005). Rauhut (2003) suggested that the two more recently named species of Ceratosaurus from the Morrison Formation were not sufficiently differentiated from the type species, and Carrano and Sampson (2008) formally synonymized C. magnicornis and C. dentisulcatus with C. nasicornis. Although this taxon is only identified from teeth out of the Mygatt-Moore Quarry they are tentatively referred to Ceratosaurus nasicornis, because there are no differences between the Mygatt-Moore material and the other specimens.

ALLOSAURUS FRAGILIS
Bones of Allosaurus are the most abundant vertebrate fossils at the Mygatt-Moore Quarry (Foster and others, 2007). This theropod is represented by more than 240 elements and at least 360 teeth in the quarry (figures 16B and 16D). Represented elements include skull, girdle, and limb elements, with dorsal and caudal vertebrae particularly abundant (figure 17). The material is identified as Allosaurus based on the presence of (1) a single lateral pneumatic fossa on the cervical centra (versus two in Ceratosaurus), (2) absence of prespinal chonos on the dorsal vertebrae (versus presence in Ceratosaurus), (3) tapering of neural spines of anterior-to mid-caudal vertebrae so that the distal end is narrower than the base in lateral view ("beveled cranial margin of the spine" in Holtz and others, 2004; distal end and base same width in lateral view in Ceratosaurus), (4) scapula with a less expanded proximal end than in Ceratosaurus, and (5) tibia with the astragalus contact ridge on the antero-distal surface that rises well up the shaft (versus very low in less derived theropods). None of the theropod material out of the Mygatt-Moore Quarry is referable to the Megalosauridae or Torvosaurus. Teeth are identified as Allosaurus based on the lack of lingual ridges on the anterior teeth and on the smaller size of the tooth crown, less lateral compression, and moderate-size serrations relative to Ceratosaurus and Torvosaurus (Bakker and Bir, 2004).
There are several named species of Allosaurus, and although few authors agree on which are valid (e.g., in Paul, 1988;A. fragilis andA. maximus in Smith, 1996, 1998), it is possible that only three species of allosaurids were present during Morrison times: the stratigraphically older-ranging Allosaurus n. sp. (Chure, 2000), the more common (and more robust) A. fragilis (Madsen,1976), and the rare and stratigraphically high Saurophaganax maximus (Chure, 1995;see Rauhut, 2003, andHoltz andothers, 2004). Dalman (2014) named A. lucasi based on material from southwestern Colorado, although this taxon has not yet been addressed by others. Carpenter (2010) documented significant variation in the Allosaurus fragilis population from the Cleveland-Lloyd Dinosaur Quarry, which suggests that some of the variation used to define species of Allosaurus in the Morrison Formation may be suspect. The material of the Mygatt-Moore Quarry is not large and robust enough to be S. maximus but is more robust than Allosaurus n. sp. The complete dorsal vertebrae from Mygatt-Moore lack the accessory horizontal processes along the bases of the neural spines seen in S. maximus (Chure, 1995). Although autapomorphies of Allosaurus nov. sp. are missing from the Mygatt-Moore material due simply to lack of elemental overlap, the overall robustness of the vertebrae and girdle elements differentiates this material from that species. Additionally, a left lacrimal of Allosaurus from the Mygatt-Moore Quarry preserves the pneumatic recesses at the base of the cornual process; the presence of these is one of the autapomorphies of A. fragilis (Chure, 2000). The Mygatt-Moore Quarry material is therefore referred to this species.

Suborder SAUROPODA Family DIPLODOCIDAE Subfamily APATOSAURINAE Genus APATOSAURUS
A large number of the sauropod elements out of the Mygatt-Moore Quarry may be referred to the diplodocid genus Apatosaurus, with all regions of the skeleton present and cervical and caudal vertebrae particularly abundant ( figure 18). The generic identification is based on the following characters: (1) short and robust cervical ribs, (2) cervical ribs set well below and lateral to the centrum, (3) anterior caudal vertebrae have "wing-like" Paleontology, Taphonomy, Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 caudal ribs (similar to Diplodocus and Barosaurus) but generally lack well-developed lateral pneumatic fossae (pleurocoels) in the centra, (4) coracoids with roughly quadrangular lateral profile, and (5) proximal condyles of the ulna sub-equal in length (Upchurch and others, 2004). The material from the Mygatt-Moore Quarry may be referred to Apatosaurinae based on the following synapomorphies of Tschopp and others (2015;numbers in parentheses refer to their synapomorphy identifiers): (1) cervical ribs project well beneath centrum so that the diapophysis/tuberculum is approximately the same length as the posterior process, (2) no paired pneumatic fossae on the ventral surface of anterior cervical vertebrae, and (4) anterior process of posterior cervical ribs is absent or reduced to a short bump-like process. Because we treat the clade Apatosaurinae of Tschopp and others (2015; Apatosaurus + Brontosaurus) as consisting of several species but all within Apatosaurus , we identify the Mygatt-Moore material as belonging to Apatosaurus. (2004) criteria, the Apatosaurus material from the Mygatt-Moore Quarry may best be referred to the species A. cf. louisae, first identified from Dinosaur National Monument (Gilmore, 1936). Among the recognized species of Morrison apatosaurs, A. ajax and A. parvus may be ruled out as candidate species for the Mygatt-Moore material due to the cervical vertebrae not having a width:height ratio of 1.5 or greater and to the scapular acromial ridge being at less than a 90-degree angle to the long axis of the scapular blade, respectively. Foster (2015), however, noted that the interspecific variation in these characters among apatosaurines was slight and that, regardless, the small sample size left the characters potentially statistically insignificant. The Mygatt-Moore material is in some respects similar to A. excelsus by Upchurch and others' (2004) criteria but bears greater similarity to material of A. louisae. As in A. excelsus, the cervical ribs terminate anterior to the posterior end of the centrum; as in A. louisae, the proximal ends of the sacral ribs are on the anterior part of their respective centra, the ante-rior caudal centra are ventrally keeled (a "ventral midline ridge"), and the beveled glenoids of the scapulae face anteriorly, ventrally, and medially (Upchurch and others, 2004). The Mygatt-Moore material is thus tentatively assigned to Apatosaurus cf. louisae.

