Anatomy and systematics of the sauropodomorph Sarahsaurus aurifontanalis from the Early Jurassic Kayenta Formation

Abstract

Sarahsaurus aurifontanalis, from the Kayenta Formation of Arizona, is one of only three sauropodomorph dinosaurs known from the Early Jurassic of North America. It joins Anchisaurus polyzelus, from the older Portland Formation of the Hartford Basin, and Seitaad reussi, from the younger Navajo Sandstone of Utah, in representing the oldest North American sauropodomorphs. If it is true that sauropodomorphs were absent from North America during the Late Triassic, the relationship among these three dinosaurs offers a test of the mechanisms that drove recovery in North American biodiversity following the end-Triassic extinction event. Here we provide the first thorough description of Sarahsaurus aurifontanalis based on completed preparation and computed tomographic imaging of the holotype and referred specimens. With new anatomical data, our phylogenetic analysis supports the conclusion that Sarahsaurus aurifontanalis is nested within the primarily Gondwanan clade Massospondylidae, while agreeing with previous analyses that the three North American sauropodomorphs do not themselves form an exclusive clade. A revised diagnosis and more thorough understanding of the anatomy of Sarahsaurus aurifontanalis support the view that independent dispersal events were at least partly responsible for the recovery in North American vertebrate diversity following a major extinction event.

Osteological redescription of the Late Triassic sauropodomorph dinosaur Thecodontosaurus antiquus based on new material from Tytherington, southwestern England

“…9A). This crest is similarly expanded in other basal sauropodomorphs (Langer, 2003;Martínez and Alcober, 2009;Ezcurra, 2010;Sereno et al, 2013;Cabreira et al, 2016;Langer et al, 2019), becomes less prominent in plateosaurians (Martínez, 2009;Apaldetti et al, 2013;Otero and Pol, 2013;McPhee et al, 2015;Marsh andRowe, 2018, Tsai et al, 2018), and is greatly reduced in Lessemsaurus and eusauropods (Pol and Powell, 2007). The pubic peduncle is longer than the ischial peduncle and projects anteroventrally.…”

“…The cuboid fossa is also well developed in Saturnalia (Langer et al, 2007) and other basal sauropodomorphs, unlike the absent or poorly developed fossa of basal saurischians such as Herrerasaurus (Sereno, 1994) and Tawa (Burch, 2014). There is no evident fossa olecrani on the posterior surface of the distal humerus between the condyles, a feature seen in other sauropodomorphs such as Saturnalia (Langer et al, 2007) and Sarahsaurus (Marsh and Rowe, 2018). Both the entepicondyle and the ectepicondyle present striations on their posterolateral surfaces, possibly associated with the origin of digital flexors and extensors, respectively (Fig.…”

“…8A, B). The mediolateral width of the proximal end is 55% the proximodistal length of the bone, relatively broader than the MC I of Macrocollum (Müller et al, 2018a), but narrower than those of Eoraptor (Sereno et al, 2013) and more derived sauropodomorphs such as Unaysaurus (McPhee et al, 2019), Adeopapposaurus (Martínez, 2009), Sarahsaurus (Marsh and Rowe, 2018), or Mussaurus (Otero and Pol, 2013). Because manual elements are isolated, the proximal inset of MC I into the carpus, characteristic of basal sauropodomorphs (Sereno, 2007;Martínez, 2009;Otero and Pol, 2013;Sereno et al, 2013), cannot be determined.…”

A comprehensive anatomical and phylogenetic evaluation ofDilophosaurus wetherilli(Dinosauria, Theropoda) with descriptions of new specimens from the Kayenta Formation of northern Arizona

“…The proximal surface of the ascending process is thin and subrectangular where it articulates with the anterolateral process of the tibia (Figs. 23.3, 34.7), unlike that of early sauropodomorphs in which it is broad and elliptical (Cooper, 1981;Langer, 2003;Moser, 2004;Marsh and Rowe, 2018) and the plate-like ascending process of early averostrans (Madsen, 1976;Madsen and Welles, 2000). Welles (1954Welles ( , 1984 located what he determined to be a sutural contact around the base of the ascending process and the main body of the astragalus of the holotype specimen (Fig.…”

“…With the help of Bill Rush and Ed Cott, Sam Welles opened the quarry of the holotype and paratype specimens of Dilophosaurus wetherilli (Welles, 1954(Welles, , 1970 within the Kayenta Formation. Welles collected a larger individual in 1964 near the holotype locality (Field Notes 1964-1968, The Samuel P. Welles Papers, Series 5: Field Notes, Box 10-11, University of California Museum of Paleontology Archives; Welles, 1984) and later teams from the University of Texas at Austin collected two individuals near Gold Spring, AZ, ∼30 miles southeast from the type quarry (Rowe et al, 2011;Marsh and Rowe, 2018). Thanks to nearly 80 years of research, Dilophosaurus wetherilli has captured the interest of the public and paleontologists alike who are drawn to its silver screen stardom, its characteristic cranial anatomy, and its ecological role as the earliest large-bodied theropod dinosaur in North America.…”