Miocene ape evolution: Where does Oreopithecus fit in?

Oreopithecus bambolii Gervais, 1872, from the Late Miocene of Tusco-Sardinia, is the latest non-cercopithecoid catarrhine from Europe. Its geographic and phylogenetic origins remain uncertain despite being well known from craniodental and postcranial remains. Currently, there is a general agreement about its hominoid status (ape and human clade) but uncertainties persist regarding its specific relationships with other fossil and living apes. In the 1990s, Oreopithecus was considered a stem hominid (great ape and human clade) likely derived from dryopithecines (Middle to Late Miocene hominids from Europe). In contrast, recent cladistic analyses recovered Oreopithecus as a derived nyanzapithecid (Early to Late Miocene putative stem hominoids from Africa). In turn, other studies hinted at a closer link with hylobatids (lesser apes). Given seemingly abundant homoplasy (false homology) in features related to orthogrady (upright body posture and locomotion), the Oreopithecus postcranium is compatible with being a stem or a crown hominoid. Craniodental evidence, in contrast, is at odds with a dryopithecine origin. A link with African nyanzapithecids seems more plausible based on dental morphology but hypothesized homologies deserve further investigation. In addition, preliminary analyses of tooth endostructure suggest similarities between Oreopithecus and pliopithecoids (putative stem catarrhines from the Miocene of Eurasia). The main branching topology of the hominoid total group (the divergence of hylobatids relative to putative stem hominoids from the Miocene of Africa) is far from being conclusively resolved due to abundant missing data and pervasive postcranial homoplasy between hylobatids and hominids, which might be causing a long-branch attraction problem. Hence, the hypothesized phylogenetic link between Oreopithecus and nyanzapithecids must not necessarily imply a stem hominoid status: given the long ghost lineage of hylobatids and the aforementioned long-branch attraction problem, a stem hylobatid status cannot be ruled out for nyanzapithecids. Previous difficulties to conclusively determine where Oreopithecus fits in hominoid phylogeny might simply stem from the need to shoehorn this taxon into broadly inaccurate Miocene ape phylogenetic schemes. Rather than considering Oreopithecus an oddball that deserves ad hoc explanations, this Late Miocene ape might be one of the key pieces needed to decipher the as yet unresolved puzzle of Miocene ape phylogeny.