DINOSAUR FAUNAS OF THE WESTERN UNITED STATES: SEQUENCE AND PALEOBIOGEOGRAPHIC SIGNIFICANCE

The western United States contains the most extensive record of dinosaurian evolution on Earth. We recognize nine dinosaurian faunas: (1) Late Triassic I (Otischalkian-Adamanian lvfs) includes rare herrerasaurids indicating a South American connection, the oldest ceratosaur and ornithischians and prosauropods, which occur globally; (2) Late Triassic II (Revueltian lvf) characterized by increased numbers of dinosaur specimens, the first coelophysid, the last herrerasaurids, and a near-absence of prosauropods that dominate other coeval faunas globally; (3) Late Triassic III (Apachean lvf) includes abundant remains of the ceratosaurian Coelophysis and ichnotaxa such as Grallator and "Pseudotetrasauropus," which are common in Europe and South Africa; (4) Early Jurassic (Sinemurian) includes pandemic taxa such as Massospondylus, Scelidosaurus and Syntarsus; (5)Late Jurassic (Comobluffian lvf) from the Morrison Formation, dominated by sauropods (principally camarasaurids and diplodocids), allosaurids and stegosaurids, and similar to coeval Gondwanan faunas; (6) Early Cretaceous I (Buffalogapian lvf) with sauropods, iguanodontians and ankylosaurs similar to coeval faunas of Europe; (7) Early Cretaceous II (Cashenranchian lvf) dominated by endemic North American taxa such as Tenontosaurus and Sauropelta; (8) Late Cretaceous I (Mussentuchitian-Aquilan lvfs) includes the last pre-Campanian North American sauropods (Cenomanian) and documents immigration from Asia and diversification of the hadrosaurians, neoceratopsians and tyrannosaurids; and (9) Late Cretaceous II (Judithian-Lancian lvfs) with further diversification of hadrosaurians, neoceratopsians and tyrannosaurids and with other immigrants from Asia (e.g., pachycephalosaurs) and from South America (titanosaurids). North American dinosaur faunas show varying pandemism in the Triassic-Jurassic, true endemism in the Early Cretaceous followed by evolutionarily significant immigration in the later Cretaceous.