By Upchurch and others
Identifying the Mygatt-Moore material to species based on synapomorphies identified by Tschopp and others (2015) is more ambiguous. The quarry's apatosaurine material includes few skull elements, but of four autapomorphies of A. ajax, three are in the skull; the fourth, "elliptical depression between the lateral spinal lamina of caudal neural spines and the postspinal lamina, " is true of most anterior caudal vertebrae from the site. (A possible juvenile apatosaur basioccipital-basal tubera, figures 21A and 21B, does not seem to match autapomorphy 3 for A. ajax.) However, there are 28 autapomorphies for A. louisae (Tschopp and others, 2015) and of these the Mygatt-Moore material is clearly lacking characters 11 and 12 (the rest of them are unclear in the preserved material or cannot be confirmed because the material was found disarticulated). Of seven autapomorphies of Brontosaurus excelsus listed by Tschopp and others (2015), most are ambiguous regarding the disarticulated Mygatt-Moore material, but character 1 does not match the type and referred material (some Mygatt-Moore cervical vertebrae do have median tubercles) and character 3 is highly variable within the Mygatt-Moore material. The Mygatt-Moore apatosaurine material does not match characters 5, 8, or 9 for B. parvus (Tschopp and others, 2015; character 1 is true only of a few anterior cervical vertebrae, but not most cervicals among Mygatt-Moore material), and it matches character 5 but not character 2 for B. yahnahpin (Tschopp and others, 2015).
Based on these results, it is difficult to assign the Mygatt-Moore material with confidence to any one apatosaurine species defined in Tschopp and others (2015). Most elements out of the quarry either lack the right data for most characters or their disarticulated condition prevents certainty when characters depend on skeletal position. Most importantly, for many characters for which we can identify their states, the material is contradictory by matching some autapomorphic characters, but not matching others. This would not be Paleontology, Taphonomy, Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 unexpected if there are multiple species in the sample, but we assume the Mygatt-Moore apatosaurine material represents a single species because most other large dinosaur quarries in the Morrison Formation contain a single species of each genus of sauropod (Foster, 2003).
We therefore are relying, until contradictory evidence is found, on the autapomorphies of Upchurch and others (2004) and identify the Mygatt-Moore apatosaurine material as A. cf. louisae. Discussion -Some of the material assigned to Apatosaurus consists of juvenile elements, including at least one cervical rib, a femur and a scapula (see SAU-ROPODA MISC. section for figures). Juvenile elements have been recognized from the Mygatt-Moore Quarry for some time, and juvenile material in general is not especially rare in the Morrison Formation (Foster, 2005a). A very large, complete femur from the quarry (1876 mm in length) may belong to Apatosaurus ( figure  18K) and represents the second longest complete femur ever found in the formation (after the type specimen of Brachiosaurus FMNH 25107). In 2016, Museums of Western Colorado crews collected the nearly complete skull of an apatosaurine (with cervicals 1-3 nearly in articulation) from Mygatt-Moore (McHugh and others, 2017); this specimen is still being prepared.

Subfamily DIPLODOCINAE DIPLODOCINAE INDET.
Eight elements, all caudal vertebrae, may be referred to non-apatosaurine diplodocids based on the "winglike" caudal ribs, deep pleurocoels, and ventral excavations of the anterior caudal vertebrae. The mid-caudal vertebrae are elongate with small pleurocoels and groove-like ventral excavations ( figure 19). These caudals are distinct from those of Apatosaurus and are similar to Diplodocus and Barosaurus (Hatcher 1901;McIntosh, 1990;Ostrom and McIntosh, 1999). Distinguishing caudals of Diplodocus and Barosaurus is rather difficult, as most of the differences between these two Morrison diplodocines are in the cervical vertebrae and the number of dorsals (Curtice, 1996;McIntosh, 2005). Barosaurus may have mid-caudal centra that are relatively shorter and less deeply excavated ventrally than in Diplodocus (McIntosh, 1990;Foster, 1996), but quantifying these differences is difficult. The material from the Mygatt-Moore Quarry is therefore identified only as Diplodocinae indet. Numerous isolated teeth of diplodocids are found in the quarry, and some of them may have belonged to Diplodocus or Barosaurus individuals, but Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H. (Foster, 2003), is rare at the Mygatt-Moore Quarry. It is represented by 23 bones, mostly caudal vertebrae, a braincase, two dorsal vertebrae, a scapula (distally expanded), three pelvic elements, several limb elements, a dentary fragment with teeth of a juvenile, and 35 shed teeth, mostly of adults, but with some juveniles represented. These elements are characteristic of Camarasaurus within the Morrison Formation in having (1) anterior caudals with simple caudal ribs, no pleurocoels, rounded unexcavated ventral outlines, and short transversely expanded dorsal ends of the neural spines, (2) dorsal vertebrae with transversely expanded ("fan-shaped") neural spines (figure 20), (3) particularly elongated metacarpals, (4) robust, spoon-shaped teeth, and (5) antero-posteriorly narrow, transversely elongate supratemporal fenestra, compared with diplodocids (Osborn and Mook, 1921;Gilmore, 1925;Berman and McIntosh, 1978;Madsen and others, 1995). The two dorsals do not preserve the neurocentral sutures well enough to determine if they are the high-pedicel-bearing type of Camarasaurus grandis or the low morphology of C. lentus (McIntosh, 1990), but all specimens are smaller than those assigned to C. supremus. The caudal neural spines appear to be gradually transversely expanded near the distal ends, in the manner of C. lentus and not like C. grandis or C. supremus (Ikejiri, 2005).

SAUROPODA MISC.
Many specimens cannot be assigned to lower taxonomic levels but are clearly sauropod elements. Most of these belong to adult sauropods, but a number of juveniles are represented, some of them very young. Juvenile sauropod elements represented in the quarry include a diplodocid basioccipital (figures 21A to 21C), a Camarasaurus dentary fragment (figure 21D), a possible Apatosaurus cervical (figure 21E), a possible Camarasaurus dorsal (figure 21G), and several Apatosaurus elements (figures 21H to 21J). The sample represents at least three juvenile sauropods (MNI).

MYMOORAPELTA MAYSI
The polacanthid ankylosaurian Mymoorapelta is represented by more than 160 elements from all parts of the skeleton (figure 22; Kirkland and Hunt-Foster, in preparation), making this genus the third most abundant vertebrate in the Mygatt-Moore Quarry. Particularly abundant are lateral spines, osteoderms, dorsal and caudal vertebrae. Based on comparative size of elements, there are at least two individuals present; occurrence of some ankylosaurs osteoderms stratigraphically high within the quarry suggests that more than two individuals may in fact be represented. Mymoorapelta was the first Jurassic ankylosaur identified in North America (Kirkland and Carpenter, 1994). Polacanthid anky-   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 losaurs have since been found at a number of other sites in other areas of the Morrison Formation, including western Colorado, Utah, and Wyoming (e.g., Kirkland and others, 1998;Tremaine and others, 2015).

Clade NEORNITHISCHIA OTHNIELOSAURUS CONSORS
Othnielosaurus was found in the quarry in 2008, the first new dinosaur taxon for the quarry in 18 years at the time. It is represented by a small maxilla or dentary fragment with three teeth in matrix (figure 23; MWC 5966). The teeth are tiny (0.9-1.4 mm in mesiodistal length), diamond-shaped, and contain two to four denticles each, characteristic of some Othnielosaurus from other localities (Galton, 1983(Galton, , 2007 and perhaps distinct from other small neornithischians of the Morrison Formation such as Drinker (Bakker and others, 1990), and very distinct from Dryosaurus (Galton, 1981). The teeth also lack the cingula of Stegosaurus and ankylosaurs. Carpenter and Galton (in press) suggest that Othnielosaurus is a junior synonym of Drinker nisti.

