The role of climate on the emergence of giant caimanines (Crocodylia, Alligatoroidea) from the Miocene western Amazonian region

Extant caimanines include the six modern species of caimans, which occur predominantly in South and Central America and are mostly medium-sized crocodylians. Nevertheless, the fossil record of the group reveals a significantly higher diversity, with remarkable body size variation. In particular, the giants Purussaurus and Mourasuchus, from the Miocene western Amazonian region, are two of the most prominent representatives. Previous work has demonstrated a correlation between the body size of crocodylians and abiotic factors throughout the Cenozoic; however, this relationship is poorly understood, particularly within the Caimaninae lineage. Here, we explore evolutionary body size patterns within Caimaninae, investigating the potential influence of climatic factors. Using a phylogenetically-informed method, we estimated the body size of 33 caimanine specimens, coupled with climatic variables from a General Circulation Model to reconstruct deep-time patterns. Our results indicate that giant Miocene caimanines are restricted to warmer conditions, with significantly less seasonal temperature variation. This suggests that the unmatching climatic conditions of the Miocene western Amazonian region possibly allowed the emergence of unique palaeoecosystems, favouring the sustenance of these very large crocodylians.