Distal humerus and ulna of Parapresbytis (Colobinae) from the Pliocene of Russia and Mongolia: phylogenetic and ecological implications based on elbow morphology

Abstract

Parapresbytis eohanuman is a colobine known from two middle Pliocene localities southeast of Lake Baikal, northern East Asia. This study examined the morphology of postcranial specimens, a distal humerus and an ulna, of Parapresbytis. A total of 18 and 13 linear measurements were taken from the humerus and the ulna, respectively, and compared with those of extant and European fossil colobines using principal components analysis. The distal humeral specimen of Parapresbytis is slightly larger than those of male Semnopithecus and Nasalis, while the ulnar specimen is much larger than those of extant colobines and is nearly as large as that of a male Papio ursinus. Morphologically, the distal humeri and ulnae of terrestrial colobines such as Semnopithecus, Dolichopithecus, and Mesopithecus can be distinguished from those of arboreal colobines. The morphology of the Parapresbytis elbow is within the range of the arboreal colobines, contradicting previous suggestions that this genus is terrestrially adapted and is phylogenetically close to Dolichopithecus. Because the arboreally adapted elbow is shared by most extant colobines, the morphology of the elbow does not provide evidence for phylogenetic proximity between Parapresbytis and a particular taxon of arboreal colobine such as Rhinopithecus. The elbow morphology suggests that Parapresbytis was adapted to arboreal locomotion as much as the extant arboreal colobines are. This is congruent with the paleoenvironmental evidence, which indicates the presence of forests in the Lake Baikal area during the middle Pliocene.