Faunal replacement in the Triassic of South America

The history of South American therapsids and archosaurs during the Early, Middle and Late Triassic is analyzed. Improvements in the locomotor apparatus are discernible in seven groups of archosaurs and indicate that a great adaptive radiation took place before the oldest recorded fauna dominated by archosaurs appeared. Competition between therapsids and archosaurs began at the outset of the Triassic, when the proterosuchids were cosmopolitan and ceased 10 m.y. before the end of the Triassic. The rich Dicroidium flora and the variety of types of both therapsids and archosaurs were synchronous. The flora probably triggered the radiation in both reptilian groups. It is assumed that the extinction of therapsids was caused by: (a) the locomotor superiority of the Middle and Late Triassic archosaurs; (b) the capability of archosaurs to increase their body size during the Triassic; (c) the appearance of herbivorous archosaurs at the outset of the Late Triassic; and (d) the competitive pressure imposed by rhynchosaurs on medium-sized therapsids.