Variation in enamel thickness and cusp area within human maxillary molars and its bearing on scaling techniques used for studies of enamel thickness between species

Abstract

Thirty-nine unworn maxillary molars ( M 1 = 18, M 2 = 12, M 3 = 9), representing 37 individuals of a Slavic population, were sectioned through the mesial cusps in a plane perpendicular to the cervical margin of the crown. Whether the range of variation in enamel cross-sectional area between individuals corresponds to the range of variation found between teeth within a species was investigated. Total tooth crown area, as determined by planimetry, was used as a scaling factor on the basis that a biologically meaningful scaling procedure should take into account both enamel thickness and total tooth crown area, rather than the size of the dentine-enamel junction. Uni- and multivariate statistics revealed that the enamel cross-sectional area of second and third molars is absolutely and relatively larger than it is in first upper molars and that it scales close to isometry with tooth crown area. Conversely, first maxillary molars, with their relatively and absolutely thinner enamel, exhibit positive allometry between enamel and tooth crown area. These patterns were retained even when differences in overall tooth size were adjusted for. Hence, the notion that there exists a ‘species-specific’ amount of enamel over the tooth crown must be reconsidered, and one should refrain from pooling different tooth types for taxonomic purposes. At least two scaling factors are needed to describe human maxillary molars alone. However, this increase in enamel cross-sectional area from anterior to posterior is not unexpected if one bears in mind that posterior teeth occupy a more advantageous position relative to the chewing muscles. It was concluded that the strong relation between enamel cross-sectional area and topography of the masticatory muscles would suggest that only teeth that occupy a comparable position within the orofacial skeleton should be used for comparative purposes. Furthermore, with regard to humans, the findings indicate that reduction in tooth size from anterior to posterior has not resulted in a concomitant loss of function.