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.

The petrosal and basicranial morphology ofProtoceras celer

“…3A), a feature shared with other protoceratids, with ruminants, and with anoplotheriids (Fig. 6) [7,25,38,46]. The presence of this ridge in P. celer indicates that a clear cerebral/cerebellar division was maintained throughout protoceratid evolution.…”

“…Artiodactyls generally have a vestibular aqueduct that originates either at the base of the common crus or just anterior to the common crus [e.g., 38,41,45], so the position of the P. celer vestibular aqueduct is as expected. Not much can be said about the morphology of the semicircular canals given that only one canal is preserved in AMMH-VP 53523.…”

“…The bony labyrinth morphology of other purported tylopods is not well-known; morphologies have only been described from Cainotherium [33,37], Diplobune [38], and Bathygenys [39].…”

“…However, there have been extensive descriptions of extinct and extant ruminant bony labyrinths [40][41][42][43][44], and the bony labyrinths of the early artiodactyl Diacodexis ilicis and the extant suid Sus scrofa have also been described [39,45]. The cochlea of P. celer has 2.75 turns, which is more turns than Diplobune, moschids, cervids, and bovids, but fewer turns than Cainotherium and S. scrofa [37,38,40,44]. It is most comparable to the tragulids; most tragulids have 3.0 turns or more, but Moschiola meminna can range from 2.75 to 3.25 turns [41,42,45].…”

“…The vestibule of P. celer is typical of artiodactyls. Most taxa have a slightly inflated saccule and utricle with a clear distinction between the two structures [e.g., 38,41,45], although this is not the case of Bathygenys [39]. The vestibular aqueduct appears to originate from the common crus, but the medial end of the aqueduct could not be identified in P. celer.…”