| This dissertation has two major components: (1) detailed analysis of thousands of microvertebrate fossils from seven localities of Otischalkian (early late Carnian) to Adamanian (latest Carnian) age in the Upper Triassic Chinle Group; and (2) a comprehensive study of the fossil record, biostratigraphy, biochronology, and evolution of Triassic dinosaurs.; The localities are the Trilophosaurus quarry and two sites near Kalgary in West Texas, Ojo Huelos and Sixmile Spring in New Mexico, and the Blue Hills and Dying Grounds in Arizona. Sampled depositional environments include channels, floodplains (including paleosols) and lakes. The fossils include chondrichthyans, osteichthyans, amphibians, and amniote tetrapods, and include six new species in four new genera and 38 new records of family- or lower-level taxa. Here I name the hybodont shark Diplolissodus murryi n. gen. et sp., the sphenodontian lepidosaurs Llanosaurus fraseri n. gen. et sp. and Planocephalosaurus lucasi n. sp., and the ornithischian dinosaurs Protecovasaurus lucasi n. gen. et sp., Crosbysaurus harrisae n. gen. et. sp. and Revueltosaurus hunti n. sp. The new records include selachians (5), osteichthyans (9), amphibians (2), and 22 new records of procolophonids, cynodonts, sphenodontians, Trilophosaurus buettneri, diverse archosauriform reptiles, and dinosaurs.; The first appearance of dinosaurs is essentially synchronous during the Otischalkian. Dinosaurs diversify rapidly and occur in strata of Adamanian age across Pangea. During Revueltian (early-mid Norian) time dinosaurs are locally common components of terrestrial faunas. By Apachean (latest Norian) time dinosaurs are the most common tetrapods in non-equatorial terrestrial basins.; The evolution of dinosaur size, locomotion, dentition and physiology, as well as paleoclimatology, tetrapod paleobiogeography and putative extinction events, demonstrate that dinosaurian success in the Triassic is complex, but not an opportunistic response to extinction events. Dinosaurs are part of a diapsid-dominated radiation. Within Diapsida, lepidosauromorphs remain small (<10kg) while archosaurs, with their upright posture, fill large-bodied (>10kg) niches. Among archosaurs, dinosaurs undergo additional locomotor evolution favoring agility (theropods) and great size (prosauropods) and are the only group to evolve two lineages of upright herbivores (prosauropods and ornithischians). Consequently, the rise of the dinosaurs was no “accident,” but instead an adaptive radiation into previously unoccupied ecospace during the Late Triassic. |
