Ulnar morphology of Pliobates cataloniae (Pliopithecoidea: Crouzeliidae): Insights into catarrhine locomotor diversity and forelimb evolution

Pliobates cataloniae is a small-bodied crouzeliid pliopithecoid from the Miocene (∼11.6 Ma) of Abocador de Can Mata (ACM; Vallès-Penedès Basin, NE Iberian Peninsula). It exhibits a mosaic of primitive (stem catarrhine) and derived (modern hominoid-like) postcranial features. The holotype partial skeleton, from locality ACM/C8-A4, includes an almost complete ulna—a bone that plays a critical role in forearm flexion-extension and pronation-supination. Here, we use landmark-based three-dimensional geometric morphometrics to evaluate the closest morphometric affinities of the Pliobates ulna and explore its implications for the locomotor repertoire of this taxon. The comparative sample includes 156 specimens corresponding to 35 species from 20 anthropoid genera, three lorisid genera, and 10 fossil catarrhines. Our results indicate that the trochlear notch morphology of Pliobates resembles that of stem catarrhines and other nonhominoid primates. However, Pliobates differs from cercopithecoids (especially terrestrial taxa) in radial notch features related to enhanced pronation-supination capabilities, closely resembling the condition displayed by crown hominoids, Ateles, and Loris. In turn, the distal ulna of Pliobates does not overlap with any extant group and differs from the other fossils analyzed, most closely resembling that of hylobatids and lorisids. Pliobates probably had an extensive range of movement in the distal forearm, as indicated by the incipiently expanded semilunar ulnar head, the relatively short styloid process, the deep fovea, and the hooklike styloid process. This suggests that Pliobates would have frequently displayed nonstereotypical limb postures and slow locomotor behaviors. Overall, the ulnar morphology of Pliobates suggests that its locomotor repertoire may have combined cautious above-branch quadrupedalism and eclectic climbing with nonagile suspensory behaviors resembling those of Ateles.