Contextualizing the evolution of theropod dinosaurs in western North America using U-Pb geochronology of the Chinle Formation and Kayenta Formation on the Colorado Plateau

The Chinle Formation and Glen Canyon Group represent one of the most complete terrestrial sedimentary sequences of the Late Triassic and Early Jurassic epochs in the world. This time period spans the end-Triassic mass extinction and the Triassic-Jurassic boundary, and the rocks preserve fossils that include some of the earliest members of extant vertebrate groups. While the Late Triassic age of the Chinle Formation is well-supported thanks to extensive radioisotopic dating, the fossiliferous Kayenta Formation lacks tuffaceous units with which to determine U-Pb ages. Further, the evolutionary relationships of certain vertebrate taxa from the Chinle Formation and Kayenta Formation are not well known. My work helps to contextualize the evolution of early theropod dinosaurs by describing and estimating the evolutionary relationships of Late Triassic and Early Jurassic theropods. In addition, U-Pb detrital zircon geochronology provides chronostratigraphic ages for early Mesozoic sedimentary units on the Colorado Plateau, and a basis for time-calibrating phylogenies of taxa living at that time. The redescription of the holotype specimen of Chindesaurus bryansmalli corrects historical identifications, identifies previously-unknown elements, and provides novel morphological characters. Chindesaurus bryansmalli is the sister taxon to the non-neotheropod Tawa hallae and is not a herrerasaurid. Missing data in the form of incomplete specimens does not alter tree topology of early theropods. U-Pb detrital zircon ages support the lithostratigraphic correlation of the Sonsela Sandstone bed near the Sonsela Buttes in northeastern Arizona to the Jasper Forest bed within the Sonsela Member of the Chinle Formation at Petrified Forest National Park. A reassessment and description of every specimen of Dilophosaurus wetherilli reveals novel characteristics of the cranial crest unique to Dilophosaurus as well as apomorphic serial variation in the cervical vertebral laminae that may be associated with an increase in body size, and the evolution of the avian air sac system. All specimens of Dilophosaurus wetherilli form a clade of non-averostran neotheropods and support the monotypy of the taxon. U-Pb detrital zircon geochronology of the Kayenta Formation indicates a Sinemurian-Toarcian age and support the hypothesis that Cordilleran arc-derived zircons are excluded from the upper part of the Kayenta Formation where it interfingers with the Navajo Sandstone erg sands