Incisor heteromorphy within anthropoids and its significance to the Sivapithecus - Pongo Clade

Abstract

Incisor heteromorphy, the size of l(^1) relative to I(^2), has been considered a synapomorphy of the Sivapithecus - Pongo clade. Recent examinations of this condition within the fossil record, however, indicate the presence of this character within a number of fossil hominoid species. Dryopithecus, Ankarapithecus and Ouranopithecus, for example, appear to retain some form of this condition. No research, to date, has attempted to clarify the frequency of incisor heteromorphy within fossil hominoids and extant anthropoids. The purpose of this research is to examine incisor heteromorphy, in the wider context of anthropoids to shed light on the phylogenetic and functional importance of this characteristic. In total, 383 individual sets of measurements, from 24 anthropoid genera were analyzed. To discern the phylogenetic usefulness of incisor heteromorphy it was coded and then optimized through existing cladograms. To examine the functional aspects of incisor heteromorphy this character was correlated with dietary consumption percentages using standard regression techniques and independent contrasts. The analysis of incisor heteromorphy indicates widespread presence of this characteristic within extant anthropoids. Other than moderate / high incisor heteromorphy appearing to be the primitive condition for anthropoids, this characteristic is of little phylogenetic use. The phylogenetic analysis indicates a large number of anthropoid genera independently acquired this condition after the last common ancestor. Widespread homoplasy indicates the susceptibility of this characteristic to the immediate environment. The analysis of diet supports this fact. It appears that frugivoiy is the main force behind the adaptation of incisor heteromorphy. Frugivores, in general, have a higher degree of heteromorphy than folivores. Within frugivores the broader and more abrasive the diet, the more incisal preparation hence the higher the degree of incisor heteromorphy. These results indicate that hyper- incisor heteromorphy within anthropoids and, in particular, in Sivapithecus and Pongo, is due to frugivory rather than shared ancestry.