DINOSAURIA? INDET. Eggshell Angustiprismatic Morphotype
A 43 x 34 mm fragment of what was estimated to have been an originally 10 cm diameter egg was found in the Mygatt-Moore Quarry during the 1990s ( figure  24). This calcitic eggshell is of an unnamed type that is considered a dinosauroid-prismatic basic type, and is also unique to the Mygatt-Moore Quarry within the Morrison Formation (Bray and Hirsh, 1998).

Preserved Taxa-"Fish Layer"
Phylum MOLLUSCA Class GASTROPODA Several indeterminate gastropods have been identified in the "fish layer" beds. These are not well preserved and are difficult to identify.

Phylum ARTHROPODA
Clade CRUSTACEA Class BRANCHIOPODA Order DIPLOSTRACA A number of "conchostracans" have been found in the claystone just below the "fish layer" (Kirkland and Carpenter, 1994) (figure 25). All are in light gray mudstone devoid of plant material. The term conchostracan has been abandoned after determination that the group was paraphyletic. The taxa formerly within "Conchostraca" are now placed within several families within the Diplostraca (Martin and Davis, 2001).    Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 These bivalve crustaceans are capable of dormancy in dry habitats and are thus able to survive in ephemeral wetlands.

Class MALACOSTRACA Order DECAPODA Unnamed crayfish
A nearly complete crayfish fossil was found in the "fish layer" during the 1990s, and this specimen has not yet been described or named. A second fragmentary crayfish specimen was discovered at the site in 2005 (figure 26I).

Phylum CHORDATA Superclass OSTEICHTHYES Clade PALAEONISCOIDEA MORROLEPIS SCHAEFFERI
Several specimens of the coccolepid palaeoniscoid Morrolepis have been found since the description of the type specimen by Kirkland (1998) (figures 26B to 26D). Some of these specimens show details of caudal and dorsal fin morphology better than the type, due to better preservation. Another specimen preserves a skull, possibly of Morrolepis (figure 26G).

Class ACTINOPTERYGII Subclass NEOPTERYGII Order INDET. "HULETTIA" HAWESI
The type specimen of "Hulettia" hawesi is from the Fruita Paleontological Area, but the species was soon after identified from the "fish layer" at the Mygatt-Moore Quarry. There are now at least five specimens known from the "fish layer, " including a scale patch, two scale patches with partial caudal fins, an anterior third of a fish with the skull, and a nearly complete fish except for the skull (figure 26A). The latter two specimens have been found since 2005. This species probably belongs in a separate genus from Hulettia, from the marine Sundance Formation (J. Kirkland, Utah Geological Survey, verbal communication, 2003), which is why the generic name is in quotations here.

ACTINOPTERYGII INDET.
Several other specimens are known from the "fish layer," most consisting of partial or isolated fins, and these generally cannot be identified to a lower taxonomic level. One partial fish skeleton, preserved in the left lateral view in its posterior portion and in oblique right-dorsal view in its anterior portion (figure 26F), is somewhat more complete but still awaits identification.

Sample Size
Over 30 years of work at the Mygatt-Moore Quarry, several thousand bones have been collected and mapped. The quarry map contains nearly 2400 mapped elements, whereas the census of the data in the Museums of Western Colorado collections consists of more than 900 elements.

Relative Abundances of Fossil Vertebrates
The sample of 897 identifiable bones indicates that the Mygatt-Moore Quarry is dominated by sauropod dinosaurs and the theropod Allosaurus fragilis (figure 27). By number of elements, the most abundant vertebrate taxa by far are indeterminate sauropods and Apatosaurus cf. louisae, followed by Allosaurus and then Mymoorapelta ( figure 27A). All other taxa are rare, including the otherwise common Morrison sauropod Camarasaurus. By minimum number of individuals, Allosaurus is most abundant followed by Apatosaurus and then indeterminate sauropods ( figure 27B).
Allosaurus is also particularly abundant based on the tooth sample out of the quarry (figure 28). The tooth sample indicates a normal distribution of basal crown length of Allosaurus teeth and a larger sample, contrary to what was found previously with a smaller example from the quarry by Foster (2005b).

Preserved Elements
The total sample inventoried in the Museums of Western Colorado, including taxonomically unidentifiable material, consisted of nearly 1900 elements (table  2). Although fragmentary bones were most abundant,     Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 teeth, vertebrae, and ribs were well represented ( figure  29). Girdle, limb, and dermal bones were about equally represented, and skull elements were comparatively rare. An analysis of the preserved elements by modified Voorhies bone distribution groups (Voorhies,1969;Carpenter, 2013;see Methods) for the sauropods and Allosaurus subsamples of the inventoried collection (figure 30) indicates that the percentage representation of elements in highly transportable (Group 1) to trans-port resistant (Group 3) categories closely matches the representation of the elements in the respective (sauropod and Allosaurus) skeletons. This suggests that the better-preserved, identifiable material from the quarry was not likely transported into the quarry area as an assemblage nor winnowed to its current condition with   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 highly transportable elements washed elsewhere. Rather, the Voorhies element analysis results seem to indicate that at least the identifiable material was essentially autochthonous to the local quarry area (fragmentary and rounded; i.e., transported allochthonous material, however, is also abundant in the quarry).

Age Distribution
The main bone layer sample includes, by minimum number of individuals of dinosaurs and other vertebrates, four juveniles, one sub-adult, and 17 adult-sized animals (figure 31). Such a distribution with many adults, fewest number of sub-adults, and moderate representation of juveniles is suggestive of an attritional morality assemblage with delayed burial (Eberth and others, 2007, their figure 5.7). This argues against a non-selective catastrophic mortality for the animals preserved and indicates that the skeletal material was    Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 exposed for some time before burial, which is consistent with other taphonomic factors noted about the site (see below).

