Assemblage-level structure in Morrison Formation dinosaurs, Western Interior, USA

  • John A. Whitlock Department of Science and Mathematics, Mount Aloysius College
  • Kelli C. Trujillo Haub School of Environmental and Natural Resources, The University of Wyoming
  • Gina M. Hanik Department of Science and Mathematics, Mount Aloysius College
Keywords: Jurassic, Morrison, dinosaurs

Abstract

­The Upper Jurassic Morrison Formation is both geographically extensive and well-sampled, making it an ideal candidate for biogeographic analysis at both coarse and finer scales. Historically, however, this has not translated into a consensus on patterns of ecological structure and connectivity, particularly with regard to the characteristic dinosaur faunas. Here, we use both traditional (genus richness, alpha and beta diversity) and bipartite network-based (biogeographic connectivity, local endemism, and average occurrence) measures to examine patterns of structure on a per-locality basis. Given the broad geographic range of the formation, we subdivide the Morrison Formation into four discrete regions based roughly on latitude and lithology—north (Montana, South Dakota, and northern Wyoming), west (Utah and western Colorado), east (central and eastern Colorado and southern Wyoming), and south (Arizona, New Mexico and Oklahoma). Further investigation revealed many coeval sites (ca. 152 Ma) in the east and west regions. Presence-absence data were also compared using network analysis to determine the presence and content of discrete subassemblages within the larger region-level assemblages. Based on our results, we favor reconstructions of the Morrison Formation as a ‘mosaic’ type environment over most of its depositional history, with patches of open environments interspersed with more closed, forested regions. ­is is suggested by relatively low rates of local endemism (patches are consistent in plant and animal structure) and connectivity across the majority of the formation, as well as the recovery of three non-overlapping assemblages dominated by dierent guilds of herbivorous dinosaurs.

Outcrop of Brushy Basin Member of the Morrison Formation in southeast Utah. Photograph by ReBecca Hunt-Foster.
Published
2018-03-15
How to Cite
Whitlock , J., Trujillo , K., and Hanik , G., 2018, Assemblage-level structure in Morrison Formation dinosaurs, Western Interior, USA: Geology of the Intermountain West, v. 5, p. 9-22., doi: 10.31711/giw.v5.pp9-22.