Description and taxonomic assessment of fossil Cercopithecidae from the Pliocene Galili Formation (Ethiopia).

The Mount Galili Formation in the Afar region, Ethiopia, samples a critical time in hominin evolution, 4.4 to 3.8 Ma, documenting the last appearance of Ardipithecus and the origin of Australopithecus. This period is also important in the evolution of cercopithecids, especially the origin of Theropithecus in general and Theropithecus oswaldi lineage in particular. Galili has provided a total of 655 cercopithecid specimens that include crania, mandibles, isolated teeth and postcrania. All the fossils were recovered from the Lasdanan (5.3-4.43 Ma), Dhidinley (4.43-3.9 Ma) and Shabeley Laag (∼3.92-3.8 Ma) Members. Here, we described and analyzed 362 fossils employing both qualitative and quantitative methods. Descriptions of the material were supplemented with dental metrics and cranial shape analysis using three-dimensional geometric morphometrics. Results indicate the presence of at least six cercopithecid taxa: Theropithecus oswaldi serengetensis (n = 28), Theropithecus sp. (n = 2), three non-Theropithecus papionin groups (n = 134) and one colobine-size group (n = 58). The T. o. serengetensis represents the earliest form of the lineage, documented from ∼3.9 Ma Galili sediments. The three Galili papionins include a smaller taxon, a medium-sized taxon comparable to Pliopapio alemui and a large papionin overlapping in size with Soromandrillus, Gorgopithecus and Dinopithecus. The majority of Galili colobines have closest affinities to Kuseracolobus aramisi and some overlap with other taxa. Papionins dominate the Galili cercopithecid collection, although colobines are still fairly common (approximately 25% of the sample). Thus, Galili sample is like Kanapoi (4.2-4.1 Ma) and Gona (5.2-3.9 Ma) localities but distinct from Aramis, suggesting paleoecological similarity to the former sites. On the other hand, Theropithecus is less abundant at Galili than geologically younger Hadar (3.4-3.2 Ma) and Woranso-Mille (3.8-3.6 Ma) sites. Whether this difference is due to sampling, time or landscape variation requires further investigation.

Biochronology of South African hominin-bearing sites: A reassessment using cercopithecid primates.

Despite recent advances in chronometric techniques (e.g., Uranium-Lead [U-Pb], cosmogenic nuclides, electron spin resonance spectroscopy [ESR]), considerable uncertainty remains regarding the age of many Plio-Pleistocene hominin sites, including several in South Africa. Consequently, biochronology remains important in assessments of Plio-Pleistocene geochronology and provides direct age estimates of the fossils themselves. Historically, cercopithecid monkeys have been among the most useful taxa for biochronology of early hominins because they are widely present and abundant in the African Plio-Pleistocene record. The last major studies using cercopithecids were published over 30 y ago. Since then, new hominin sites have been discovered, radiometric age estimates have been refined, and many changes have occurred in cercopithecid taxonomy and systematics. Thus, a biochronological reassessment using cercopithecids is long overdue. Here, we provide just such a revision based on our recent study of every major cercopithecid collection from African Plio-Pleistocene sites. In addition to correlations based on shared faunal elements, we present an analysis based on the dentition of the abundant cercopithecid Theropithecus oswaldi, which increases in size in a manner that is strongly correlated with geological age (r2 ∼0.83), thereby providing a highly accurate age-estimation tool not previously utilized. In combination with paleomagnetic and U-Pb data, our results provide revised age estimates and suggest that there are no hominin sites in South Africa significantly older than ∼2.8 Ma. Where conflicting age estimates exist, we suggest that additional data are needed and recall that faunal estimates have ultimately proved reliable in the past (e.g., the age of the KBS Tuff).