ABSTRACT: The Early Evolutionary Diversification of Dinosaurs in the Western United States: They Began with a Bang Not a Whimper
Adrian P. Hunt
The western United States is the best place in the world to study the early diversification of the dinosaurs because it contains: (1) a sequence of at least four successive vertebrate faunas that span the late Carnian-late Norian (compared with only two faunas in Argentina and Brazil and no pre-dinosaur Carnian fauna there); (2) a range of paleoenvironments that do not change radically through this time interval (compared with Germany where faunas fluctuate from 90% amphibians to 90% dinosaurs); and (3) extensive and fossiliferous outcrops. Selected stratigraphic units containing these faunas and their dinosaur component are: (1) Lower Dockum Group (Otis Chalk)/Popo Agie Formation (early-late Carnian)--no dinosaurs; (2) lower part of the Petrified Forest Member of the Chi le Formation (PFM)/Tecovas Formation/Santa Rosa Formation (late-late Carnian)--staurikosaurid, ceratosaur, anchisaurid, and "fabrosaur"; (3) upper part of PFM/Bull Canyon Formation/Cooper Formation (early Norian)--staurikosaurid, large ceratosaurs (2), Coelophysis-like ceratosaur, Coelophysis bauri, Technosaurus smalli, Revueltosaurus callenderi, and a "fabrosaur"; (4) Redonda Formation/Rock Point Member of Chinle Formation (mid?late Norian)-- "Coelophysis bauri" and abundant Grallator footprints. Faunal differences between (2) and the late-late Carnian Santa Maria and Ischigualasto formations are ecological in origin, and both have similar dinosaur diversity. It is apparent that: (1) a diverse dinosaur fauna appeared suddenly in the late-late Carnian; (2) diversification in the early No ian was rapid (first large carnivores and diverse ornithischians) following a major faunal turnover; and (3) most of the early dinosaurs were of small body size, with large-bodied "prosauropods" restricted to inland faunas (Stubensandstein, Los Colorados Formation).
AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990