MicroCT-scans of fossil micromammal teeth: re-defining hypsodonty and enamel proportion using true volume

Abstract Both hypsodonty and proportion of enamel are important measures for reconstructing diets and environments of fossil mammals. Classically, the first is calculated using crude dimensions and the second using specific cross-sections. With the increased availability of three-dimensional imaging techniques such as (micro)CT scanning, an upgrade towards new indices using actual 3D volumes is highly appropriate. Here we present examples from fossil small mammals to illustrate a straightforward and objective protocol to calculate new volume-based indices. Both hypsodonty and enamel proportion are defined in a consistent way with regard to orientation, and both are robust against damage or loss of dentine. Whereas hypsodonty values in the studied rodents are reduced by more than one third with regard to the older methods, they are lowered by more than a factor two in taxa with a very strong dental relief, such as insectivores. Thus, relative positions of taxa on the continuum between animal and plant consumers change, by using actual dental volume and mean height instead of maximum height. Although enamel proportion and hypsodonty are expected to be positively correlated across rodents in general, the two parameters may easily get decoupled, for instance when thick enamel is needed to break down hard but high-nutrition food items such as seeds or nuts, or when thin enamel blades are needed to cut low-nutrition items, such as grass leaves.