UCLA

Mustelidae are the largest and most diverse family of carnivores, its record dates back to the Early Oligocene, and its early history occurred in western Europe; therefore, the finding of a basal mustelid in southeastern Mexico (northwestern Oaxaca) is quite important. The material found is a very small, nearly complete skull collected from the Yolomécatl Formation, a ~650 m thick lacustrine/fluvial succession interbedded by tuff sheets, yielding a40Ar-39Ar age of 40.3 ± 1.0 Ma, which places this unit and associated fauna in the latest Uintan-earliest Duchesnean NALMAs. However, a purported early Arikareean age has been proposed on the basis of uncritical/unsupported biochronological and isotopic data. The skull is referred to a new genus and species of Mustelidae on the basis of its unique combinaton of plesiomorphic and derived characters. Cladistically, it falls in an unresolved polytomy that inludes only basal European mustelids. This poses a biogeographic problem unsolvable with the evidence at hand: either the new taxon originated in Europe from as yet unknown ancestors, then migrated to southern (tropical) North America, or it evolved there, independently attained the synapomorphies of basal Mustelidae, and migrated to Europe. Finally, the new taxon fits the weasel-ecomorph, approaching the appearance of the extant Mustela frenata, thus suggesting tunnel-hunting habits.