Bone Distribution
The nearly 2400 mapped bones from the Mygatt-Moore Quarry show a pattern of disarticulation and only slight association in rare instances (figure 32). Of the entire mapped sample only eight bones are in articulation with at least one other one (three pairs of dorsal vertebrae and one pair of cervical vertebrae, all of Apatosaurus; two of the three dorsal pairs appear to be from the same sub-adult individual). This ratio of articulated bones to the total number (0.00337) is lower than for any other major quarries of the Morrison Formation studied here (table 3). The almost totally disarticulated nature of the sample out of the Mygatt-Moore Quarry has been one of its notable features since it was first worked. From the quarry map (figure 32) it is also apparent that the orientation and distribution of material in the quarry is random (see below); bones are scattered in all parts of the quarry, lightly in some areas but clustered in some others.
The main bone layer is approximately 1 m thick and the occurrences of bones within this interval are concentrated near the basal 33 cm or so. More than half of the bones collected from the 2000-2013 seasons were found in that lower third of the deposit (figure 33). Elements from the upper third accounted for only a little more than 15% of the sample. The vertical distribution of fossils in the quarry area was also illustrated qualitatively in sections by Kirkland and Carpenter (1994) and Kirkland (2006).
Based on the number of mapped bones to mapped area, there appear to be approximately 4.9 mapped bones per square meter of the quarry. This density, however, is based on the bones large enough to map and record. Because very small and fragmentary bones have been noted as common in the quarry, but have not historically been documented, an unexcavated square meter was selected from the eastern part of the quarry to excavate and collect everything found to record the density of material within this test area. This nearly cubic meter (m 3 ) of matrix contained about 100 randomly distributed bones (figure 34); mostly these are small fragments, but the sample also included four theropod teeth. The results showed a relatively random orientation within the meter but concentrated in the lower portion. The fragments were mostly unidentifiable and small, with a mean volume of 6.78 cm 3 and a maximum volume of about 47 cm 3 (figure 35). The excavated m 3 is thought to be typical of the deposit overall because it is from an area close to where large dinosaur bones have been excavated. It is from neither within the current excavation nor close to what appears to be the edge of the deposit to the east. The material was found within typical medium light-gray claystone containing abundant carbonized plant fragments. If the many small bone fragments randomly distributed within this 1 m3 indicate what is typical of the quarry, there may have been approximately 50,000 individual bone pieces within the about 500 m 2 of the quarry excavated so far. Extrapolated to the potential area of the quarry demonstrated by the drill coring (see above), the entire deposit may contain as many as 463,000 bones, based on the fully sampled single m 3 , and as many as 23,150 mappable bones, based on the density demonstrated by the quarry map (figure  Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 32). The abundance of small fragmentary bones, along with larger better preserved bones and more complete dinosaur elements in the same deposit, may suggest two sources for the material. As mentioned above, the quarry map (figure 32) appears to show a random orientation of the bones at Mygatt-Moore. Azimuth orientations of bones from the quarry, measured off the map and categorized as either bidirectional or unidirectional, suggest that indeed the orientation is random. Neither bidirectional bones nor those that are unidirectional demonstrate any strongly preferred azimuth orientation (figure 36) such as would be expected if there were current influence on the material (Toots, 1965). Random orientations of bones as demonstrated by figure 36 suggest scattering by scavenging and other process other than hydraulic current influence.

Bone Preservation and Modification
A large number of bones in the quarry are fairly well preserved and complete. Most of the identifiable elements that have been collected over the years are nearly complete, with only moderate degradation by abrasion, corrosion, weathering, and breakage. Among the bone modifications are green fractures that appear to have occurred when the bones were in or on the muddy matrix near the surface, well before lithification. Examples of these breaks include splintered, broken sauropod ribs (figures 37A and 37B), a broken and displaced Allosaurus metatarsal (figures 37E and 37F), and an Allosaurus distal tibia broken off before burial. A random survey of a subsample of the total collections inventory (446 specimens, or about 23.5% of the bone collection) indicates that approximately 61.6% of bones have post-mortem, pre-burial breaks that in many cases probably resulted from trampling. Only about 0.46% of the bones have pathologies (table 4). One of these includes a sauropod ulna with a slightly forked, distally-pointing bone spur along the shaft near the distal end of the element (MWC 5044).
Another common modification on bones is tooth marks left by carnivorous theropod dinosaurs. Approximately 4.62% of the bones out of the quarry have some    Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 indication of tooth marks (table 4), which is perhaps not surprising given the more than 400 teeth of theropods that have been found in the deposit (98.3% of these teeth appear to be shed and were probably lost during scavenging, as only a few percent of the 419 teeth counted for this study had intact roots). These teeth are so abundant that they are often found in close association with bones of herbivores. One tooth was found in close proximity to the pleurocoel of an Apatosaurus cervical vertebra (figure 37G), and in another case nine teeth were excavated in 2005 from around a single sauropod femur. Among the distinctly tooth-marked bones from the quarry are an Apatosaurus ischium ( figure 37D), an Apatosaurus pubis, and the type ilium of Mymoorapelta (Kirkland and Carpenter, 1994). Abrasion of bones was noted in about 24.5% of the bones surveyed. It was characterized commonly by single, small, shallow, and randomly oriented scratches on the bone surface in some cases, but most often by rounding of originally sharp edges of bones or broken elements of them, a common characteristic of transport modification. In some cases, abrasion was difficult to distinguish from corrosion. If a bone was not significantly broken otherwise, it was assumed that most wearing of edges was more likely due to corrosion than transport abrasion.
The corrosion of bone from Mygatt-Moore was found to be concentrated on the edges of vertebral centra and on the ends of long bones such as limb elements and ribs (figure 37C), where cortical bone is thin and trabecular bone close to the surface. Most limb boneshafts, for example, where cortical bone is thick, are unaffected by corrosion. The nature of this bone loss is confirmed as corrosion rather than abrasion by the documentation of thin and delicate spalled outer bone surfaces separated from trabecular bone in cross sections of several Mygatt-Moore bones collected and thin sectioned in the 1990s (J. Kirkland, Utah Geological Survey, verbal communication, 2013).
Weathering was noted in 25.2% of bones from the   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 sample and was generally characterized by cracking and decay of the outer surface of the bone. The degree of weathering (i.e., average weathering stage) of bones from the quarry, however, was generally low. The majority of bones surveyed were either unabraded (0) or only slightly so (1, figure 38A). A number of bones demonstrated an abrasion condition of 1 to 2 and none was at 3. Similarly, most surveyed bones were unweathered (0) and none reached class 3. Corrosion and breakage were much more common (figures 38B and 38D), and for both of these a number of bones were in classes 2 and 3. In contrast, a surveyed subsample of the collection from the Cleveland-Lloyd Dinosaur Quarry (at the Natural History Museum of Utah) indicates much less modification in all four categories (figure 39); none of those elements reached class 3 (in case of weathering, class 1 was the highest recorded for any bone). Although the degree of corrosion and breakage within the Cleveland-Lloyd subsample was higher than it was for abrasion and weathering, all were lower than for the Mygatt-Moore Quarry subsample. The Cleveland-Lloyd Dinosaur Quarry sample was, however, museum-based, meaning that there was a collecting  bias in favor of better preserved material from the start.
The sample studied from the single test meter in the Mygatt-Moore Quarry, interestingly, showed a different pattern from the collection inventory subsample. The test meter consisted of a full recording of every bone piece within a square meter, regardless of size or identification potential. Because of that fact, we perhaps should not be surprised that the results for the test meter (figure 40) are quite different from the quarry collections subsample, and from the Cleveland-Lloyd Dinosaur Quarry subsample, for that matter. In fact, recent detailed work at the Cleveland-Lloyd Quarry indicates a similar pattern in that most bones in the matrix (generally uncollected) are small and fragmentary (Gates, 2005;Peterson and others, 2017), and thus the collections sample surveyed here is skewed in favor of better preservation by collection bias. Many bones at Mygatt-Moore are abraded, more are corroded, a number are lightly weathered, and a large majority are broken quite badly, many rounded into a "bone pebble" shape. The test meter results may indicate that numerous allochthonous, fragmentary bones were transported into the site, although these were not necessarily transported long distances, nor out of the home ranges of the animals they belonged to (see Behrensmeyer and Rogers, 2017).
Histological slides of two very rounded bone "pebbles" (containing no outer bone surface), and of two bone fragments containing sections of the outer surface of the elements as well, indicate very different states of preservation at a microscopic scale. Whereas at fine scale, some bone specimens appear nearly pristine (figure 41C), and others are very well preserved (figure 41B), others have some or a great number of micro-fractures in the bone cell structure (figures 41A and 41D). The range of preservation of these elements suggests that these apparently allochthonous bone fragments may come from several different source areas. Especially interesting is that most rounded and abraded bone "pebble" specimens (figures 41C and 41D) are, respectively, the best and worst preserved pieces at a microscopic scale among the four that were thin sectioned; there was no way to predict this prior to sectioning. The nearly perfect preservation of the osteons in one specimen (figure 41C) and the heavily fractured nature of the same in another (figure 41D) suggest very different pre-burial (or possibly digenetic) histories of these elements and perhaps a complex set of origins for the "background" majority of small bone fragments that comprises the bone sample out of the quarry.

