Discovery of the fossil otter (Carnivora, Mammalia) in Mexico reconciles a palaeozoogeographic mystery.

The North American fossil otter is thought to be partially convergent in ecological niche with the living sea otter , both having low-crowned crushing teeth and a close association with marine environments. Fossil records of are found in mostly marginal marine deposits in California and Florida; despite presence of very rich records of fossil terrestrial mammals in contemporaneous localities inland, no fossils are hitherto known in interior North America. Here we report the first occurrence of outside of Florida and California, in a land-locked terrestrial mammal fauna of the upper Miocene deposits of Juchipila Basin, Zacatecas State, Mexico. This new occurrence of is at least 200 km from the modern Pacific coastline, and nearly 600 km from the Gulf of Mexico. Besides providing further evidence that was not dependent on coastal marine environments as originally interpreted, this discovery leads us to propose a new east-to-west dispersal route between the Florida and California populations through central Mexico. The proximity of the fossil locality to nearby populations of modern neotropical otters suggests that trans-Mexican freshwater corridors for vertebrate species in riparian habitats may have persisted for a prolonged period of time, pre-dating the Great American Biotic Interchange.