Variability at the carpometacarpal and midcarpal joints involving the fourth metacarpal, hamate, and lunate in catarrhini.

Abstract

Intraspecies variability is investigated in two regions of the wrist, for the purpose of determining whether patterns may be discerned in the variability that may be compared in the functional and phylogenetic analysis of living and fossil catarrhines. In the midcarpal joint region, two lunatohamate configuration patterns are found, and at the fourth carpometacarpal joint four types of configuration are identified. These two sites previously were reported to show almost continuous variability in humans, thus precluding comparison with other species. The different types of configuration in our study are delineated on the basis of their relation to differences in joint function. At the lunatohamate site there is a strong tendency in each species examined for one type to dominate in frequency. At the fourth carpometacarpal joint there is a tendency for one type or for two related types to predominate in each species. The chimpanzee sample exhibits the least variability of all species studied in joint configuration at the two sites. Australopithecus afarensis has a combination of joint types in these regions likely to be found today in only a small percentage of living Hominoidea. We conclude that patterns may be discerned at some joints in what was formerly considered to be a continuum of variation. Since these patterns (joint types) differ in their relative frequencies among living species, the frequency differences may be useful as a guide to the reconstruction of phylogenetic relationships and of potential wrist functions in fossil species.