The stem-archosaur evolutionary radiation in South America

Abstract

The oldest archosauromorphs (dinosaurs, birds, crocodiles, and their stem-taxa) arerecorded in middle−upper Permian rocks, but it was not after the Permo−Triassic massextinction that the group shows a substantially high taxonomic richness andecomorphological disparity. The early evolutionary history of the Archosauromorphaduring the Early and Middle Triassic is mainly based on fossils recovered from rocks insouthern Africa, Europe and Asia, whereas South America possesses a morecomplete fossil record of the group only in the Late Triassic. Here we revisit, discuss,and reanalyse the non-archosaurian archosauromorph fossil record of the current-daySouth America. The Early Triassic archosauromorph record in this continent is stillscarce, but it documents the early evolution of the group in western Pangaea and iscrucial to understand more globally the biotic recovery after the Permo−Triassic massextinction. The Middle Triassic record is extremely scarce, but the Late Triassicarchosauromorph assemblage of South America is among the most diverse andabundant worldwide. The last decade has witnessed a considerable improvement inour knowledge of the record, taxonomy, phylogeny, and macroevolution of the groupwith the input from the South American fossils. Nevertheless, a considerable amount ofresearch is needed and ideally should be focused on some particular aspects of theTriassic evolutionary radiation of Archosauromorpha. Among them, the Early Triassicrecord should be expanded, more numerous and more complete Middle Triassicarchosauromorph specimens are crucial to have a more complete picture of theevolution of the group, and the taxonomy of groups like proterochampsids andhyperodapedontine rhynchosaurs should be clarified through detailed anatomical work.