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

Abstract

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).

Keywords: Pleistocene; Pliocene; Theropithecus; chronology.

Fig. 1.

Ranges of select African Plio-Pleistocene cercopithecids from radiometrically dated, stratified sites in eastern Africa. For a complete list of all cercopithecid species and their ranges see SI Appendix, Table S3. Vertical axis indicates geological age in millions of years (Ma); a geomagnetic polarity timescale (81) is also shown for reference. Boxes represent chronological ranges of eastern African occurrences based on well-constrained and readily taxonomically diagnosable material, while whiskers indicate possible extensions due to less taxonomically diagnostic and/or less well-constrained occurrences. Note C. coronatus includes Cercopithecoides kimeui (30). Left column indicates African Cercopithecid zones identified in this analysis.

Fig. 2.

Distribution of select cercopithecids across South African sites. Left lists localities analyzed here in approximate chronometric order from oldest to youngest based on results from this study. Placement of different sites in African Cercopithecid zones from Fig. 1 are indicated on the Right.

Fig. 3.

Model for evolution of molar size in T. oswaldi lineage used to estimate geological age based on analysis of covariance using age in millions of years (Ma) as the independent variable (horizontal axis) and log₁₀ of molar area (mm²) as the dependent variable (vertical axis). Area calculated as indicated in the Lower Right Inset (length × the average of the mesial and distal breadths). Upper molars are shown by solid circles with solid trend lines and lowers by hollow circles and dashed trend lines. Color denotes tooth position: gold, 1; orange, 2; and red, 3. Approximate geological age ranges for recognized subspecies of the T. oswaldi lineage are shown by the alternating shaded boxes and labeled at Top. See SI Appendix, SI Text and Table S1 for further details.

Fig. 4.

Consensus estimates for the ages of select South African fossil sites indicated by vertical bars, black for sites in the Cradle, blue for others. Vertical axis indicates geological age (Ma), with the geomagnetic polarity timescale (after ref. 82) and shading indicating flowstone forming (FGI) and sedimentary (SED) intervals (after ref. 2) for reference. Red stars mark mean age estimates based on T. oswaldi lineage molar sizes. Yellow dots mark U/Pb estimates. Black and white dots mark paleomagnetic samples. For further details on data behind estimates see SI Appendix, SI Text.