Biogeographic and Systematic Implications of a Caimanine from the Late Miocene of Southern Mexico.

An isolated supraoccipital from the late Miocene of Chiapas, southern Mexico, can be referred to Caimaninae, a group including the living caimans and their closest extinct relatives. The specimen shares a polygonal shape, indicating lateral contact with the squamosals, with extant Caiman and Melanosuchus, but some extinct caimanine lineages had a similar morphology. This is the northernmost known caimanine occurrence during the Neogene, suggesting that members of this salt-intolerant lineage were present in North America possibly before the Isthmus of Panama was complete. It might also indicate that extant lineages within Caiman, including those found in Mesoamerica today, were distinct earlier than generally believed. [ABSTRACT FROM AUTHOR]