Molar microwear in Praeanthropus afarensis: evidence for dietary stasis through time and under diverse paleoecological conditions

Molar microwear fabrics in extant mammals vary with diet and, more particularly, the physical properties of the items that are consumed. Praeanthropus afarensis is well represented in the fossil record over a prolonged and radiometrically controlled temporal span, and reasonably robust paleoecological reconstructions are available for the various localities from which it is known. We therefore examined molar microwear in this species to determine whether diet varied in relation to time or in response to different ecological conditions. Of more than 70 specimens of Pr. afarensis that contain one or more worn permanent molars, only 19 were found to be suitable for microwear analysis. These derive from eight temporal horizons in the Laetolil Beds and Hadar Formation spanning approximately 400kyr (3.6-3.2Ma). Six paleoecological categories have been reconstructed for these horizons, and these were ranked on the basis of floral cover. None of the microwear variables observed for Pr. afarensis is significantly associated with either temporal or paleoecological rank. Thus, microwear and, by extension, diet does not appear to have altered significantly in Pr. afarensis through time or in response to different paleoecological circumstances. The wear pattern that appears to have characterized Pr. afarensis overlaps extensively that of Gorilla gorilla beringei and differs notably from the fabrics of extant primates (e.g., Cebus apella and Cercocebus albigena) that consume hard objects. The high proportion of scratches on Pr. afarensis molars suggests the inclusion of fine abrasives in or on the food items consumed by those individuals sampled in this study. Although Pr. afarensis may have been morphologically equipped to process hard, brittle items, the microwear data suggest that it did not necessarily do so, even in the face of varying environmental circumstances. Explanatory scenarios that describe Pr. afarensis as part of an evolutionary trajectory involving a more heavily masticated diet with an increased reliance on hard, brittle items need to be reconsidered. However, fallback foods that were consumed during relatively short, albeit critical periods may have exerted sufficient selective pressure to explain the evolution of the comparatively robust Pr. afarensis trophic apparatus. Because it is unlikely that many individuals from such restricted temporal intervals would be sampled in the paleontological record, we suggest that the most productive approach to the elucidation of paleodiet is the integration of genetic (morphological) and epigenetic (microwear and isotopic) lines of evidence.