A non-averostran neotheropod vertebra (Dinosauria: Theropoda) from the earliest Jurassic Whitmore Point Member (Moenave Formation) in southwestern Utah

ABSTRACT

Theropod dinosaurs are minor components of Late Triassic ecosystems in North America, comprising coelophysoids and various non-neotheropods from the Chinle Formation of Arizona, Utah, Colorado, and New Mexico and the Dockum Group of western Texas. By the Sinemurian (Early Jurassic), the coelophysoid “Syntarsus” kayentakatae and the large-bodied non-averostran neotheropod Dilophosaurus wetherilli from the Kayenta Formation were the dominant terrestrial predators. Theropods are virtually unknown from the intervening Rhaetian–Hettangian Moenave Formation, with the exception of two partial coelophysoid pelves from somewhere within the Dinosaur Canyon Member, which includes the Triassic–Jurassic boundary and end-Triassic mass extinction. Here we describe an anterior trunk vertebra from a non-coelophysoid, non-averostran neotheropod from the uppermost Whitmore Point Member of the Moenave Formation in southwestern Utah, which is Hettangian in age. The vertebra has prominent vertebral laminae and associated pneumatic fossae, and anterior and posterior ‘shoulders’ on the neural spine that are similar to those found in Dilophosaurus wetherilli. This vertebra belongs to a theropod that may be as many as 15 million years older than Dilophosaurus wetherilli from the middle of the Kayenta Formation in Arizona. This theropod is associated with Grallator, Eubrontes, and Characichnos theropod traces made on the shores of the Early Jurassic Lake Whitmore that are abundant in the Whitmore Point Member in southwestern Utah. Its occurrence in the Hettangian roughly coincides with the appearance of Eubrontes tracks in North America, indicating that not all contemporaneous theropod traces were made by coelophysoids.

ACKNOWLEDGMENTS

We thank D. Freeman for discovering and collecting SGDS 768, and the Washington County School District for allowing us to continue research on their land. Funding for the excavation and preparation of SGDS 768 was provided by the St. George Dinosaur Discovery Site at Johnson Farm and the Utah Geological Survey. Thanks to X. Jenkins and M. Zahner for thoughtful reviews. Thanks also to the Utah Geological Survey reviewers S. Carney, M. Hylland, and G. Willis. This is Petrified Forest National Park Contribution No. 79. The conclusions presented here are those of the authors and do not represent the views of the United States Government.