Anatomy, phylogeny and palaeobilogy of Early Archosaurs and their kin

Abstract

Archosauria include two extant clades, crocodylians and avians. However, the diversification of the avian and crocodylian crown groups are relatively recent events that began during the Late Cretaceous (Brochu 2001, 2003; Clarke et al. 2005), less than 100 Ma ago. These crown diversifications are shallow in time compared to the much deeper divergence of the crocodylian and avian stem lineages in the Triassic, approximately 250 Ma ago (Butler et al. 2011; Nesbitt 2011; Nesbitt et al. 2011). The early archosaurian diversification began in the wake of the devastating end-Permian mass extinction, and the divergence of both lineages and disparate body forms was rapid, occurring less than 10 Ma after the first archosaur (Nesbitt et al. 2010). This initial diversification was severely affected by the end-Triassic mass extinction approximately 50 Ma later, and only three major archosaur clades – pterosaurs, dinosaurs and crocodylomorphs – survived to diversify during the rest of the Mesozoic.