The morphology of the oval window in Paranthropus robustus compared to humans and other modern primates.

The oval window (OW) is an opening connecting the inner and middle ear. Its area has been shown to consistently scale with body mass (BM) in primates, and has been used alongside semi-circular canal (SCC) size to differentiate Homo sapiens and fossil hominins, including Paranthropus robustus. However, while the morphology of other inner ear elements, such as cochlea and SCCs, has been extensively studied in primates, OW shape has received little attention. In this study, we assess OW morphological variability in extant primates, and compare P. robustus to extant hominids. The potential of OW size to predict BM is also assessed. For this, measurements were performed on 3D scans from extant primate species and of P. robustus from the sites of Kromdraai, Swartkrans, and Drimolen. Size was assessed using perimeter (OWP), area (OWA), and centroid size (OWCS). Shape was assessed using geometric morphometric methods. The OW has no sexual dimorphism; there is no size difference between juveniles and adults, but there is a slight shape difference between human juveniles and adults, with a seemingly opposite ontogenetic trajectory compared to other primates. P. robustus has an intermediary OW shape between apes and humans, with more ape-like specimens from Kromdraai and more human-like ones from Drimolen. Overall, OW morphology discriminates primate species well enough, especially H. sapiens. BM is well explained by OWA, but OWA is not reliable as a BM proxy due to high prediction errors. Nonetheless, the OWA of P. robustus suggests a BM close to that of a chimpanzee.