Crocodilian diversity in space and time: the role of climate in paleoecology and its implication for understanding K/T extinctions

The taxonomic diversity of crocodilians (Crocodylia) through the last 100 m illion years shows a general decline in the number of genera and species to the present day. But this masks a more complex pattern. This is investigat ed here using a comprehensive database of fossil crocodilians that provides the opportunity to examine spatial and temporal trends, the influence of s ampling, and the role of climate in regulating biodiversity. Crown-group crocodilians, comprising the extant families Alligatoridae, Cro codylidae and Gavialidae, show the following trend: an initial exponential diversification through the Late Cretaceous and Paleocene that is restricte d to the Northern Hemisphere until after the K/T boundary; relatively const ant diversity from the Paleocene into the middle Eocene that may be an arti fact of sampling, which might mask an actual decline in numbers; low divers ity during the late Eocene and Oligocene; a second exponential diversificat ion during the Miocene and leveling off in the late Miocene and Pliocene; a nd a precipitous drop in the Pleistocene and Recent. The coincidence of dro ps in diversity with global cooling is suggestive of a causal link-during t he initial glaciation of Antarctica in the Eocene and Oligocene and the Nor thern Hemisphere glaciation at the end of the Pliocene. However, matters ar e complicated in the Northern Hemisphere by the climatic effects of regiona l uplift. Although the global trend of diversification is unperturbed at the K/T boun dary, this is largely due to the exceptionally high rate of origination in the early Paleocene. Nonetheless, the survival of such a demonstrably clima te-sensitive group strongly suggests that a climatic explanation for the K/ T mass extinctions, especially the demise of the dinosaurs, must be reconsi dered.