Dinosaur Skin
At least two patterns of dinosaur skin have been found in the Mygatt-Moore Quarry (Foster and Hunt-Foster, 2011), one identified as that of a sauropod consists of ~30 mm diameter hexagonal scales ( figure  42A) and another consisting of much smaller polygonal scales (figures 42B and 42D) that belongs to an indeterminate dinosaur. These skin sections are preserved as either carbonized patches or similarly partially carbonized impressions of scales in matrix. In some cases, the     Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 skin pieces are folded over within the matrix and are separated by up to 1 cm of matrix.

Paleoenvironmental Setting
The stratigraphic sections around Rabbit Valley suggest that the quarry area itself was surrounded by an alluvial system of poorly-drained to well-drained floodplains and sandy to very gravelly river channels. These channels may have been meandering to anastomosing (e.g., Cooley and Schmitt, 1998), as they are only traceable laterally for at most several hundred meters and are noticeably less laterally extensive than the channel sandstones of the underlying Salt Wash Member. The sections also indicate a great degree of lateral variability within the Brushy Basin Member strata as mudstone   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 bed, channel sandstone, and the "fish layer" beds were all documented to change or pinch out laterally within tens to hundreds of meters. The lateral variability is matched by almost universal changes in the vertical stacking patterns (figure 4). These vertical components document changing paleoenvironments over time, and within the quarry area itself we can document several changes. A transition from a permanent or ephemeral overbank topographic low into an ephemeral pond and then to a permanent but small (and probably shallow) lake is indicated by the lithologic variability of the quarry interval. Later, a return to floodplain deposition with a nearby river represented by the channel sandstone at the Camarasaur Stop.
The main quarry layer demonstrates features of a pond or pool. The lack of bones east of trench 5 and the reddish-colored mudstone at the quarry level at the   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 Caudal Draw stratigraphic section, the different character of trench 5 in both trench stratigraphy and thin sections, and the fact that drill holes MM-1 and MM-2 showed that the main quarry layer is thinner and slightly higher stratigraphically, all suggest that the quarry layer is a convex-bottomed, laterally restricted lens. The minimum area of the deposit is approximately 18,200 m 2 and may be the deepest near the present northern part of the quarry (dimensions about 130 x 140 m, mea-sured drill hole MM-1 to trench 5 and drill hole MM-2 to trench 4). Although this is an approximately rectangular area, the actual deposit is probably roughly oval.

