Cranial morphology of Australopithecus afarensis: a comparative study based on a composite reconstruction of the adult skull.

The Pliocene hominid species Australopithecus afarensis is represented by cranial, dental, and mandibular remains from Hadar, Ethiopia, and Laetoli, Tanzania. These fossils provide important information about the cranial anatomy of the earliest known hominids. Because complete crania or skulls are not known, we produced a composite reconstruction of an adult male skull based on 13 specimens from the Hadar Formation. The reconstruction serves as a testable hypothesis regarding functional relationships in the A. afarensis skull and is the basis for the comparative study presented here. We examine six major aspects of cranial and mandibular anatomy. We combine our results with those of White et al. (1981) in a discussion of alternate hypotheses of early hominid phylogeny. In the cranium, jaws, and teeth A. afarensis exhibits a morphological pattern that we interpret as primitive for the Hominidae. Homo habilis retains a number of these primitive features for which A. africanus, A. robustus, and A. boisei share derived character states, particularly in the masticatory apparatus. Homo and "robust" species of Australopithecus share a suite of derived cranial base features. These shared traits may relate to upper facial orthognathium which is also common to these taxa and are probably indicative of parallelism rather than a close phylogenetic relationship. The cranial base characteristics of A.L. 333-45 do not, contrary to Olson's (1981) claims, provide evidence for an A. afarensis--"robust" Australopithecus sister group. When the range of mastoid variation in extant African pongids and A. afarensis is examined thoroughly, the Pliocene hominid appears to retain a primitive, rather than derived, morphology.