The anatomy and palaeobiology of the early armoured dinosaur Scutellosaurus lawleri (Ornithischia: Thyreophora) from the Kayenta Formation (Lower Jurassic) of Arizona

Research output: Contribution to journalArticlepeer-review

Authors

  • Benjamin Breeden
  • Thomas Raven
  • Richard Butler
  • Timothy Rowe
  • Susannah Maidment

External organisations

  • University of Brighton
  • The Natural History Museum
  • University of Utah
  • Natural History Museum of Utah
  • The University of Texas at Austin

Abstract

The armoured dinosaurs, Thyreophora, were a diverse clade of ornithischians known from the Early Jurassic to the end of the Cretaceous. During the Middle and Late Jurassic, the thyreophorans radiated to evolve large body size, quadrupedality, and complex chewing mechanisms, and members of the group include some of the most iconic dinosaurs, including the plated Stegosaurus and the club-tailed Ankylosaurus; however, the early stages of thyreophoran evolution are poorly understood due to a paucity of relatively complete remains from early-diverging thyreophoran taxa. Scutellosaurus lawleri is generally reconstructed as the earliest-diverging thyreophoran and is known from over 70 specimens from the Lower Jurassic Kayenta Formation of Arizona, USA. Whereas Scutellosaurus lawleri is pivotal to our understanding of character state changes at the base of Thyreophora that can shed light on the early evolution of the armoured dinosaurs, the taxon has received limited study. Herein, we provide a detailed account of the osteology of Scutellosaurus lawleri, figuring many elements for the first time. Scutellosaurus lawleri was the only definitive bipedal thyreophoran. Histological studies indicate that it grew slowly throughout its life, possessing lamellar-zonal tissue that was a consequence neither of its small size nor phylogenetic position, but may instead be autapomorphic, and supporting other studies that suggest thyreophorans had lower basal metabolic rates than other ornithischian dinosaurs. Faunal diversity of the Kayenta Formation in comparison with other well-known Early Jurassic-aged dinosaur bearing formations indicates that there was considerable spatial and/or environmental variation in Early Jurassic dinosaur faunas.

Bibliographic note

Funding Information: The first ornithischian dinosaur remains recognized from the Kayenta Formation were discovered in June 1971 by Mr. David Lawler, then a summer intern at the MNA. He collected a nearly complete postcranial skeleton of an armoured ornithischian dinosaur with a few cranial fragments that would become the holotype specimen of Scutellosaurus lawleri Colbert, 1981 (MNA.V.175) southeast of the topographic landmark Rock Head at MNA Locality 219-1. A second larger, but less complete, specimen of Scutellosaurus lawleri (MNA.V.1752) was discovered along Gold Spring Wash in July 1977 by Mr. William Amaral as part of MCZ in a joint expedition with MNA, under permits issued to MNA and funded by the National Science Foundation. This locality (MNA Locality 291-5) is approximately 16 km to the northwest of the holotype locality (figure 1). These two specimens were described by the late Edwin H. Colbert, who had become a Research Associate of the MNA following his retirement from a long career as Curator of Fossil Reptiles at the American Museum of Natural History [18]. Colbert [18] also referred to a femur from a much larger ornithischian than Scutellosaurus lawleri collected during that time; however, he did not cite a specimen number for the femur. We infer that this specimen is MNA.V.109, a large left femur referred to Dilophosaurus wetherilli by Gay [60]; however, we agree with Colbert’s initial assignment of this specimen to Ornithischia. Funding Information: Ethics. We describe fossil specimens collected from the lands of the Navajo Nation, which require the written permission from the Navajo Nation Minerals Department prior to publication. We sent a draft of our manuscript to the Navajo Nation Minerals Department for approval to publish on 14 August 2020, and we received approval in writing dated 17 August 2020. Data accessibility. A detailed inventory of referred specimens is available as electronic supplementary material. Authors’ contributions. B.T.B., R.J.B. and S.C.R.M. conceived of the study; B.T.B., T.J.R., R.J.B. and S.C.R.M. wrote the primary draft of the manuscript and assembled the figures. All authors edited and approved the final version of the manuscript. Competing interests. We declare we have no competing interests. Funding. B.T.B. was funded for this work by the Ernest L. and Judith W. Lundelius Endowment in Vertebrate Paleontology and the Jackson School of Geosciences at The University of Texas at Austin and the Doris O. and Samuel P. Welles Research Fund at the University of California Museum of Paleontology; T.J.R. was funded by a University of Brighton Science Scholarship; R.J.B. was funded by a NERC PhD studentship during the early stages of this work; T.B.R. was funded by National Science Foundation grants EAR 1258878 and IIS-9874781; and S.C.R.M. was funded by a University of Cambridge Domestic Research Studentship during the early stages of this work. Acknowledgements. First and foremost, we acknowledge the people of the Navajo Nation for facilitating continued palaeontological research on their land. The specimens described herein were collected by the MNA, UCMP, and MCZ between the years of 1971 and 1983 during fieldwork on the lands of the Navajo Nation under permits issued to MNA. We thank Akhtar Zaman, Bradley Nesemeier, Richard Carlton and Rowena Cheromiah of the Navajo Nation Minerals Department for their assistance and support of this research. 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 phone number +1 (928) 871-6588. Publisher Copyright: © 2021 The Authors.

Details

Original languageEnglish
Article number201676
Number of pages43
JournalRoyal Society Open Science
Volume8
Issue number7
Publication statusPublished - 21 Jul 2021

Keywords

  • Dinosauria, Ornithischia, Thyreophora, Kayenta Formation, Jurassic, Scutellosaurus lawleri