Paleoenvironment of the Main Quarry Layer
The base of the main quarry layer (with abundant plant debris and bone) appears to begin with deposition in a quiet, perennial pond, in that the very base of the   Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 quarry (at the contact with the underlying light-green silty claystone) is a layer that contains several viviparid gastropods. The sediments that contain these gastropods, however, do not have any plant fragments. So, these gastropods occur in a basal layer or unit rather different than the actual main bone layer just a few centimeters higher in the section. The main quarry layer has some seemingly contradictory taphonomic characteristics. Some characteristics suggest very wet conditions, possibly perennial standing water; others suggest wet or damp conditions but only ephemeral water. Some evidence suggests the bones in the quarry are locally autochthonous, whereas other rounded and abraded elements seemingly indicate transport a moderate distance; it is unlikely even heavily modified bones were transported great distances or mixed into areas outside the ranges of the animals they derive from (Behrensmeyer and Rogers, 2017). Although there is no clear evidence of miring of dinosaurs at the Mygatt-Moore Quarry as there is at some other sites (e.g., Howe Quarry, Farlow in Bird, 1985), the abundance of theropod material might at first suggest a predator trap. Even though the sample is dominated by just two of the present dinosaur taxa, the age profile 74 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado-Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 suggests an attritional assemblage (see below). The Mygatt-Moore Quarry "bone layer" may represent an ephemeral pond surrounded by conifers, similar to a wooded vernal pool, although there is a paucity of direct evidence to confirm this. Amphibians, often characteristic of modern vernal pools may be very rare at the site due to taphonomic bias. The unit geometry (see above) and the mudstone (with some silt) lithology indicate that it is not a channel and is likely some type of overbank deposit. The presence of many bone fragments (almost a 'background' of small, nearly rounded fragments), calcium carbonate "pebbles, " and small clayballs indicates that a significant portion of the material was washed in, probably during flooding of a nearby channel. The range of micro-preservation characteristics in the histological bone samples, the appearance of two 75 Paleontology, Taphonomy, and Sedimentology of the Mygatt-Moore Quarry, a Large Dinosaur Bonebed in the Morrison Formation, Western Colorado-Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 distinct geochemistry signals in the calcium carbonate nodules, and the different colors and range of the silt and clay content in the clayballs, all in the lower main quarry layer, suggest that there were multiple sources for the allochthonous bones, "pebbles, " and clay clasts in the quarry. In addition, these sources mixed during a single phase of emplacement, though probably not a single episode. Variation in clay clast, nodule, and bone characteristic does not appear to stratigraphically correlate. Despite the "background" of small fragmentary bones (allochthonous), a significant number of nearly complete, well-preserved, but often rotted (corroded), bones are present in the main quarry layer, which probably represent an autochthonous component. Small, elongate plant debris that is often aligned might indicate light (possibly oscillatory) water influence during deposition of the bone layer. The fact that the rose diagrams both show no preferred orientation suggests that whatever current there was, it was not a unidirectional current and not strong enough to align dinosaur skeletal material.
The total lack of fish from the main quarry layer, the fact that not a single turtle element has ever been confirmed from the layer, and the extreme paucity of neosuchian material all suggest that permanent water was probably not present at the site at the time of deposition of the main bone layer. The restriction of "prosobranch" gastropods to layers above and below the main bone layer indicates that the quarry area was a locality of perennial water, but there is no direct evidence to indicate this with certainty from the bone layer itself. That "conchostracans" are present just below the "fish layer" suggests that the area may have been an ephemeral water hole at times, but again there is no direct evidence of these ephemeral pond inhabitants in the bone layer. Actinopterygian fish are restricted to the overlying "fish layer" and indicate perennial water with certainty, but only well after dinosaur quarry (bone layer) deposition was over.
In addition, the fact that other sites such as the Small Quarry (Garden Park), Quarry 9 (Como Bluff), and the Little Houston Quarry (Black Hills) seem to represent ponds, and also preserve abundant turtles, neosuchians, and fish (Foster, 2001(Foster, , 2003, suggests that if these aquatic and semi-aquatic taxa were present at the Mygatt-Moore Quarry at the time, they would have been preserved. Their absence may be due to a mechanical taphonomic bias that filtered out smaller animals (Behrensmeyer and others, 1979), but this bias seems unlikely for taxa as robust as turtles. If these animals were present in large numbers at the Mygatt-Moore Quarry, it would seem at least a few would be preserved.
The abundance of plant material and carbonized plant fragments throughout the main quarry layer at the Mygatt-Moore Quarry suggest that the soil was acidic. The high degree of corrosion (or "rot" along with resulting spalling) in the quarry is also probably indicative of acidic conditions in the quarry mud at the time, as bone preservation can deteriorate in lower-pH conditions (Baxter, 2004). The effects of low pH on bones can sometimes be quite variable even between samples in the same soil, however (Nicholson, 1996). Preservation of dinosaur skin, not just as impressions in matrix, but more often as carbonization, suggests occasional dysoxia in the mud of the deposit, if not necessarily in any standing water that may have been present. Dysoxic or anoxic conditions may have enhanced the frequency of plant preservation also (Briggs, 1999). The presence of contemporary wood-rotting fungi in a number of the wood specimens from the quarry (Tidwell and others, 1998) would also seem to suggest that the environmental setting was wet and possibly humid.
Like the Cleveland-Lloyd Dinosaur Quarry, the Mygatt-Moore Quarry bone layer matrix is elevated in several elements such as Pb, As, Mn, K, and S (Peterson and others, 2017). It is unknown if this phenomenon is due to original conditions in the deposit, diagenetic factors, or perhaps is a result of decomposition of carcasses at the site (Peterson and others, 2017). As discussed above (matrix XRF analysis), it seems more likely that the elevation of several elements is the result of diagenetic processes at the Mygatt-Moore Quarry.
Perhaps perennial water was present at the Mygatt-Moore Quarry during deposition of the bone layer, but were there environmental conditions that kept smaller aquatic and semi-aquatic animals out? The acidic soils and dysoxic conditions apparent at the site during deposition of the main quarry layer might have been Paleontology, Taphonomy, Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 such factors to make the area unappealing to these taxa. However, the fact that turtles, crocodylians, and fish are abundantly preserved in the dark, carbonaceous (presumably somewhat acidic) mudstone and the (sometimes dysoxic; mummifying) sandstone beds of Late Cretaceous deposits suggest that such conditions were neither always avoided by the animals nor detrimental to their preservation. The paucity of neosuchians and the lack of fish and turtles from the main quarry layer may well be a paleobiological pattern then, and the possibility remains that the mud that now comprises the main quarry layer was only seasonally under water. The presence of trampled bones suggests that living animals frequented the deposit area, stepping on bone already in the mud during times of low (or no) water. This trampling is evidenced by many bones broken clearly after the element was in the mud but still exposed at or near the surface; unhealed, splintered fractures are relatively common and were documented in the field as having occurred prior to collection by matrix adjacent to broken surfaces. Documented field evidence, such as vertical changes in mudstone bedding within the quarry and post-depositionally broken bones, also suggests that many bones were churned in the soft mud, again presumably by trampling. Among observed elements that seem to have been preserved in soft wet mud were multiple bones preserved at an angle to horizontal, thin bones with fresh unabraded edges preserved in the matrix on edge, and at least one shed theropod tooth found on end (vertically) with the tip down in the matrix (no associated skull material).
The frequent presence of carnivores and abundant scavenging in the quarry area are indicated by the abundance of Allosaurus bones, the abundance of shed theropod teeth, and the moderate amount of tooth marks on bones. The extremely high rate of disarticulation and significant disassociation compared with other large quarries in the Morrison Formation may be due to trampling by dinosaurs (as with scatter of modern elephant bones by elephants; Haynes and Klimowicz, 2015) and scattering by scavenging theropods. Some modern bones, however, can also be found scattered by lake waters, surface sheet flow, and possibly wind along lacustrine margins in autochthonous deposits (Co-bo-Sánchez and others, 2014). Therefore, a number of factors may have been acting in scattering the material at the Mygatt-Moore Quarry.
Several of the plant types preserved in the quarry are associated with wet environments, including horsetails, Czekanowskia (Tidwell and others, 1998), and lycopodiophytes. Lycopodiophytes today are common elements of some vernal pools (Keeley, 1990) and, in some settings, live in slightly acidic water (Wang and others, 2002). Ferns and conifers are also particularly abundant, and plenty of ginkgoes and cycadophytes are also present. The fact that podocarpaceous wood and pollen are both found in the quarry (Tidwell and others, 1998;Hotton and Baghai-Riding, 2010) indicates that some of the abundant fossil wood at Mygatt-Moore is almost certainly local and therefore that the surrounding area was probably wooded. Among the plants common at the Mygatt-Moore Quarry, Equisetum and ginkgoes are two of the better candidates for plant fodder for herbivorous dinosaurs, whereas Podocarpaceae were probably less appealing (Gee, 2010).
Vertebrate age profiles in hypothetical standing populations would have many juveniles and fewer adults, and non-selective catastrophic mortality would result in a similar profile (Eberth and others, 2007). Attritional mortality could result in an age profile with many juveniles and adults and fewer mid-age classes. Rapid burial of such a profile would result in a similar distribution, whereas delayed burial would favor preservation of adult-size classes (although Myers and Storrs, 2007, noted that with sauropod remains, rate of burial may have less influence on preservation effects than with smaller taxa). Still, delayed burial of an attritional assemblage may result in an age profile with few mid-sized animals, more juveniles, and an abundance of older animals (Eberth and others, 2007) and this is what is observed at the Mygatt-Moore Quarry. The extremely high degree of disarticulation of the material seen at the quarry (lowest measured articulation radio among large Morrison sites) is also consistent with delayed burial (Myers and Fiorillo, 2009). Despite the unusual abundance of two taxa (Allosaurus and Apatosaurus) in the quarry, the age profile seems to indicate an attritional assemblage with non-rapid burial. Therefore, Western Colorado-Implications for Upper Jurassic Dinosaur Preservation Modes Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 the deposit does not appear to be the result of a catastrophic mortality of dinosaurs.
The slow burial would seem support the existence of an ephemeral overbank pond, but the high abundance of just two taxa is harder to explain. Among several scenarios, it is possible that the deposit built up over multiple environmental stresses (e.g., occasional droughts), with the high abundance of Apatosaurus resulting from a population frequenting the water source repeatedly during times when the rest of the floodplain was drying out. That apparently-shed sauropod teeth (mostly Diplodocidae but with a number of camarasaurids) are relatively abundant at the site suggests that in fact the animals were feeding nearby, and it is estimated that population densities on the Morrison landscape would have allowed for a handful of individuals of each sauropod species per km 2 (Farlow and others, 2010). Possibly, a regional apatosaur population tended to frequent the area seasonally, coincidentally when stresses on the herd were highest, while other sauropod taxa migrated through during other, less stressful times (J. Kirkland, Utah Geological Survey, verbal communication, 2013). There seems to be some evidence of regional migration among sauropods from the Morrison (Fricke and others, 2011). At times when the population was in the area it may have had higher densities of animals because the group was absent at other times, and because presumably at times when the apatosaurs were there, the other sauropods were away. An alternative hypothesis is that a local apatosaur population frequented the area yearround, preferring an environmental or dietary aspect of the surrounding area, to the general competitive exclusion of other sauropods. In this scenario, the attritional assemblage would also have been built up over time as the area was populated long-term by a high percentage of Apatosaurus. Although we were able to discern an attritional versus catastrophic origin for the bone assemblage, distinguishing between the two above scenarios to explain the abundance of Apatosaurus relative to Camarasaurus and Diplodocinae indet., may be difficult. The facts that the macro-plant and pollenspore assemblages are thus far almost unique among plant sites in the Morrison (Hotton and Baghai-Riding, 2010), and that the Mygatt-Moore Quarry is one of the only apatosaur-dominated sites in the formation, suggest that perhaps something about the flora of the local habitat was attractive to these particular sauropods. Still, determining whether this attraction was seasonal or perennial may be impossible.

