1Institute of Human Origins and School of Human Evolution and Social Change, Arizona State University, 2The University Museum, University of Tokyo, 3Rift Valley Research Service, Addis Ababa, 4Human Evolution Research Center and Department of Integrative Biology, University of California, Berkeley, 5Dept. of Anatomy, Sackler School of Medicine, Tel Aviv University
Thursday 2:00-2:15, Ballroom C
The early Pliocene African primate Ardipithecus ramidus was diagnosed as a hominin/d based on non-honing canines, relatively foreshortened cranial base, and postcranial characters related to facultative bipedality. However, postcranial traits related to arboreal quadrupedality, such as a grasping hallux and elongated ischium, have raised suspicions that this taxon instead exemplifies putative widespread homoplastic evolution of humanlike traits among apes around the time of the chimpanzee-human split.
Because the basicranium has undergone profound structural change during human evolution, we examined this region of the Ar. ramidus cranium (ARA-VP 1/500) for additional clues to its phylogenetic position with reference to African apes, humans, and Australopithecus. Besides the relatively anterior foramen magnum, humans differ from apes in the lateral shift of the carotid foramina and mediolateral abbreviation of the tympanic elements, which reflect a relative broadening of the central basicranium, a derived condition among catarrhines. These differences underlie changes in shape of the tympanic and the extent of its contact with the petrous. Ar. ramidus shares with Australopithecus each of these humanlike modifications.
We used the preserved morphology of ARA-VP 1/500 to estimate the missing basicranial length, drawing on consistent proportional relationships in apes and humans. Ar. ramidus is confirmed to have a relatively short basicranium. A short, broad cranial base with an anterior foramen magnum is otherwise found exclusively in Homo and Australopithecus among catarrhines, warranting identification of Ar. ramidus as a hominin/d. Reorganization of the cranial base is among the earliest morphological markers of this clade.