A new atoposaurid crocodylomorph from the Morrison Formation (Upper Jurassic) of Wyoming, USA

A left mandible of a small crocodyliform found in the Upper Jurassic Morrsion Formation of northeastern Wyoming represents the first occurrence of the atoposaurid Theriosuchus in North America. The specimen demonstrates lower jaw morphology, including heterodonty (as indicated by alveolus shape), similar to Theriosuchus and Knoetschkesuchus, but autapomorphies and a unique combination of characters among these taxa indicate that it is a distinct, new species of Theriosuchus.


INTRODUCTION
Fossil crocodylomorphs are diverse in the Upper Jurassic Morrison Formation of western North America, with seven species of terrestrial and semi-aquatic forms occurring, with collective widespread distribution and high abundance (e.g., Clark, 2011;Pritchard and others, 2013;Foster and McMullen, 2017). In 2004, the left mandible of a small crocodyliform was collected from the Little Houston Quarry in the Morrison Formation of the Black Hills, northeastern Wyoming (figure 1). This specimen was found along with a diverse assemblage of dinosaurs and microvertebrates, the latter including mammals, fish, amphibians, a lizard, sphenodontians, the choristodere Cteniogenys, and turtles, among others (Foster, 2001). The dinosaurs included particularly abundant basal neornithischians, theropods, and diplodocine and camarasaurid sauropods. The crocodyliform specimen was initially described as a juvenile goniopholidid that would have demonstrated strong allometric growth in the relative elongation of the lower  USA Foster, J.R. jaw through ontogeny (Foster, 2006). Re-examination of the specimen (MWC 5625) indicates heterodonty in the lower jaw and strong similarities to Theriosuchus, and a redescription is provided here.

INSTITUTIONAL ABBREVIATIONS
MWC -Museums of Western Colorado, Dinosaur Journey Museum, Fruita, Colorado; BMNH -The Natural History Museum, London, England.

Type Horizon
Morrison Formation undifferentiated; thin local Morrison section of only ~23 m (Mapel and Pillmore, 1963); exact stratigraphic level and intraformational correlation with other localities in Wyoming unknown.

Etymology
For its occurrence in the Morrison Formation.

Diagnosis
Theriosuchus species with the following unique combination of characters (*denote autapomorphic for species relative to Theriosuchus and Knoetschkesuchus): greatly enlarged D2 and D3 alveoli*; dramatic reduction in mesiodistal diameter of alveoli from D3 to D4*; overall structure of mandible similar to T. pusillus in depth:length ratio, lack of external mandibular fenestra, retroarticular process angle, and dentary dorsal profile in two "waves" in lateral view; however, dentary nutrient foramina row and symphysis orientation relative to tooth row both more similar to K. guimarotae.