Cleveland-Lloyd Dinosaur Quarry
The major quarry to which the Mygatt-Moore Quarry compares most closely is the Cleveland-Lloyd Dinosaur Quarry in the Morrison Formation of the San Rafael Swell of east-central Utah (Stokes, 1985;Miller and others, 1996;Bilbey, 1999;Gates, 2005;Peterson and others, 2017). Similarities to the Mygatt-Moore Quarry include mudstone matrix, many disarticulated bones, highly mixed arrangements of the bones, many Allosaurus elements (MNI = 49), and a relatively thin bone layer (Richmond and Morris, 1996;Gates, 2005;Hunt and others, 2006;Foster and others, 2016). Differences of Cleveland-Lloyd Dinosaur Quarry compared to Mygatt-Moore Quarry include (1) lack of carbonized plant material in the mudstone, (2) higher degree of articulation, and (3) greater dominance by Allosaurus primarily. Additionally, as found in this study, the Mygatt-Moore Quarry has a much higher percentage of shed carnivore teeth (versus rooted), has slightly more tooth marked bones, has more fragmented, weathered, abraded, and corroded bone, and has more allochthonous clasts and bone fragments. Mygatt-Moore Quarry also demonstrates less evidence of hydraulic sorting and more random azimuth orientations of the bones as compared to the Cleveland-Lloyd Dinosaur Quarry (see Gates, 2005). The Cleveland-Lloyd Dinosaur Quarry has only a single Apatosaurus element , whereas the Mygatt-Moore Quarry has no elements clearly belonging to Stegosaurus; the reasons for these taxa being rare or missing are unclear, as they are otherwise very common overall in the Morrison Formation. The Mygatt-Moore Quarry also has a lower percentage of juveniles and sub-adults than does the Cleveland-Lloyd Dinosaur Quarry. Paleontology, Taphonomy, Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5

Howe Quarries
The Howe Quarry (Bird, 1985) and the Howe-Stephens Quarry (Ayer, 1999) are large, multi-taxic sites in the Morrison Formation of northern Wyoming. Both differ from the Mygatt-Moore Quarry in that the matrix is a silty to slightly sandy mudstone with more direct evidence of fluvial influence (more so at Howe-Stephens). The Howe Quarry contains an abundance of diplodocine sauropod material relative to other taxa, and in this sense, it is similar to the Mygatt-Moore and Cleveland-Lloyd Dinosaur Quarries. The Howe Quarry also preserve a relative abundance of dinosaur skin (Czerkas, 1992).

Little Houston Quarry
The Little Houston Quarry is in the thin Morrison Formation of the Black Hills in northeastern Wyoming (Foster and Martin, 1994;Foster, 2001). This quarry is in a laterally restricted, ribbon-like abandoned channel deposit with interbedded green claystone and laminated gray-green siltstone. Articulation of dinosaur bones is moderate, density of the bone material is high, and microvertebrate material is very abundant, even in among the sauropod bones in some levels. This site preserves a diversity of large dinosaurs and especially small reptiles, mammals, and fish, along with an abundance of turtles and neosuchian crocodyliforms. This preservation mode is common in the northern and eastern parts of the Morrison Formation (Foster and McMullen, 2017) and is characteristic of the Small Quarry and Quarry 9 as well (Foster, 2001(Foster, , 2003. This mode is taphonomically very different from the Mygatt-Moore and Cleveland-Lloyd mode of preservation.

Carnegie Quarry, Dinosaur National Monument
The Carnegie Quarry at Dinosaur National Monument in Utah is a good representative of the taphonomic mode of preservation so common among large dinosaur quarries in the Morrison Formation in that it occurs in a thick, pebbly sandstone (Lawton, 1977;Carpenter, 2013). The site preserves a diversity of dinosaurs, most of the reasonably common ones in the formation, in fact, and in high numbers. There are thousands of bones in this quarry and probably more than 120 individuals represented. The degree of articulation is the highest measured for this study. Sedimentologically and taphonomically channel sandstone deposits like the Carnegie Quarry are very different from large sites in mudstone like Mygatt-Moore and Cleveland-Lloyd Dinosaur Quarries.

