ANATOMY, SYSTEMATICS, AND GEOCHRONOLOGY OF DILOPHOSAURUS WETHERILLI (DINOSAURIA: THEROPODA) FROM THE EARLY JURASSIC KAYENTA FORMATION OF ARIZONA

Dilophosaurus wetherilli was the largest animal to have ever lived on land in North America during the Early Jurassic. Despite its charismatic presence in pop culture and dinosaurian phylogenetic analyses, little clarity exists of the skeletal anatomy, taxonomy, ontogeny, and evolutionary relationships of this dinosaur. The various skeletons have been collected from the middle and near the base of the Kayenta Formation in the Navajo Nation of Arizona, however the unit is assumed to be time-transgressive and superpositional relationships must be verified with radiometric ages. Redescription of the holotype, referred, and previously undescribed specimens of Dilophosaurus wetherilli supports the existence of a single species of crested, large-bodied theropod from the Kayenta Formation of Arizona. The parasagittal nasolacrimal crests are uniquely constructed by a small ridge on the nasal process of the premaxilla, dorsoventrally expanded nasal, and tall lacrimal that includes a posterior process behind the eye. The cervical vertebrae exhibit serial variation within the posterior centrodiapophyseal vertebral lamina; it bifurcates and reunites down the neck such that the posterior centrodiapophyseal lamina present on the third vertebra is not homologous to that found on the ninth. Many of the features of the postcranial skeleton are derived compared to Late Triassic theropods and may be associated with an increase in body size in Theropoda through time or in Dilophosaurus as it grew. Phylogenetic analyses conducted using each specimen of Dilophosaurus wetherilli as a terminal taxon result in each specimen being the sister taxon to the holotype. When all five specimens are included in such an analysis, they form a monophyletic group of non-averostran neotheropods in a grade of other stem-averostrans like Cryolophosaurus and Zupaysaurus. U-Pb detrital zircon geochronology used to determine the maximum age of deposition of the Dilophosaurus-bearing horizons in the Kayenta Formation suggests that the time contained within the unit includes more of the Early Jurassic than previously hypothesized using vertebrate biostratigraphy, and that Dilophosaurus wetherilli persisted for at least 14 million years.