The enamel-dentine junction of the upper premolars of Homo naledi and other hominins: a morphometric study.

ABSTRACT.Homo naledi is a hominin species first described in 2015 based on remains from the Dinaledi Chamber in the Rising Star cave system in South Africa. There has been much debate about the taxonomy and phylogenetic relationships of H. naledi because of its unique mix of primitive and advanced features. It has a primitive body size, chest, shoulders and hips and the brain size is small both absolutely and in relation to its body size. However, it does show more advanced features in the wrist, foot and thumb. The dentition also shows mixed features, being small, as in modern humans, but retaining certain primitive features in morphology.In this study, geometric morphometric analysis was applied to the enamel-dentine junction (EDJ) of the upper premolars of H. naledi and a comparative sample of living and fossil hominins. The aims of the study were to determine whether this technique can distinguish between the upper premolars of different hominin taxa and of tooth position within taxa and, if so, to use the method to elucidate the taxonomic relationships of Homo naledi. Additionally, the study identified qualitative morphological features of the EDJ. Their frequency of expression between species and tooth position was analyzed and the differences between group mean frequencies of expression was summarised by multidimensional scaling.The principal components of shape, derived from a Procrustes analysis, were able to differentiate well between hominin taxa and between tooth position within taxa. There is a morphological trend from Paranthropus robustus at one extreme to modern Homo sapiens at the other. This is also an allometric trend, as the size of the EDJ is correlated with the first principal component of shape. At the Paranthropus extreme, the EDJ is short and broad, especially in the bucco-lingual direction. The EDJ ridge has a square or rectangular outline with expansion of the talon in a disto-lingual direction. At the other end of the trend, the EDJ is taller and narrower, with a more triangular outline due to a flatter disto-lingual portion of the EDJ ridge. Third upper premolars differ from fourth upper premolars by having taller, more distally placed buccal dentine horns.H. naledi has a unique morphology within the genus Homo. The EDJ is small but, has retained a more primitive morphology. It has a prominent distal accessory buccal dentine horn and a short main buccal dentine horn with prominent disto-lingual extension of the talon, leading to a relatively flat and broad EDJ profile. In addition, the size of the fourth upper premolar is greater than that of the third, as is seen in Australopithecus and Paranthropus and unlike other species of Homo, where the teeth are more equal in size or the third upper premolar is larger. This autapomorphy could suggest that H. naledi was adapted to a different dietary lifestyle than other members of the genus Homo.Qualitative trait analysis identified and described some features at the EDJ for the first time in the hominin study sample. Third upper premolars of the genus Homo are associated with a notch and mesial 'bulge' in the mesial EDJ ridge together with a depression or concavity in the mesial surface of the EDJ (except for H. naledi where there is a bulge but no notch); fourth upper premolars of all hominin species have a tendency to form a distinct distal accessory buccal dentine horn, except for H. neanderthalensis, where there is a distinctive distal buccal shoulder instead; A. africanus teeth are associated with prominent ridges on the buccal surface of the tooth together with extension of the buccal cingulum and lingual cingulum but P. robustus is more strongly associated with a shelf-like lingual cingulum; third upper premolars of H. neanderthalensis have a distinctive boss-shaped bulge in the buccal cingulum. P. robustus and, to a lesser extent A. africanus, show a tendency towards molarisation, with an oblique ridge, distal accessory lingual dentine horn and talon expansion. A transverse crest is almost universally present in the EDJs of all Homo species except for H. sapiens (both fossil and modern) and in H. naledi, where there is a distinctive bucco-mesial ridge in place of the transverse crest in 43% of third upper premolars. This contrasts with the outer enamel surface (OES), where transverse crests are rarely observed in Homo species.