Relative placement of the mandibular fossa in great apes and humans

Several researchers have investigated, or commented on, the relative placement of the hominin mandibular fossa with regard to brain expansion and masticatory function. Two confounding factors are identified in this previous work. First, a number of different measurement techniques have been applied, confusing comparisons between studies. Second, the effects of squamous thickening due to temporal bone pneumatization are shown to influence measurements based relative to the ectocranial margin of the skull. To investigate the influence of these factors, a sample of adult human (n=12), chimpanzee (n=12), gorilla (n=15), and orang-utan (n=8) skulls from the Cleveland Museum of Natural History, University of Wisconsin Zoology Museum, and University of Wisconsin Anthropology collections, were CT scanned. Coronal scans were horizontally aligned and measured on a personal computer using ImageJ (NIH). To identify fossa placement, fossa breadth was measured as the projected distance in the coronal plane between the tip of the entoglenoid to lateral margin of the articular surface. A second distance, from the tip of the entoglenoid to a sagittal plane, tangent to the lateralmost margin of the endocranial surface was taken to indicate the extent of medial placement of the fossa. By eliminating the influence of pneumatization, these data unambiguously confirmed the medial placement of the human fossa and show all great apes as having a laterally placed fossa. Similar measurements on three fossil hominins, KNM-BC 1 (Homo sp. indet.), OH 5 and KNM-ER 23000 (Paranthropus boisei) demonstrate that, while all specimens demonstrate a broad fossa, only KNM-BC 1 is characterized by a relatively medial placement while the latter two display lateral placement.