Assessment of upper premolar morphological traits as reliable phylogenetic indicators

Abstract

Upper premolar size and cusp heteromorphy have been used in several studies that argue for alternative phyletic placements of early and middle Miocene fossil hominoids relative to extant primate clades. The underlying interpretation is that upper premolar enlargement relative to the first and/or second molar and a reduction in upper premolar cusp heteromorphy are characteristic of extant apes. The aim of the present study is to test the strength of the phylogenetic signal contained within these characters to determine whether they diagnose the groups of living primate taxa for which they are proposed. The hypotheses are evaluated by means of character state analysis performed on seven metric characters derived from associated upper premolar and molar data collected from seventeen extant and fifteen extinct catarrhine species. The computer programme MacCIade is used to reconstruct hypothetical ancestral nodes using the phylogenetic method of character optimization. The results indicate that there is only a very weak phylogenetic signal contained within upper premolar size and cusp heteromorphy. Both characters fail to unambiguously diagnose groups of living apes as clades. Further analyses suggest that relative upper premolar enlargement is an adaptation to hard object feeding, but the functional significance of upper premolar cusp heteromorphy remains unclear. These findings imply that phylogenetic analyses that incorporate one or both dental traits to develop a phylogenetic framework within which to place fossil taxa relative to extant anthropoids are fundamentally flawed, because neither upper premolar size nor cusp heteromorphy provide convincing evidence of common ancestry relationships.