Functional and phylogenetic implications of molar flare variation in Miocene hominoids

Comparative analyses of molar shape figure prominently in Miocene hominoid evolutionary studies, and incomplete understanding of functional and phylogenetic influences on molar shape variation can have direct consequences for the interpretation of fossil taxa. Molar flare is a shape trait whose polarity, phylogenetic distribution, and functional significance have been sources of contention. To clarify the determinants of molar flare variation in the hominoid radiation, a combination of statistical methods was employed to investigate the effects of diet, phylogeny, and geologic age upon several measures of molar shape, to identify interactions among these factors, and to estimate their relative influence. Classic indices of molar crown shape and cusp relief are highly significantly associated with diet and show no clear phylogenetic or temporal patterning. Correlations with diet are insignificant when phylogenetic effects are controlled, a result which is interpreted as an artifact of the distribution of folivory in the Miocene hominoid radiation. Possession of pronounced molar flare was found to be the primitive condition for Miocene hominoids, but molar flare reduction cannot be considered a crown hominoid synapomorphy. Molar flare is strongly correlated with geologic age but differs significantly among dietary categories when the effects of time are controlled. Among contemporaneous taxa, hard-object feeders consistently show the highest levels of flare. Molar flare reduction is hypothesized to arise from realignment of cusp positions to maximize molar shearing and increase working occlusal surface area, while variation in flare among contemporaneous taxa may be due, at least in part, to enamel thickness variation. The pronounced molar flare of Otavipithecus is interpreted as a primitive retention, although alternative dietary and phylogenetic interpretations cannot be excluded. A dramatic reversal of molar flare reduction in Mio-Pliocene hominins is interpreted as a synapomorphy of the crown hominin clade, thus supporting the hominin status of the Lukeino hominine. The last common ancestor of the Pan-Homo clade is predicted to have possessed relatively non-flaring molars, and implications of this hypothesis for early hominin recognition are discussed.