Late Miocene to Pliocene carbon isotope record of differential diet change among East African herbivores

Abstract

Stable isotope and molecular data suggest that C(4) grasses first appeared globally in the Oligocene. In East Africa, stable isotope data from pedogenic carbonate and fossil tooth enamel suggest a first appearance between 15-10 Ma and subsequent expansion during the Plio-Pleistocene. The fossil enamel record has the potential to provide detailed information about the rates of dietary adaptation to this new resource among different herbivore lineages. We present carbon isotope data from 452 fossil teeth that record differential rates of diet change from C(3) to mixed C(3)/C(4) or C(4) diets among East African herbivore families at seven different time periods during the Late Miocene to the Pliocene (9.9-3.2 Ma). Significant amounts of C(4) grasses were present in equid diets beginning at 9.9 Ma and in rhinocerotid diets by 9.6 Ma, although there is no isotopic evidence for expansive C(4) grasslands in this part of the Late Miocene. Bovids and hippopotamids followed suit with individuals that had C(4)-dominated (>65%) diets by 7.4 Ma. Suids adopted C(4)-dominated diets between 6.5 and 4.2 Ma. Gomphotheriids and elephantids had mostly C(3)-dominated diets through 9.3 Ma, but became dedicated C(4) grazers by 6.5 Ma. Deinotheriids and giraffids maintained a predominantly C(3) diet throughout the record. The sequence of differential diet change among herbivore lineages provides ecological insight into a key period of hominid evolution and valuable information for future studies that focus on morphological changes associated with diet change.

Fig. 1.

Map of northern Kenya showing the three fossil localities in the Suguta Valley and the Turkana Basin.

Fig. 2.

Lithostratigraphy, magnetostratigraphy, and radiometric ages from Nakali, the Samburu Hills, and Lothagam. Geomagnetic Polarity Time Scale (GPTS) changes at the break between 9.0 and 8.0 Ma. The uppermost age from the Nakali Formation (10.10 ± 0.12 Ma) is from a pyroclastic flow and essentially synchronous with the two slightly younger ages below. Modified from refs. , , and –.

Fig. 3.

Histograms of δ¹³C values of fossil enamel from all herbivore taxa by age. Note the different vertical axis for the 9.9 Ma population.

Fig. 4.

The δ¹³C value vs. age (Ma) for nine herbivore families and for all taxa. Data points represent individual samples. Lines are the 20th, 50th (dashed), and 80th percentile δ¹³C values for fossil (f) populations. White circles to the right of the fossil data for each family indicate the 20th, 50th (dashed), and 80th percentile δ¹³C values for extant (x and EXT) herbivores from Northern Kenya (32). The δ13C values from extant herbivores are corrected by +1.5‰ for recent changes in atmospheric δ¹³C values.