THE PERMO-TRIASSIC DIVERSIFICATION OF ARCHOSAUROMORPHS

EZCURRA, M. D.; BUTLER, R. J.; SOOKIAS, R. B.

Haarlem

Congreso; XIV Annual Meeting of the European Association of Vertebrate Palaeontologists; 2016

Utrecht University

One of the key evolutionary events during the Triassic was the rise of archosaurs, thegroup that includes dinosaurs, birds, crocodylomorphs and pterosaurs. The crowngroup Archosauria forms one component of a broader evolutionary radiation ofdiapsid reptiles, Archosauromorpha, but this broader context has often been ignoredin analyses of Triassic faunal turnover events. Here, we present an overview of ourrecent and ongoing work revising and clarifying the diversity and phylogenetics ofearly archosauromorphs, and documenting and quantifying temporal and spatialchanges in their morphological diversity and species richness. Archosauromorphsoriginated by the middle?late Permian (c. 260?255 Ma), but just four Permianspecies are currently known. However, their broad palaeogeographic distribution andphylogenetic interrelationships suggest that substantial archosauromorphdiversification occurred before the end of the Permian, possibly in response to theend-Guadalupian extinction event. Archosauromorph fossil remains increase inabundance in the immediate aftermath of the Permo-Triassic mass extinction (PTME;c. 252 Ma), forming part of a short-lived, broadly distributed, earliest Triassic ?disasterfauna? with low morphological diversity. A major increase in archosauromorphmorphological diversity, abundance, species richness and maximum body sizeoccurred around the transition between the Early and Middle Triassic (Olenekian?Anisian boundary, c. 247 Ma), including the origin of large, hypercarnivorouserythrosuchids. This evolutionary event may reflect the post-PTME recovery andstabilization of global ecosystems, following the unstable ?boom and bust? world ofthe Early Triassic. The earliest archosaur body fossils, the sail-backedctenosauriscids, occur at this time, with a broad palaeogeographic distribution.Phylogenetic relationships and footprints suggest that much of the Early Triassicdiversification of archosaurs remains to be discovered. Cranial disparity of nonarchosaurianarchosauromorphs in the Anisian was notably high, and exceeded thatof the crown archosaurs. However, non-archosaurian archosauromorphs declined inthe early Late Triassic, coincident with the morphospace expansion anddiversification of crown archosaurs. This turnover may reflect a competitivereplacement. Only crown archosaurs survived into the Jurassic, going on to dominatethe Mesozoic world.