Morphometric Study on the Mandible of Colobine Fossil (Mesopithecus pentelicus) Found in East Asia, a Comparison With Extant Taxa

Abstract

A tooth-bearing mandible fossil of a colobine monkey discovered at Shuitangba, Zhaotong, Yunnan, China, was morphometrically analyzed and compared with extant Asian colobines. Our previous qualitative and quantitative descriptions indicate that it can be safely attributed to Mesopithecus pentelicus, a Miocene fossil colobine widely found in Europe and South Asia. The present research aims to explore fossil association with extant colobines and functionally propose its dietary preferences based on multivariate morphometric analyses of mandibular morphology. The results indicate that this fossil species presents a mosaic association with extant langurs (Presbytis, Trachypithecus, and Semnopithecus) and odd-nosed monkeys (Pygathrix, Nasalis, and Rhinopithecus), with most similarities of size-related traits and size-adjusted shapes, and a notable difference from extant Rhinopithecus, due to the increased body size of the latter since the Pliocene. The allometric analysis showed that, like the fossils of the same species in other sites, its mandibular structure appears more adapted to crushing hard seed shells than chewing leaves. Moreover, our findings also imply that the Asian colobines significantly modified their dietary preferences over the last 6 million years following the ecological and environmental changes triggered by the accelerated uplift of the Qinghai-Tibet Plateaus and severe monsoons. Morphologically, the more developed condyle length and moment arms of the temporomandibular joint and medial pterygoid muscles appear to accommodate a more folivorous dietary selection, functionally related to the intake of rigid fibers in leaves that require frequent one or two-side mandibular chewing and grinding.