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IntroductionPapionins are a well-studied and morphologically diverse clade of cercopithecid monkeys. Understanding how craniodental morphology varies in this clade has implications for interpreting taxonomic variation in the fossil record and for our understanding of primate evolution. MethodsWe quantified the phenotypic relationship between facial length and dental proportions in N=314 cercopithecid individuals across 10 species (six papionins, two cercopithecins, and two colobines) using dental ratios MMC (molar module component, ratio of the lengths of the third and first molars) and PMM (premolar-molar module, ratio of the lengths of the second molar and the fourth premolar) and two metrics of facial length: palatal length and prosthion – glabella. ResultsFacial length and molar dental proportions are significantly correlated interspecifically across cercopithecids (PGLS, p<0.01), where species with longer faces have relatively longer maxillary and mandibular third molars. These traits are generally not correlated intraspecifically in the cercopithecids sampled, with some exceptions. DiscussionOur data demonstrate that prognathic faces evolved convergently at least twice in papionins, with parsimony supporting that Papio/Theropithecus shared a prognathic ancestor after the divergence of Lophocebus. Additionally, this study lends support to the hypothesis that facial reduction and third molar reduction in human evolution were coordinated and may have been the result of pleiotropy alongside changes in diet. 

Modern papionin monkeys are a diverse group that encompasses a broad range of morphologies, behaviors, and ecologies. A fossil genus known from African Plio-Pleistocene deposits, Parapapio, is widely regarded as a candidate ancestor to later African papionins. However, despite general agreement that this genus sits at or near the base of the African papionin clade, the taxonomy within Parapapio remains highly contentious. This project evaluates the species-level taxonomy of Parapapio with an explicit hypothesis-based approach to interpreting morphological variation in this sample of fossils. We tested two hypotheses: (H₁) the craniodental variation within Parapapio does not cluster into three groups that reflect the three known species, and (H₂) all the Parapapio fossils can be accommodated within the craniodental shape and size variation observed for a single extant species of papionin. To test the first hypothesis, we assessed a subset of relatively complete and well-preserved Parapapio crania (n=16), intentionally without reference to previous taxonomic identifications. Specimens were sorted by similarity in cranial features and results were then compared with published taxonomic classifications. Our results demonstrate that morphological traits do not cluster consistently according to the current species categories within Parapapio, failing to reject our first hypothesis. To test our second hypothesis, we examined variation in cranial and dental metrics within Parapapio (n=64) relative to three extant papionin samples (n=310). Our results fail to reject the hypothesis that all Parapapio specimens could belong to a single species and suggest that the three-species paradigm does not reflect the anatomical variation of this genus. We recommend subsuming all Parapapio specimens within Parapapio broomi, the species name with taxonomic priority. The results of this hypothesis-testing approach to taxonomy carry substantial implications for the taxonomy of Parapapio, as well as for biochronological and paleoecological studies more generally, including the taxonomy and paleobiology of hominids recovered from these same deposits. 

Abstract ObjectivesThe goals of this study were to describe and interpret two new fossil assemblages of cercopithecin monkeys (n = 328), one from the Faro Daba beds (ca. 100,000 years) and the other one from the Chai Baro beds (>158,000 years old), in the Afar Rift of Ethiopia. Materials and MethodsWe describe the two assemblages and compare them to extant cercopithecin species and the smaller fossil assemblage from Asbole, Ethiopia (ca. 600 ka). We use a population‐based approach to the taxonomy given the unusually large number of specimens. Craniodental and postcranial anatomy are presented. Evidence of locomotor habitus is described and evaluated in a framework of hybridization and postcranial plasticity. ResultsWe attribute all cercopithecin specimens from both beds to cf.Chlorocebusand conclude that the Faro Daba and Chai Baro assemblages likely sample single species at each time horizon. Subtle differences between the two assemblages, mostly in postcranial morphology, are insufficient to justify separation at the species level. DiscussionThe large sample sizes and unique preservational aspects of these two assemblages open a new window into the recent evolution of guenons. Our data indicate that these fossil populations may be ancestral to the cercopithecins currently living in the Afar region of Ethiopia.