COLORADO’S OLDEST DINOSAURS: THEROPODS FROM THE “RED SILTSTONE FORMATION” OF THE CHUGWATER GROUP (UPPER TRIASSIC) OF THE EAGLE BASIN, AND AN OVERVIEW OF LATE TRIASSIC DINOSAUR BIOCHRONOLOGY IN WESTERN NORTH AMERICA

The Upper Triassic “red siltstone formation” of the Chugwater Group in the Eagle Basin of Colorado contains a diverse dinosauromorph assemblage, which includes the oldest known dinosaurs from Colorado, one of the first reported occurrences of Triassic dinosaurs from the Chugwater Group, and the northernmost vertebrate assemblage from the Revueltian estimated biochronozone. Basal theropods are represented by a proximal femur referrable to Chindesaurus bryansmalli. Coelophysoids (basal neotheropods) are known from cervical vertebrae, ilia, femora and an astragalus representing a new taxon, Coloradivenator carpenteri. C. carpenteri is diagnosed by two autapomorphies (the complete absence of a dorsolateral trochanter on the femur and a tapering postacetabular process on the ilium) and a combination of several ancestral and derived character states (a weakly developed brevis shelf on the ilium, a pubic peduncle ventral to the ischial peduncle, a weakly projecting tibiofibular crest on the femur, the presence of a rimmed embayment behind the ascending process of the astragalus and the lack of a medial sulcus on the astragalus). C. carpenteri is the first formally named Revueltian coelophysoid, and the second Revueltian basal neotheropod to be named after Gojirasaurus quayi. A review of western North American ornithodiran biochronology suggests that both lagerpetids and silesaurids may have been absent by the latest Triassic, but that both basal theropods and coelophysoids have a fossil record spanning all four of the Upper Triassic land vertebrate biochronozones. Late Triassic dinosaur diversity was greatest in the Revueltian basal theropods and coelophysoids were joined by large-bodied basal neotheropods on the line to Averostra, here named Prothalodraconta. Currently, prothalodracontans are unknown from the Apachean biochronozone of western North America, suggesting a latest Triassic large-bodied theropod hiatus in that region. These patterns of Late Triassic dinosaur diversity are consistent with the overall picture of significant faunal reorganization after the Adamanian-Revueltian turnover.