Quantifying mental foramen position in extant hominoids and Australopithecus: implications for its use in studies of human evolution.

The location of the mental foramen on the mandibular corpus has figured prominently in debates concerning the taxonomy of fossil hominins and Gorilla gorilla. In this study we quantify the antero/posterior (A/P) position of the mental foramen across great apes, modern humans and Australopithecus. Contrary to most qualitative assessments, we find significant differences between some extant hominoid species in mental foramen A/P position supporting its potential usefulness as a character for taxonomic and phylogenetic analyses of fossil hominoids. Gorilla gorilla, particularly the eastern subspecies, with a comparatively longer dental arcade and fossil and extant hominins with reduced canines and incisors tend to exhibit more anteriorly positioned mental foramina. Conversely, Pan troglodytes exhibits more posteriorly positioned mental foramina. Variation in this character among Gorilla gorilla subspecies supports recent taxonomic assessments that separate eastern and western populations. In all taxa other than Pan troglodytes the A/P position of the mental foramen is positively allometric with respect to dental arcade length. Thus, within each of these species, specimens with longer dental arcades tend to have more posteriorly positioned mental foramina. Those species with greater sexual dimorphism in canine size and dental arcade length (i.e., Gorilla gorilla and Pongo pygmaeus) exhibit more extreme differences between smaller and larger individuals. Moreover, among great apes those individuals with greater anterior convergence of the dental arcade tend to exhibit more posteriorly positioned mental foramina. Dental arcade length, canine crown area and anterior convergence are all significantly associated with mental foramen A/P position, suggesting that these traits may influence taxonomic variation in the A/P position of the mental foramen.