Revised Description (modified from Foster, 2006)
Specimen MWC 5625 is 141 mm long, with a pitted to rugose lateral surface and a relatively deep dorsoventral dimension (figure 2). The lingual side of the jaw (particularly along the ventral margin) is not well preserved in the central area, but the articular region and the rostral third are in good condition. The tooth row is relatively short. There are alveoli for 16 teeth, although all teeth are missing, and the lateromedial widths of the alveoli for the caudal seven of those teeth have been reduced somewhat by postmortem crushing. There is no external mandibular fenestra. In lateral view, the retroarticular process is short, and its dorsal surface is steeply inclined in a caudoventral direction. Although sutures of the bones of the lower jaw are difficult to distinguish in many areas, the splenial is involved in the caudal part of the oval symphysis. A deep foramen intermandibularis oralis occurs just caudal to the symphysis. A row of foramina extends along the occlusal surface of the dentary just lingual to the alveoli of D2 to D9 (figure 3). The edges of the alveoli are vertically festooned around D2 through D5 and are flat posterior to that.
There is slight damage to the anterior end of the dentary so the nature of the alveolus for D1 is obscured to some degree. Diameters of alveoli D2 and D3 are greatly enlarged relative to surrounding tooth positions; diameters of D4 through D9 alveoli are significantly reduced; and mesial-distal diameters of D10 through D16 are somewhat enlarged, some as large as D2 and D3 (table 1;  ing of the jaw posterior (caudal) to D8, the mesial-distal diameters of the alveoli do not appear to have been significantly altered, and the crushing appears to have been relatively minor at the caudal end of the tooth row. Alveoli for D10 through D16 are so significantly elongate mesial-distally, compared to their labiolingual diameter, that even prior to crushing, the alveoli for these posterior tooth positions appear to have been very different in shape compared with more anterior (rostral) ones. Given that the preserved widths of D10 through D16 are about 1 mm each, with a mesiodistal length of ~3 to 4 mm for each, and about 1 mm of labiolingual crushing, these alveoli still appear to have been nearly twice as long mesiodistally as wide labiolingually. Thus, from the moderate-sized D1 alveolus posteriorly: D2 and D3 are very large and nearly circular, D4 through D9 are very small and circular, and D10 through D16 are seemingly large and elongate-oval  ; table 1). The interalveolar spaces between positions D10 through D16 appear to have extremely thin septae (at least three positions) or to lack them entirely (confluent), a feature still apparent despite some crushing. The preserved septae are oriented diagonally between alveoli due to crushing (originally perpendicular to the tooth row), but they are generally only about 1 mm in length, again suggesting that teeth D10 through D16 were laterally compressed. These great size differences and apparent change in shape suggest that the tooth row exhibited significant heterodonty and that the dentary possessed at least three tooth types with conical teeth anteriorly and possibly laterally compressed teeth posteriorly.

