Isotopic equifinality and rethinking the diet of Australopithecus anamensis.

OBJECTIVES

Australopithecus anamensis has comparable δ C values to Ardipithecus ramidus, and both have been characterized as C feeders in open woodland habitats similar to "savanna" chimps. Unlike Ar. ramidus and "savanna" chimps, A. anamensis shows a derived dentognathic morphology for tough foods and a dental microwear pattern similar to the C -C -mixed-feeding A. afarensis. Here I test the hypothesis that changing the variables (ε , δ C δ C values) used to calculate the percentage of dietary C foods (%C diet) by 1-2‰ does not make a substantial difference for hominin diet reconstructions [van der Merwe, Masao, & Bamford, 2008, South African Journal of Science 104:153-155].

MATERIALS AND METHODS

I estimate vegetation structures for A. anamensis with pedogenic carbonate and faunal enamel δ C values from the Pliocene Omo-Turkana Basin (4.2-3.9 Ma). I recalculate A. anamensis' %C diet based on new body size-dependent estimates of the ε value and alternative δ C and δ C values.

RESULTS

The Pliocene Omo-Turkana Basin shows evidence for dietary resources with a wide range of δ C values including canopy-driven C depleted ground C foods. Alternative equation variables changed by 1-2‰ yield higher C estimates for A. anamensis (15-31%) than previously thought (0-10%). The choice of δ C and δ C values for estimating %C is not a perfunctory task and potentially explains the δ C isotopic equifinality of A. anamensis and "savanna" chimps.

DISCUSSION

My integrative diet model reconciles the carbon isotopic data with the dentognathic and microwear evidence of A. anamensis' diet and suggests that "savanna" chimps are not proper dietary analogs of A. anamensis. A foraging strategy across heterogeneous habitats of the Pliocene Omo-Turkana Basin incorporating an array of C-depleted and C-enriched C foods and a portion of C resources may have served as one of the selective pressures for A. anamensis, the earliest habitual biped [van der Merwe et al., 2008, South African Journal of Science 104:153-155].