Postosuchus kirkpatricki (n.g., n.sp.) is a large, gracile, carnivorous thecodontian from the late Triassic Dockum Group of Texas. The species is characterized by its erect and facultative bipedal gait, highly advanced pelvis with slightly open acetabulum and footed pubis. The teeth are large, highly serrated, deeply set in sockets, and show a wave-like replacement pattern. Some flexibility in the palate and the lower jaw permitted the animal to swallow large chunks of flesh. Postosuchus was the arch predator of its time, and lived in the interfluves or upland regions along with aetosaurs, coelurosaurs, and ictidosaurs. The quarry's thanatocoenose probably reflects autochthonous burials of a Dockum community, perhaps in consequence of a catastrophic event such as a flash flood. Although Postosuchus exhibits many carnosaurian traits, its ankle joint is of `crocodile-normal' pattern, which suggests its pseudosuchian affinity. All Triassic reptiles previously thought to be carnosaurs are considered herein as advanced thecodontians; true carnosaurs probably appeared in post-Triassic time. Postosuchus is grouped with Poposaurus, Teratosaurus, Arizonasaurus and the unnamed `Warwick archosaur' in the family Poposauridae. The Poposauridae and Rauisuchidae are placed in the infraorder Rauisuchia. Tyrannosaurs probably evolved from poposaurids, as indicated by the correlation between skull, pelvis and ankle structure.