The petrosal and bony labyrinth of Diplobune minor, an enigmatic Artiodactyla from the Oligocene of Western Europe.

Abstract

Anoplotheriinae are Paleogene European artiodactyls that present a unique postcranial morphology with a tridactyl autopodium and uncommon limb orientation. This peculiar morphology led to various hypotheses regarding anoplotheriine locomotion from semiaquatic to partly arboreal or partly bipedal. The petrosal bone, housing the organs of balance, and hearing, offers complementary information to postcranial morphology on the ecology of this uncommon artiodactyl. Here, we investigate the middle ear and bony labyrinth of the small anoplotheriine Diplobune minor based on four specimens from the Early Oligocene locality of Itardies (Quercy, France). A macroscopic study coupled with a μCT scan investigation of the petrosal anatomy provides novel information on the bony labyrinth, stapes, and innervation and vasculature of the inner ear of this enigmatic taxon. The petrosal of D. minor exhibits a mosaic of plesiomorphic characters and peculiar features that shed new light into the anatomy of this poorly studied taxon of an obscure taxonomic clade. We can confidently reject that D. minor was a semiaquatic species based on the petrosal morphology: presence of a large mastoid process and nonpachyostotic tegmen tympani do not support underwater hearing. On the other hand, the average semicircular canal radius points to a slow or medium slow agility for D. minor, and fully rejects it was a fast moving animal, which is congruent with its postcranial anatomy.