Protostylid variation in Australopithecus

Recent advances in computed tomography (CT) and genetics provide new insights into the morphology and biology of anatomical traits, particularly in the dentition. As we move towards a fuller understanding of the genetic and developmental bases for dental traits, we need to reassess the taxonomic and evolutionary variation of established characters. Quantitative genetic analyses indicate that the degree of expression of upper and lower primate cingular remnants are genetically interdependent. This has serious evolutionary implications that need to be explored for fossil hominids. Studies of Carabelli's cusp, a cingular remnant on hominid upper molars, have been advanced through both genetic and CT analyses setting the stage for such an investigation. But its mandibular morphological homologue, the protostylid has not been similarly studied. This paper represents the first step towards a quantitative understanding of the variation and evolution of this trait in early hominids. Since the first discoveries of Australopithecus specimens in South Africa more than sixty years ago, cingular features on lower molars have played a significant role in the description and comparison of hominid taxa. This largely qualitative history is reviewed. Because the modern human classification system for protostylid variation does not adequately describe the variation seen in Australopithecus samples, a quantification scheme with six expression states is established. Using this new protocol, protostylid variation in six species of Australopithecus is assessed. Results from these analyses show that the distribution of the degree of protostylid expression in these species is highly varied. When first, second, and third molar samples are considered separately, the distribution of expression states is found to differ considerably within the same species. These results provide a foundation for further genetic and developmental research on the evolutionary history of the hominid dentition.