DINOSAURIAN EVOLUTION IN THE REVUELTIAN (LATE TRIASSIC: NORIAN) OF THE SOUTHWESTERN UNITED STATES (NEW MEXICO, TEXAS, PETRIFIED FOREST NATIONAL PARK)

The Bull Canyon Formation (BCF) of eastern New Mexico and west Texas is of Revueltian age (Late Triassic: early-middle Norian) and includes the type fauna of the Revueltian land vertebrate faunachron. The BCF includes two superposed faunas that can be discriminated on the basis of dinosaurs. The lower BCF fauna (R1) has a diverse dinosaur fauna including an indeterminate prosauropod, the ornithischians Revueltosaurus callenderi and Technosaurus, the herrerasaur Chindesaurus and a new genus, and more derived theropods including Gojirasaurus, Protoavis? (partim), “Coelophysis” (sensu UCMP 129618) and at least 2 other taxa. The upper BCF fauna (R2) includes an indeterminate prosauropod, the ornithischian Lucianosaurus and a new herrerasaur. Revueltian faunas occur at Petrified Forest National Park (PEFO) in the Painted Desert Member of the Petrified Forest Formation. At PEFO an R1 fauna occurs below the Black Forest Tuff (BFT) and includes the ornithischian R. callenderi, the herrerasaur Chindesaurus and the ceratosaurian theropod “Coelophysis” (sensu UCMP 129618). An R2 fauna occurs in and above the BFT, lacks R. callenderi and contains a derived theropod. R. callenderi is the most useful dinosaur for distinguishing R1 from R2 as it is both common and readily identifiable on the basis of isolated teeth. Revueltian dinosaurs from the American Southwest are the most diverse Late Triassic dinosaur faunas known. They differ from the Revueltian faunas of Western Europe in: (1) overall paucity of dinosaur specimens; (2) paucity of prosauropods; (3) higher diversity of theropods; and (4) presence of herrerasaurs. R1 and R2 may be equivalent to the Sellosaurus and Plateosaurus biochrons of Western Europe. The major dinosaurian turnover between R1 and R2 indicates a faster rate of evolution in dinosaurs in the Late Triassic than in most tetrapod groups (e. g., crurotarsans, metoposaurs) which show no taxonomic changes within the Revueltian.