Early catarrhine evolution in the light of semicircular canal and vestibule shape

The phylogenetic relationships of early catarrhines (Old World monkeys and apes) are contentious, particularly regarding pliopithecoids, which are considered a clade of Miocene stem catarrhines more derived than most of their Oligocene counterparts. However, their debated affinities and the uncertain phylogenetic relationships of the Oligocene catarrhine Saadanius leave their systematic position unresolved. Notably, Saadanius displays a mosaic morphology, coupling primitive craniodental traits with a fully ossified ectotympanic (as opposed to the annular/incompletely ossified ear tube of stem catarrhines and early stem hominoids), a characteristic customarily considered as a shared-derived trait of crown catarrhines. To clarify the early evolutionary history of catarrhines, we analyzed the semicircular canal and vestibule morphology of Saadanius, the pliopithecoid Pliobates, the stem cercopithecoid Victoriapithecus, and the stem hominoid Ekembo. Using diffeomorphic deformation-based geometric morphometrics on a broad anthropoid sample, we quantified shape variation and inferred ancestral morphotypes. Our results reveal that Saadanius retains a primitive, platyrrhine-like inner ear, lacking all crown catarrhine synapomorphies. We also recover similarities between Pliobates and the other analyzed pliopithecoid (Epipliopithecus), as these taxa share some derived traits with crown catarrhine (particularly, in the anterior semicircular canal), while missing most hominoid and cercopithecoid synapomorphies. Furthermore, Victoriapithecus displays a derived, cercopithecoid-like morphology indistinguishable from extant Old World monkeys, whereas Ekembo exhibits unexpected derived, ape-like features. These findings support that pliopithecoids are stem catarrhines more derived than Saadanius and imply that its tubular ectotympanic must have evolved independently from crown catarrhines, thus representing a remarkable instance of homoplasy.