Isotopic equifinality and rethinking the diet of Australopithecus anamensis

Abstract

Objectives: Australopithecus anamensis has comparable δ¹³ C_enamel values to Ardipithecus ramidus, and both have been characterized as C₃ 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 C₃ -C₄ -mixed-feeding A. afarensis. Here I test the hypothesis that changing the variables (ε^*_enamel-diet , δ¹³ C_C3, δ¹³ C_C4 values) used to calculate the percentage of dietary C₄ foods (%C₄ 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 δ¹³ C values from the Pliocene Omo-Turkana Basin (4.2-3.9 Ma). I recalculate A. anamensis' %C₄ diet based on new body size-dependent estimates of the ε^*_enamel-diet value and alternative δ¹³ C_C3 and δ¹³ C_C4 values.

Results: The Pliocene Omo-Turkana Basin shows evidence for dietary resources with a wide range of δ¹³ C values including canopy-driven ¹³ C depleted ground C₃ foods. Alternative equation variables changed by 1-2‰ yield higher C₄ estimates for A. anamensis (15-31%) than previously thought (0-10%). The choice of δ¹³ C_C3 and δ¹³ C_C4 values for estimating %C₄ is not a perfunctory task and potentially explains the δ¹³ 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 ¹³ C-depleted and ¹³ C-enriched C₃ foods and a portion of C₄ 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].

Keywords: Australopithecus anamensis; Omo-Turkana Basin; enamel carbon isotopes; isotopic equifinality; percent C4 diet.