Ardipithecus fossils found in late Miocene and early Pliocene deposits in the Afar region of Ethiopia, along with Sahelanthropus tchadensis from Chad and Orrorin tugenensis from Kenya, are among the earliest known human ancestors and are considered to be the predecessors to the subsequent australopithecines ( Australopithecus anamensis and Australopithecus afarensis ). Current paleoenvironmental reconstructions suggest a wooded habitat for both Ardipithecus kadabba and Ardipithecus ramidus but more open and varied environments for other hominids living in Africa during the late Miocene and early Pliocene. To further evaluate the environmental context of Ardipithecus , we present stable carbon isotope data of 182 fossil herbivore teeth from Ardipithecus -bearing fossil deposits in the Gona Paleoanthropological Research Project area, in the Afar region of Ethiopia. The sampled teeth include representatives of all major fossil herbivore taxa and the majority of the mammalian biomass that lived in the same time and place as the hominids. When compared to extant herbivores from East Africa, the spectra of isotopic results from herbivores found in late Miocene Ar. kadabba and early Pliocene Ar. ramidus sites at Gona are most similar to isotopic values from extant herbivores living in bushland and grassland regions and dissimilar to those from herbivores living in closed-canopy forests, montane forests, and high-elevation grasslands. The tooth enamel isotopic data from fossil herbivores make it clear that Ardipithecus at Gona lived among a guild of animals whose diet was dominated by C 4 grass, and where there is no record of closed-canopy vegetation.

Abstract The Fremont River drainage basin has a variety of geologic and geomorphic features that provide insight into the long-term landscape development of the catchment. Volcanic rocks that are ca. 26 to 4 Ma and are offset by Basin-and-Range style normal faulting underlie the western third of the drainage basin. Fish Lake Hightop and Boulder Mountain show evidence of Pleistocene glaciation. Recent mapping and surface exposure dating suggests that the glacial deposits around these two mountains were deposited during the last glacial maximum (LGM). Mass movement and fluvial deposits in the catchment are predominantly composed of volcanic boulders derived from the volcanic rocks atop Boulder and Thousand Lakes Mountains. Fremont River and tributary incision rates estimated from surface exposure dating of these deposits range from 0.20 to 0.43 m/k.y. Longer-term estimates of exhumation rates in the drainage basin based on emplacement depths of igneous rocks range from 0.10 to 0.38 km/m.y.