Metric variation and species recognition in the fossil record

One of the first and most important tasks of the paleontologist is classifying specimens into species. Species recognition commonly involves sorting specimens on the basis of qualitative and quantitative similarities and differences. Often, however, variation in simple metric characters like tooth size or jaw length plays an important role in debates about whether a sample comprises a single species or more than one morphologically similar species. For example, Simpson, Roe, and Lewontin(1) suggested that a fossil sample showing a coefficient of variation greater than 10.0 was likely to comprise more than one species. Well-known controversies over species recognition in which metric sample variation has been important have simmered for years, focusing on hominids, hominoids, and other extinct primates. Some of these have been resolved; others have not. For example, Pilbeam and Zwell(2) convincingly demonstrated multiple species among South African hominids by showing that metric tooth size variation was too great to be reasonably interpreted as sexual dimorphism. But metric variation continues to play a role in debates about whether Australopithecus afarensis(3,4) and Homo habilis(5-9) each comprise a single species or two or more separate species. Similarly, there has been steady debate about the number of species present in African Proconsul. Some favor an interpretation of a single extremely dimorphic species, (10-12) while others favor an interpretation of two or more species.(13)