Preserved Diversity by Site
One interesting aspect of the preservation of dinosaur taxa at some of the large quarries in the Morrison Formation is the variation in the relative abundance of the most common dinosaur taxa. For example, within the Morrison overall, the most abundant dinosaurs are, in descending order, Camarasaurus, Allosaurus, Stegosaurus, Apatosaurus, and Diplodocus. Each of these is slightly less abundant than the next highest in rank abundance. If we look at large quarries in mudstone and siltstones (fine-grained sites), we notice that (1) this pattern does not hold up and that the relative abundances among these five taxa are highly uneven, with some quarries missing one genus entirely, (2) one or two taxa often dominate, with the other taxa rare, and (3) the dominant taxon or taxa are variable (figure 43; Foster and others, 2016). Although the Mygatt-Moore Quarry has a number of Allosaurus individuals, it is dominated by Apatosaurus by number of identified elements (as opposed to MNI). Cleveland-Lloyd Dinosaur Quarry is even more dramatically dominated by Allosaurus, and the Howe Quarry is dominated by diplodocines. The pattern demonstrated by large quarries in channel sandstones, however, more closely reflects the pattern in the Morrison Formation overall (figure 43). All of the "big five" dinosaur genera are present at all sites, relative abundances are much more even, and no site is dominated by one taxon to the degree that the fine-grained sites are. These different relative abundances probably reflect the relative size of the sampled geographic area and the amount of time averaging within each deposit. The geographic area from which large Morrison river channels were drawing skeletons could well have been rather extensive, particularly considering the possible areas of the individual drainage basins on a relatively Paleontology, Taphonomy, Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H. Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 flat floodplain at the time (Behrensmeyer, 1982). Although sauropod bones and carcasses probably would not have been transported great distances in recognizable form (Myers and Storrs, 2007), it would not be surprising if the area from which a dinosaur deposit in a channel sandstone was drawn was in fact far broader than that of a local pond or overbank topographic low. There is variation in the amount of time represented by different sites within the fine-grained site subsample, with the Mygatt-Moore Quarry perhaps representing an attritional and possibly more time-averaged sample than Cleveland-Lloyd Dinosaur Quarry, for example. The amount of time-averaging in an attritional mudstone site was probably still far less than that of a channel sandstone deposit like the Carnegie Quarry, which appears to include several lag beds superimposed in a thickness of several meters. Both Mygatt-Moore and Cleveland-Lloyd Dinosaur Quarries are much thinner beds of about 1 m thickness. This pattern of uneven relative abundances at mud-stone sites and more diverse and evenly represented taxa at sandstone sites (figure 43) suggests that large quarries of dinosaur bones in the Morrison Formation fall into at least two major taphonomic modes. Many sites in the Morrison consist of paucispecific deposits of a few individuals and up to a few hundred bones, sometimes also dominated by one or a few taxa, in poorly drained floodplain mudstones (e.g., Tucker, 2011). Very large deposits of multiple hundreds or thousands of bones are often either (1) diverse sandstone quarries with relatively even abundance distributions among the most common dinosaurs, or (2) less diverse fine-grained (mudstone or siltstone) quarries with sometimes wildly uneven abundance distributions among the most common dinosaurs (some missing entirely). There are undoubtedly a number of the former deposits in the Morrison Formation channel sandstone beds with many bones and many taxa (e.g., Carnegie Quarry, Dry Mesa Quarry), and undeveloped examples can be seen in the Morrison Formation at least in Colorado and Utah  Foster, J.R., Hunt-Foster, R.K., Gorman, M.A., II, Trujillo, K.C., Suarez, C.A., McHugh, J.B., Peterson, J.E., Warnock, J.P., and Schoenstein, H.E. Geology of the Intermountain West 2018 Volume 5 (e.g., Split Rock, Mill Canyon). Perhaps there are yet undiscovered deposits of the latter kind, large potential quarries in mudstone that sampled a Camarasaurus-or ankylosaur-dominated fauna, for example. These sites have the potential to tell us a little more about the vagaries of preservation and dinosaur paleobiology than do more time-averaged and geographically blended sites.

CONCLUSIONS
The Mygatt-Moore Quarry is Late Jurassic in age (152.18 +/-0.29 Ma), very close to the Kimmeridgian-Tithonian boundary. The Mygatt-Moore Quarry is in a claystone that has slightly higher (and coarser) silt content than the surrounding claystone beds. Claystone beds that are lateral to the quarry deposit are different in color and silt content and, in some cases, include sandstone beds near quarry level away from the site. The bones were probably deposited on a topographically low part of the floodplain, possibly within 800 to 1000 m of a river channel to the east (figure 4).
The quarry was at least about 130 m by 140 m across, possibly an oval shape. The area may be as much as 18,200 m 2 as currently known from excavations, trenches, and drill cores. The thickest part of the quarry is in the northern area of the current excavation; it thins to the east, southwest, and northwest. The base slopes upward in those latter directions as well, so that the bone layer is somewhat lens-shaped (convex down).
The quarry preserves both autochthonous and allochthonous bone components. There is a group of mostly fragmented and almost rounded bones that was probably washed in early in the deposit's history from a significant distance and a second group of more intact bones that were sourced from skeletons of animals that died near or at the point of deposit itself.
As a probably perennially damp to seasonally flooded low area of the floodplain, the quarry area was very likely occupied and immediately surrounded by a greater abundance of conifers, horsetails, and, to a lesser degree, ferns than the next most outlying areas. Abundant plant material in the quarry bone layer matrix was likely preserved due to acidic soil conditions, which are conductive to plant preservation. Relatively low pH during deposition of the bone deposit seems likely, although at the earliest stages when the "pebble layer" was deposited, pH may have been higher. Dampness from high moisture content in conifer wood in the quarry area led to rotting of wood by fungi. Preservation of carbonized skin of dinosaurs (and plant material) suggests the mud of the deposit may have been dysoxic to anoxic.
The calcium carbonate nodules that are abundant near the base of the bone layer appear to be washed in from at least two different source areas, but how far they were transported is difficult to assess. They formed in place as incipient soil nodules but were then reworked and deposited at the quarry after transport. Their origin could be from a different area than the rock clasts and clayballs in the matrix of the main bone layer. Much of the initial input to the bone layer (basal portion) thus appears to have been from elsewhere.
The abundance of Allosaurus bones and teeth is almost certainly a result of a combination of environmental stresses on the animal (drought?) causing slightly higher mortality of the population and increase in the number of individuals in the area, probably drawn in to scavenge on carcasses of both herbivorous and carnivorous dinosaurs (figure 44). Although the abundance of shed teeth and tooth-marked bone clearly indicates scavenging, there is no direct evidence of predation occurring at the site.
The abundance of Apatosaurus appears to be from attritional mortality of a resident population of animals that frequented the area either year-round or seasonally. If these animals were around all year, it may have been due to an affinity for the Mygatt-Moore Quarry area's unique mix of plant types. Other sauropods like Camarasaurus were probably competitively excluded or simply did not like the fodder in the area and so would be preserved less frequently.
Although it is possible that acidity or oxygen-poor conditions excluded small aquatic taxa or caused them not to be preserved, this is not the case at other, very wet sites in the Morrison Formation (e.g., Little Houston, Quarry 9). We believe the near total lack of aquatic and semi-aquatic animals represents their actual rarity during deposition of the deposit due to infrequent (possibly only seasonal) presence of standing water at