Morphology, diet, and stable carbon isotopes: On the diet of Theropithecus and some limits of uniformitarianism in paleoecology.

Geladas were long supposed to be the only living primates feeding almost entirely on graminoids and accordingly display dramatic dental and manual adaptive traits. A recent study of Theropithecus gelada, the first in a relatively undisturbed habitat, revealed a more diverse diet, also incorporating large quantities of forbs. The peculiar adaptive traits of T. gelada are also observed in extinct Theropithecus as early as 3.7 Ma. Stable carbon isotopic data of extinct Theropithecus from eastern Africa indicate that specimens older than 3 Ma consumed a significant proportion of C3 plants (on average ca. 40% of total food intake) whereas specimens younger than 2 Ma consumed more C4 plants (on average ca. 80%). Recent paleobotanical evidence suggests that C3 herbaceous plants were still present in non-negligible proportions in Plio-Pleistocene lowland tropical ecosystems. Together, the shared morphological adaptive traits of extant and extinct Theropithecus and the varied diets of extant T. gelada suggest that the paleodiets of Theropithecus may have been dominated by herbaceous plants, comprising both C3 forbs and graminoids and C4 graminoids. The changes in stable carbon isotopes could correspond to a replacement of C3 plants by C4 plants within the herbaceous strata rather than a shift from C3 woody vegetation to C4 graminoids. This synthesis highlights the need for a more exhaustive knowledge of the ecology of extant species to achieve meaningful paleodietary and paleoenvironmental reconstructions. A strong selectivity for food resources that are rare in the landscapes (as in T. gelada) should also be considered when interpreting stable carbon isotopes of extinct African mammals (and notably hominids).