New Specimens of Scutellosaurus Lawleri Colbert, 1981, from the Lower Jurassic Kayenta Formation in Arizona Elucidate the Early Evolution of Thyreophoran Dinosaurs

ABSTRACT

We describe new specimens of the ornithischian dinosaur Scutellosaurus lawleri Colbert, 1981, from the Lower Jurassic Kayenta Formation of Arizona and discuss their systematic importance. The new specimens represent at least 46 individuals and include two associated skeletons that preserve regions that were poorly known until now, including the skull and pelvis. Computed tomography (CT) assisted our interpretation of these specimens. Using an ornithischian data matrix, we first tested whether the two associated skeletons were justifiably assigned to Scutellosaurus lawleri and found that they group unequivocally with the holotype and paratype specimens. This enabled scoring of 35 character states that were previously unknown, raising the scoring completeness of Scutellosaurus lawleri from 52% to 67%. The results recovered Lesothosaurus diagnosticus as the basal-most member of Neornithischia, while corroborating the monophyly of Thyreophora and Scutellosaurus lawleri as its most basally branching member. In terms of numbers of specimens, Scutellosaurus lawleri is now the most abundant dinosaur known in any Early Jurassic vertebrate fauna. The presence of a second thyreophoran in the Kayenta Formation, along with the presence of Early Jurassic thyreophorans in Europe and Asia, suggests that Thyreophora may have originated in the northern hemisphere. The ornithischians from the Kayenta Formation support a pattern of dinosaurian diversification after the end-Triassic extinction in North America, if not a broader area, that was fueled by independent northward dispersals from the southern hemisphere, supporting dispersal as an early driver of dinosaurian evolution.

ACKNOWLEDGMENTS

All specimens described herein were collected on the Navajo Nation. Fieldwork on the Navajo Nation was conducted under permits from the Navajo Nation Minerals Department. Any persons wishing to conduct geologic investigations on the Navajo Nation must first apply for, and receive, a permit from the Navajo Nation Minerals Department, P.O. Box 1910, Window Rock, Arizona 86515, and telephone number [phone]. We thank A. Zaman, B. Niessemeyer, R. Carlton, and L. Becenti of the Navajo Nation Minerals Department for their assistance and support of this research.

We thank W. Amaral, B. Andres, J. Franzosa, R. Gary, E. Gordon, F. Jenkins Jr., T. Macrini, P. Owen, C. Schaff, G. Grellet-Tinner, and R. Tykoski for their field assistance collecting the specimens described herein. We thank K. Bader and E. Yarmer for their preparation of these specimens. We thank M. Colbert and J. Maisano (UTCT) for CT scanning TMM 43663-1 and TMM 43664-1. Thanks to J. Cundiff and S. Pierce (MCZ), D. Gillette and J. Gillette (MNA); M. Brown and C. Sagebiel (The University of Texas at Austin); and P. Holroyd and K. Padian (University of California Museum of Paleontology) for facilitating collections access. We thank C. Bell and R. Martindale for revising an early draft of this manuscript. M. Baron, L. Brenskelle, M. Brown, C. Boyd, R. Butler, J. Clarke, C. Griffin, R. Irmis, J. Lively, S. Maidment, A. Marsh, K. Melstrom, J. Merck, S. Nesbitt, K. Padian, T. Raven, R. Tykoski, and Z. Wistort all provided valuable conversations and insight throughout the course of this research. Funding to support this research was provided to B. Breeden by the Jackson School of Geosciences (The University of Texas at Austin), the Ernest L. and Judith W. Lundelius Endowment in Vertebrate Paleontology (The University of Texas at Austin), and the Doris O. and Samuel P. Welles Research Fund (UCMP); and to T. Rowe by NSF EAR 1561622 and NSF EAR 1258878.