A complete skeleton of a Late Triassic saurischian and the early evolution of dinosaurs

Characterizing the evolutionary history of early dinosaurs is central to understanding their rise and diversification in the Late Triassic. However, fossils from basal lineages are rare. A new theropod dinosaur from New Mexico is a representative of the early North American diversification. Known from several nearly complete skeletons, it reveals a mosaic of plesiomorphic and derived features that clarify early saurischian dinosaur evolution and provide evidence for the antiquity of novel avian character systems including skeletal pneumaticity. The taxon further reveals latitudinal differences among saurischian assemblages during the Late Triassic, demonstrates that the theropod fauna from the Late Triassic of North America was not endemic, and suggests that intercontinental dispersal was prevalent during this time. By the Late Triassic (~230 million yearsago), Dinosauria had diversified intothree major lineages: Sauropodomorpha, Theropoda, and Ornithischia (1, 2). In compari-son to the later Mesozoic, fossils of Triassic early dinosaurs and their closest relatives are generally rare, fragmentary, and incomplete (3, 4). Indeed, the record from the Ischigualasto Formation, whic