Fossil Crocodilians From The High Guajira Peninsula Of Colombia And The History Of Neogene Crocodilian Diversity In Tropical South America

The greatest diversity of Cenozoic crocodilians occurred during the Miocene in equatorial South America. However, the origin of this high diversity and its relationship to environmental factors are poorly understood. Most described species come from localities assigned to Laventan (13.8-11.8 Ma) and Huayquerian (9.0-6.8 Ma) South American land mammal ages (SALMAS), whereas the record is sparse in the early to middle Miocene and after the latest Miocene and Pliocene. Field research in the Castilletes (early Miocene-Pliocene) Formation in the High Guajira Peninsula of Colombia provides new fossil data on the origin of Neotropical crocodylian diversity. The Castilletes Formation crops out most extensively in the Cocinetas Basin, and represent depositional environments consisting of deltaic and shallow marine systems in the lower Castilletes and predominately fluvial environments in the upper Castilletes. Crocodilian fossils from the Castilletes Formation include gavialoids, alligatoroids and crocodyloids. Gavialoid remains have been recovered from both terrestrial and shallow marine deposits in the lower Castilletes. Remains of the specialized caimanines Purussaurus and Mourasuchus extend the temporal range of both lineages into the early middle Miocene (15-16 Ma). These records suggest that high diversity crocodilian assemblages were already established by the early Miocene or late Oligocene. Fossils from the upper Castilletes Formation include cranial elements identified as a non-tomistomine crocodyloid, some of them assigned to cf. Crocodylus. These records indicate that by the Pliocene, endemic assemblages were extinct, at least in the northern parts of the continent, allowing the establishment of Crocodylus. The pattern of crocodilian diversity in the Neogene of equatorial South America suggests that diversity was highly linked to hydrographic conditions. The development of high diversity assemblages developed in a time of greater connections among river drainages and mega-wetland systems. The isolation of river drainages and disappearance of mega-wetlands are correlated with the extinction of most crocodilian lineages. Aridity in peripheral drainages may have caused local extinctions outside Amazonia. The change from wetland to riverine conditions has been proposed as the cause of crocodilian extinctions in the western Amazon, but this remains to be tested.