How many landmarks? Assessing the classification accuracy of Pan lower molars using a geometric morphometric analysis of the occlusal basin as seen at the enamel-dentine junction

Abstract

Previous research has demonstrated that species and subspecies of extant chimpanzees and bonobos can be distinguished on the basis of the shape of enamel-dentine junction of lower molar crowns. Thus, there is potential for fossil taxa, particularly fossil hominins, to be distinguished at similar taxonomic levels using lower molar crown morphology. New imaging techniques allow for the collection of large amounts of shape data, but it is not clear whether taxonomic distinctiveness increases with the inclusion of more and more finely detailed aspects of crown shape. We examine whether increasing the amount of shape data collected will lead to an increase in the accuracy with which enamel-dentine junction (EDJ) shape classifies Pan lower first and second molars at the species and subspecies level. Micro-computed tomography was employed to non-destructively image the EDJ and geometric morphometric analytical methods were used to compare EDJ shape among samples of Pan paniscus, Pan troglodytestroglodytes, and Pan troglodytes verus. The results of discriminant analyses using three landmark sets (number of landmarks=8, 112, and 534 landmarks and semi-landmarks, respectively) indicate a high degree of classification accuracy for each landmark set, with small increases in accuracy as the numbers of landmarks are increased. The morphological differences in EDJ shape among the taxa are subtle, but consistent, and relate to the relative height and position of the dentine horns. Thus, EDJ shape can contribute to taxonomic analyses and the more information that can be included the better.