New evidence from the bony labyrinth and teeth of the Late Miocene ape Oreopithecus based on a partial cranium from Baccinello

Oreopithecus bambolii, from the Tusco-Sardinian archipelago, is the latest-known European Miocene ape and a longstanding evolutionary enigma, owing to its mosaic morphology. Its systematic position has been intensely debated, resulting in a broad spectrum of hypotheses, ranging from a dryopithecine great ape to a nyanzapithecid stem hominoid, a giant hylobatid, or even a pliopithecoid. We reassess these competing hypotheses based on new data from the bony labyrinth and teeth of a partial cranium (Bac 62) from Baccinello (8.3–7.7 Ma). Diffeomorphic deformation analysis of the semicircular canals (SCs) and vestibule, dental tissue proportions, and enamel-dentine junction shape were used to compare Oreopithecus with a sample of extant and extinct anthropoids. Our results confirm that the SCs and vestibule of Oreopithecus combine some derived crown hominoid synapomorphies and crown hylobatid-like characters with the retention of anthropoid symplesiomorphies that preclude a straightforward classification. In turn, the enamel-dentine junction shape of the upper molars markedly differs from that of dryopithecines and extant hominoids but only shows some similarities with that of pliopithecoids, while the analysis of tooth tissue proportions reveals thick enamel. Our results, combined with previous evidence based on cranial morphology, strongly argue against a close phylogenetic relationship with Miocene apes from Europe and do not particularly support a close relationship with hylobatids. Instead, they are more consistent with Oreopithecus being a derived stem hominoid, although the scarce available data for nyanzapithecids precludes adequately testing this hypothesis and some similarities with pliopithecoids remain intriguing and deserve further investigation.