Stemarchosaurs

Stemarchosaurs represent a paraphyletic group of archosaurs that existed during the Triassic period. This informal classification encompasses the earliest diverging lineages of the Archosauria, predating the split between the two major extant archosaurian groups, crocodiles and birds. Stemarchosaurs are characterized by a suite of primitive features retained from their earlier archosauriform ancestors, alongside a mosaic of traits that foreshadowed later archosaurian evolution.

Key members of this group include such families as the phytosaurs, often mistaken for crocodiles due to their superficial resemblance, and the aetosaurs, characterized by extensive bony armor. The errhosuchids and proterosuchids are also considered stemarchosaurs, representing some of the earliest archosaurian forms. These animals varied widely in size and ecological niche, with some being terrestrial predators and others semi-aquatic.

The study of stemarchosaurs is crucial for understanding the origins and early diversification of Archosauria. By examining their anatomy and phylogenetic relationships, paleontologists can reconstruct the evolutionary pathway that led to the development of modern crocodilians, dinosaurs, and birds. Their extinction at the end of the Triassic, often linked to the Triassic-Cretaceous extinction event, marked the end of a significant chapter in archosaur evolution, paving the way for the rise of the dinosaurs.