Specialization for amphibiosis in Brachyodus onoideus (Artiodactyla, Hippopotamoidea) from the Early Miocene of France

Abstract

A partial cranium of a very large anthracothere was unearthed during a palaeontological excavation at Saint-Antoine-de-Ficalba (Lot-et-Garonne, France; Early Miocene, ~18-17.0 Ma). The new material, referred to as Brachyodus onoideus (Gervais, 1859), documents the cranial features of this species, so far mainly known by dental and postcranial remains. The preserved part of the skull roughly coincides with the neurocranium and is remarkable for the dorsally-protruding orbits, the importance of the postorbital constriction, the small volume of the braincase, and the gigantic size of the occipital condyle relative to the other elements of the neurocranium. A very careful dissection of the left auditory region allowed extraction of the left petrosal bone and provides the first description of a petrosal for Brachyodus. The morphology of the petrosal is strikingly similar to that of extant hippos with: (1) a ventral basicapsular groove, (2) a sharp crista petrosa, (3) a wide prefacial commissure fossa, (4) a reduced mastoid, and (5) an hyperinflated tegmen tympani. Both the disposition of the orifices of the head and the petrosal morphology support a specialization of Brachyodus onoideus to an amphibious lifestyle and to potential underwater directional hearing. © 2013 Swiss Geological Society.