DISCUSSION AND CONCLUSIONS
The specimen was found in an abandoned channel-fill pond deposit in interbedded laminated siltstone and green claystone (Foster and Martin, 1994;Foster, 2001). This is a dense bone bed preserving microvertebrate bones (e.g., jaws of mammals) and macrovertebrate bones (e.g., articulated sauropods) in the same layer and just centimeters away from each other. The abandoned channel deposit occurs as an elongate, laterally restricted deposit that lies stratigraphically just above a convex-bottomed channel sand, and it consists of two 1-to 10-cm-thick laminated siltstone beds with many aquatic, semi-aquatic, and terrestrial taxa mixed in the same layers. Charophytes, horsetails, bivalves, and other non-vertebrate taxa recovered from the deposit are also indicative of a relatively wet environment. Teeth of possible goniopholidids as well as unidentified crocodylomorph osteoderms are also found in the deposit.
Specimen MWC 5625 was originally described as a juvenile goniopholidid that would have undergone dramatic allometric growth in its lower jaw through ontogeny (Foster, 2006). I proposed that Morrison goniopholidids would have greatly increased the relative length of their tooth row and greatly reduced the relative depth of their lower jaw (greatest mandible depth/ overall mandible length) as they grew to adult size. In both these ratios, however, specimen MWC 5625 is well off the trend lines set by a sample of Morrison goniopholidids (Amphicotylus and Eutretauranosuchus) and a growth series of the modern Alligator ( figure 4). The apparent growth pattern collectively shown by those three taxa was in fact the reverse of what would have been required to turn the individual represented by specimen MWC 5625 into an adult (Foster, 2006). And in shape of the symphysis of the lower jaws, specimen MWC 5625 in fact plotted among the alligators and nowhere near the region of goniopholidids of having a more anteroposteriorly elongate symphysis (Foster, 2006). My own graphs in that paper (Foster, 2006) suggested, however, that Alligator at least did not undergo such dramatic allometric growth from the same small size up to that of the largest Morrison specimen, and even the trend among the known Morrison goniopholidids (though all are significantly larger than specimen MWC 5625) did not suggest dramatic changes.
In re-examining the specimen and data in that paper, it is clear that specimen MWC 5625 shares characters with the atoposaurid crocodyliform Theriosuchus, which was first named from the Purbeck (Lower Cretaceous) of England (Owen, 1879;Salisbury, 2002) and has since had members of its genus or close relatives show up in Asia, Portugal, Spain, Romania, and possibly (from teeth) in a few other places, usually in the Late Jurassic to Early Cretaceous (Martin and others, 2010; Young and others, 2016) but also ranging into the Late Cretaceous. Theriosuchus was a small neosuchian with a short and triangular-shaped skull (top view) with large eyes and a short, narrow snout (Salisbury, 2002;Martin and others, 2014).
Specimen MWC 5625 is relatively large and differs from the dentary of Knoetschkesuchus from Portugal in lacking an external mandibular fenestra and in having a mandible less elongate relative to its maximum depth (figure 4; Schwarz and Salisbury, 2005 others, 2017). Based on alveolar diameters, specimen MWC 5625 also appears to have had more pronounced heterodont dentition anteriorly than Knoetschkesuchus. It differs from Atoposaurus, Alligatorium, and Alligatorellus in lacking homodont dentition and from Alligatorium and Alligatorellus specifically in lacking the external mandibular fenestra and smooth external surface of the mandible, respectively (Tennant and Mannion, 2014;Tennant and others, 2016). Specimen MWC 5625 also differs from species of Sabresuchus in lacking a lateral dentary concavity for reception of an enlarged 5th maxillary tooth, in lacking a diastema between D7 and D8, in lacking as short a symphysis, in not having all dentary teeth in a continuous groove, and in not having the occlusal surface of the dentary entirely lacking nutrient foramina (Tennant and others, 2016). Turner (2015) found species of Theriosuchus and Alligatorium to form an atoposaurid clade, although another recent analysis has suggested that Atoposauridae may be restricted to Atoposaurus, Alligatorium, and Alligatorellus, and that traditional species of Theriosuchus formed a polyphyletic group (Tennent and others, 2016). The same latter analysis found, however, that Theriosuchus pusillus and Knoetschkesuchus at least were sister taxa. Specimen MWC 5625 is most similar overall to a referred lower jaw of Theriosuchus pusillus (specimen BMNH 48328; figure 5) illustrated by Salisbury (2002).
Although the anterior tip of that specimen is missing, an estimation of the full jaw length suggests that the depth:length ratio of the mandible is very similar to MWC 5625 (figures 4 and 5). Additionally, the apparent tooth row length, external sculpturing, and retroarticular process of MWC 5625 are all most similar to T. pusillus; the symphysis length and orientation and the dentary dorsal profile with two "waves" also are similar to Theriosuchus (figure 5).
