The youngest known South American dyrosaurid (Late Paleocene of Colombia), and evolution of Dyrosauridae (Crocodyliformes: Tethysuchia)

Here we describe a dyrosaurid vertebra recovered from the late Paleocene Cuervos Formation in the Llanos Foothills (Colombia), which was previously unknown to yield vertebrates. This is the third locality in Colombia with records of dyrosaurids. Until now, they were described in South America possibly from the Maastrichtian, but mainly from the Danian and the Selandian. The present specimen, dated from the Thanetian, is the youngest record of dyrosaurid remains on this continent, suggesting an incomplete knowledge on the history of this group in South America. Dyrosaurids diversified during the Maastrichtian, reached their golden age during the Paleocene, and declined from the lower Eocene. The reason why they seem to begin their decline during the Ypresian remains obscure, being incongruent with the Early Eocene Climatic Optimum that should have been favourable for this tropical crocodyliform. Their diversity is best known in Africa, where their apparent Ypresian decline in diversity could be related to their comparison with the Paleocene post K-Pg crisis recovery. The Paleocene shows a higher diversity and species turn-over compared to the more stable Ypresian fauna with a comparatively lower number of species. Similar evolution is observed for North American crocodylians, and strong correlation between the evolution of their diversity, and that of African dyrosaurids, suggests that they were impacted by the same factors. The real drop in dyrosaurid diversity would be Lutetian, as in North American crocodylians, more correlated with the middle-late Eocene climatic cooling. New exploration should be focused on South American Eocene fields to evaluate if the extinction of South American dyrosaurids at the end of the Paleocene was an artifact, or if their early extinction compared to African dyrosaurids was related to regional factors.