Anatomy of the mandibular corpus of extant cercopithecids : taxonomy and variation

Abstract

This study aims to discriminate cercopithecid taxa of higher taxonomic levels (subfamily, tribe, subtribe, and genus) on the basis of corpus shape in transverse cross-section at the M₁-M₂ junction and to assess its variation using 2D geometric morphometrics. Specifically, we evaluated the effect of allometry and sexual dimorphism on differences in corpus shape at interspecific and intraspecific levels, respectively. We also investigated whether corpus variation among cercopithecids was following Brownian motion using Pagel’s λ. Taxonomic discrimination and sexual dimorphism were established using Analysis of Variance on Principal Component scores. Allometry was studied using phylogenetic least-squares regressions and partial least-squares regressions. We demonstrated that, using corpus shape, extant cercopithecids can be significantly discriminated at the subfamilial, tribal, and subtribal levels. In addition, the main axis of variation of the Principal Component Analysis follows a distribution expected under Brownian motion, validating the presence of a phylogenetic signal in corpus shape. Colobines exhibit a robust corpus (superoinferiorly short and transversely broad) with large lateral prominences while cercopithecines have a gracile corpus (superoinferiorly long and transversely thin in its distal portion) with marked corpus fossae in African papionins. Exception to the typical subfamilial or tribal shape pattern exist, with the best examples being Trachypithecus, Presbytis and Pygathrix within colobines, Allenopithecus within Cercopithecini, and Macaca, Theropithecus and Cercocebus within Papionini. Sexual dimorphism is a confounding factor in shape discrimination, as there are significant differences between sexes, notably in Papio anubis, Nasalis larvatus and Procolobus verus . Intriguingly, sexual dimorphism in corpus shape does not seem to follow the dimorphism deduced in canine and molar crown dimensions. This discrepancy is illustrated by the low degree of dimorphism in corpus shape in Piliocolobus badius, despite dimorphic canine and molar dimensions. Overall, our findings concerning corpus shape variation in cercopithecids will greatly benefit to paleontological studies that seek to identify taxa in the fossil record, and to neontological studies aiming to explore the ecomorphological value of the cercopithecid mandible.