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.
Highlights
Thyreophoran dinosaurs were important and diverse terrestrial herbivores from the Late Jurassic to the Late Cretaceous [1,2,3]
Histological thin sections were taken from one radius, one tibia and an osteoderm of University of California Museum of Paleontology (UCMP) 130580 and from one radius and one femur of UCMP 170829 in order to study the ontogeny of Scutellosaurus lawleri by Padian et al ([116]; [115])
Three lines of arrested growth in the right tibia and seven lines of arrested growth in the radius of UCMP 130580 were observed by Padian et al [116], who noted that growth appeared to be ceasing in both bones, which may indicate that UCMP 130580 was nearly fully grown
Summary
Thyreophoran dinosaurs (the armoured dinosaurs—stegosaurs, ankylosaurs and related forms) were important and diverse terrestrial herbivores from the Late Jurassic to the Late Cretaceous [1,2,3]. Detailed knowledge of the anatomy of Scutellosaurus lawleri is important for several reasons: (i) character polarization during analyses of stegosaurian and ankylosaurian phylogeny and thyreophoran functional evolution This is especially important given there are several dozen specimens of Scutellosaurus lawleri, including several relatively complete postcranial skeletons with associated cranial material, making it one of the best-known early thyreophorans; (ii) determination of the phylogenetic position of Thyreophora within Ornithischia, and of the plesiomorphic character-states for Thyreophora in broad-scale analyses of archosaurian and dinosaurian evolution; and (iii) reconstruction of the character-state changes that occurred at the Thyreophora–Neornithischia split, and during early thyreophoran evolution. In 2020, Breeden & Rowe [20] described new specimens of Scutellosaurus lawleri reposited at TMM preserving features of skeleton not present or poorly preserved in the type or referred Museum of Northern Arizona (MNA) and UCMP specimens but refrained from re-describing any of those latter specimens. Zoology (MCZ) for the first time, providing new data on non-eurypodan thyreophoran morphology and a new life reconstruction for Scutellosaurus lawleri based on new palaeobiological interpretations
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
