Paranthropus boisei: an example of evolutionary stasis?

Of the presently recognised early hominid species, Paranthropus boisei is one of the better known from the fossil record and arguably the most distinctive; the latter interpretation rests on the numbers of apparently derived characters it incorporates. The species as traditionally diagnosed is distributed across approximately one million years and is presently confined to samples from East African sites. The hypodigm has been examined for evidence of intraspecific phyletic evolution, with particular emphasis on the areas best represented in the fossil record, namely the teeth and mandible. The results of this examination of 55 mandibular and dental variables which uses the gamma test statistic for the detection of trend, and nonparametric spline regression (Loess regression) for investigating pattern and rate of temporal change, show that within Paranthropus boisei sensu stricto most evidence of temporally related morphological trends relates to the morphology of the P4 crown. There is little or no evidence of any tendency to increase in overall size through time. Fossils from the Omo Shungura Formation and West Turkana which have been recovered from a time period earlier than the P. boisei sensu stricto hypodigm resemble the latter taxon in some features, but differ from it in aspects of cranial morphology. There is insufficient fossil evidence of the earlier taxon to tell whether it changes through time, but when trends of 47 mandibular and dental variables are assessed across the combined East African "robust" australopithecine sample, there is evidence for a relatively abrupt change around 2.2-2.3 Myr in approximately 25% of the dental and mandibular remains. Some of these changes correspond with the temporal trends within P. boisei sensu stricto, but others, such as mandible height, do not. If the earlier material is ancestral to P. boisei sensu stricto, evidence from the teeth and jaws is consistent with a punctuated origin for the latter taxon.