Specimen MWC 5625 possesses the following characters from the generic diagnosis of Theriosuchus in Young and others (2016). The folloing numbered list refers to the corresponding character numbers in Young and others (2016), and the missing character numbers relate to characters not preserved in the mandible: (1) heterodont dentition with pseudocaniniform and likely labiolingually compressed teeth (judging from alveoli); (5) some of the dentary alveoli form a confluent chain from dentary alveolus D4 through D8 (D10 through D16 in MWC 5625); (7) dentary alveolar size strongly heterogeneous; and (8) external surface of dentary is ornamented with heterogeneously spaced pits, ventrolaterally rugose. Specimen MWC 5625 appears to differ in lacking a notch in the dentary for the enlarged 5th maxillary tooth (character 6 in Young and others, 2016) and in seeming to lack a progressive reduction in alveolus size from D4 through D6 (character 3; instead Figure 4. Comparison of mandible greatest depth to total length, by size, for: the atoposaurids Theriosuchus pusillus, T. morrisonensis (specimen MWC 5625, labeled), and Knoetschkesuchus (stars), several goniopholidids (squares), and a growth series of Alligator (circles), showing greater mandible depth:length of Theriosuchus. Green trendline is for Alligator series. Length of T. pusillus mandible estimated due to missing tip in BMNH 48328. Modified and updated from Foster (2006).
MWC 5625 demonstrates dramatic reduction in diameter from D3 through D4).
The external sculpting of the mandible and heterodonty were also characters listed in the revised diagnosis of Theriosuchus in Tennant and others (2016). The revised diagnosis of T. pusillus includes three mandibular characters that MWC 5625 matches: heterodont dentition (apparent indirectly in MWC 5625), absence of a mandibular fenestra, and dorsal edge of dentary with two dorsally projecting "waves" in lateral view. Specimen MWC 5625 lacks all mandibular characters listed by Tennant and others (2016) for a single speciment assigned to Theriosuchus sp. (Young and others, 2016) from the Middle Jurassic of the Isle of Skye, Scotland.
The mandibular configuration and combination of characters of MWC 5625 distinctly separate the specimen from goniopholidids, shartegosuchids, and protosuchids/sphenosuchians and suggest that MWC 5625 is within the genus Theriosuchus, closest to T. pusillus (also illustrated by Salisbury, 2002, andothers, 2017); however, it differs from that genotype species in having greatly enlarged D2 and D3 alveoli and having the splenial more evenly distributed dorsoventrally along the symphysis (not restricted to dorsal part). Specimen MWC 5625 also differs from T. pusillus, and is similar to Knoetschkesuchus guimarotae, in hav- Characters that make MWC 5625 unique among Theriosuchus and Knoetschkesuchus specimens include: (1) very large D2 and D3, relative to D4 through D9, and (2) dramatic reduction in diameter from alveoli D3 through D4.
Specimen MWC 5625 is too incomplete to run a meaningful phylogenetic analysis (only 35 of 329 characters known from Tennant and others [2016]; table 2), and preliminary assessments in TNT software and using the datasets of Turner (2015) and Tennant and others (2016) show it to be rather unstable within Neosuchia. However, its unique features and combination of numerous characters shared with the closely related Theriosuchus pusillus and Knoetschkesuchus guimarotae (Turner, 2015;Tennant and others, 2016) suggest that, if more complete, Theriosuchus morrisonensis (MWC 5625) would likely be found to lie within this clade, possibly as the sister taxon to T. pusillus. This is the first occurrence of Theriosuchus in the Late Jurassic of North America and is a new crocodyliform species for the Morrison Formation. Although isolated teeth from the Early Cretaceous Cedar Mountain Formation have been referred to atoposaurids (Cifelli and others, 1999), until now no confirmation of Theriosuchus or close relatives has been found on this continent. The discovery thus strengthens biotic ties once again between the Morrison Formation and the Late Jurassic-Early Cretaceous of Europe (e.g., Mateus, 2006). This new occurrence also increases the diversi-ty of crocodylomorphs in the Morrison Formation to eight taxa, with now at least two goniopholidids (Amphicotylus, Eutretauranosuchus), two shartegosuchids (Fruitachampsa and an unnamed form), two hallopodid sphenosuchians (Hallopus, Macelognathus), this new species of Theriosuchus, and a possible protosuchian (Hoplosuchus). The diversity of crocodylomorphs in the Late Jurassic-Early Cretaceous of Europe may have been even higher.
Atoposaurids appear to have been terrestrial or semiaquatic, depending on the species (Tennant and Mannion, 2014). With little of the postcranial skeleton known, the specific ecology of Theriosuchus is not clear, but the environmental setting of Knoetschkesuchus guimarotae in estuarine lagoons and swamps (Schwarz and others, 2017) suggests that that species at least was semi-aquatic. The occurrence of T. morrisonensis in an abandoned channel pond deposit in the apparently wetter northern region of the Morrison Formation (Turner and Peterson, 2004;Foster and McMullen, 2017) aligns with the wet paleoenvironmental settings of K. guimarotae and T. pusillus and may indicate that T. morrisonensis too was semi-aquatic in its habits.

ACKNOWLEDGMENTS
Thanks to the field crews of 2004 that helped on the excavations when we collected this specimen. Thanks to Kay Fredette (Museums of Western Colorado), who prepared MWC 5625, and to Julia McHugh (Museums of Western Colorado) for loan of the specimen. Suggestions for improvement of the manuscript are greatly appreciated and were offered by Daniela Schwarz (Museum für Naturkunde), Thomas Adams (Witte Museum), and Kelli Trujillo (Laramie County Community College), though none of them necessarily endorses my interpretations.