Occipital Condyle Development in Extant Hominids and Australopithecus afarensis

Objectives

Variation in the position and orientation of the occipital condyles is well documented in hominids and has been linked to differences in positional behavior, especially upright posture in bipedal hominins. Less is known about the curvature of the condylar joint surfaces. This study examines the ontogeny of condylar curvature in extant hominids and Australopithecus afarensis.

Materials and Methods

Condylar curvature was quantified in 147 chimpanzees, gorillas, humans, orangutans, and five specimens of A. afarensis. Specimens were grouped into juvenile, adolescent, and adult dental age categories. We used resampling methods to test for differences among age groups within genera, and among genera at each developmental stage. We also recorded the timing of fusion of the occipital synchondroses to link ontogenetic changes in curvature to structurally important developmental events.

Results

Condylar curvature increases through ontogeny in all taxa. Adult Homo and Gorilla have flatter condyles than Pan and Pongo. Juvenile morphologies do not predict the adult pattern of similarities and differences. Periods of rapid change in curvature occur prior to complete fusion of the intraoccipital synchondroses in Gorilla and Pongo. A. afarensis specimens suggest a Pan-like morphology and developmental pattern.

Discussion

Taxonomic and developmental patterns of variation do not align with differences in posture and locomotion. The similarity of A. afarensis to Pan suggests that the flatter condyles of humans and gorillas are convergent. The Pan-like morphology of A. afarensis is also consistent with other evidence indicating that the cervico-occipital region remained plesiomorphic in many ways following the shift to obligate bipedality in early hominins.