Early Archosauromorphs: The Crocodile and Dinosaur Precursors

The archosauromorphs include crocodiles, dinosaurs (containing birds) and all reptiles more closely related to them than to lepidosaurs (tuataras, snakes, lizards). The oldest archosauromorphs have been collected in middle-upper Permian rocks of Europe and Africa, and the group survived the Permo-Triassic mass extinction (c. 252 million years ago), the deadliest biotic crisis documented in the fossil record. After this mass extinction, archosauromorphs diversified and became the dominant tetrapods of continental ecosystems and dispersed across the entire planet during the Triassic Period. The evolution of archosauromorphs during the Triassic is considered an example of adaptive radiation in geologic time. The non-archosaurian archosauromorphs (a group that excludes modern forms, namely crocodiles and birds, and all descendants from their most recent common ancestor) were eclipsed by the archosaur radiation in the Late Triassic, and no groups survived the Triassic-Jurassic mass extinction (c. 201 million years ago). The last decade has witnessed a renewed interest in the paleobiology of non-archosaurian archosauromorphs, and multiple advances have been made in our knowledge of these fossil reptiles. Here, we provide an updated review of the diversity, distribution, phylogeny, ecology and long-term evolution of the important but underappreciated early archosauromorph groups that flourished before the dominance of dinosaurs.