Body Size Disparity of the Archosauromorph Reptiles during the First 90 Million Years of Their Evolution

Abstract. An important parameter in studying the macroevolutionary history of a clade is the variation of its body size. However, the analysis of this parameter in archosauromorphs has been restricted to Archosauria, and the disparity in body size has not been exhaustively explored. In the present work, we study the variation in body size of more than 400 known Permian–Early Jurassic archosauromorph species in the context of their early evolutionary radiation. We analysed the disparity of body size over time and the relationship between this parameter and the palaeolatitudinal distribution of species. From these analyses, it was found that the disparity of body size of archosauromorphs increased after the Permian/Triassic boundary. In the case of Pseudosuchia and Pan-Aves (=Avemetatarsalia), the Triassic/Jurassic extinction shows a pattern of selective extinction of medium to large-sized forms, and their body size disparity decreased significantly. In contrast, dinosaurs increased their body size and their disparity did not change significantly after the Triassic/Jurassic extinction event. Regarding the relationship between body size and geographic distribution, pseudosuchians show a pattern of body size decrease towards higher palaeolatitudes, i.e., a converse Bergmann's rule. These results could be linked to physiological factors since many groups of extant ectothermic animals show a similar pattern. These analyses help to elucidate the complex body size evolutionary dynamics in the early radiation of Archosauromorpha, as indicated by the different patterns observed across its subclades.