Date Published: May 6, 2019
Publisher: Public Library of Science
Author(s): Michael A. Berthaume, Julia Winchester, Kornelius Kupczik, Alistair Robert Evans.
Dental topography is a widely used method for quantifying dental morphology and inferring dietary ecology in animals. Differences in methodology have brought into question the comparability of different studies. Using primate mandibular second molars, we investigated the effects of mesh preparation parameters smoothing, cropping, and triangle count/mesh resolution (herein, resolution) on six topographic variables (Dirichlet normal energy, DNE; orientation patch count rotated, OPCR; relief index, RFI; ambient occlusion, portion de ciel visible, PCV; enamel surface area, SA; tooth size) to determine the effects of smoothing, cropping, and triangle count/resolution on topographic values and the relationship between these values and diet. All topographic metrics are sensitive to smoothing, cropping method, and triangle count/resolution. In general, smoothing decreased DNE, OPCR, RFI, and SA, increased PCV, and had no predictable effect on tooth size. Relative to the basin cut off (BCO) cropping method, the entire enamel cap (EEC) method increased RFI, SA, and size, and had no predictable effect on DNE and OPCR. Smoothing and cropping affected DNE/OPCR and surfaces with low triangle counts more than other metrics and surfaces with high triangle counts. There was a positive correlation between DNE/OPCR and triangle count/resolution, and the rate of increase was weakly correlated to diet. PCV tended to converge or decrease with increases in triangle count/resolution, and RFI, SA, and size converged. Finally, there appears to be no optimal triangle count or resolution for predicting diet from this sample, and constant triangle count appeared to perform better than constant resolution for predicting diet.
Dental ecology, the study of interactions between an organism’s teeth and its environment, provides a link between teeth, diet, and behavior . As teeth are the most common element of the fossil record, dental morphology has been a primary way of inferring ecology and behavior of extinct mammals (e.g., [2–4]). Advances in scanning methodologies and computer technologies have led to the development of new methods for quantifying dental morphology to better understand dental ecology, with a focus on correlating postcanine mammalian tooth shape and diet [5,6].
Our sample consisted of 209 primate lower second molars, representing two paraphyletic groups (Prosimii and Platyrrhini), 35 genera, 54 species, and five diets. Unsmoothed surface files of the teeth were downloaded from http://morphosource.org , and were previously used in other dental topographic studies [20,21,31,40] (see Supplementary Information S1 Table for sample list). Information concerning the acquisition and production of the surface files can be found in  and . The dietary classifications used in this study were taken from [20,21,31], and can be found in S1 Table, with two changes. Galago alleni was coded as an omnivore in  but was coded with the other Galagos as an insectivore in this study, and Nycticebus javanicus, which was coded as unknown in  was coded with the other Nycticebus as an omnivore in this study.
Raw data can be found in S1 Table. Reducing triangle count and lowering resolution can cause drastic changes in the digital representation of the tooth. These effects are exacerbated by smoothing, particularly at lower resolutions (Fig 1). Small teeth are more affected by smoothing, for a constant resolution, than larger teeth, because they are made up of fewer triangles. Smoothing appears to affect large and small teeth equally when triangle count is held constant.
All topographic metrics are sensitive to smoothing, cropping method, and triangle count/resolution. In general, smoothing causes DNE, OPCR, RFI, and SA to decrease, PCV to increase, and is just as likely to cause size to increase or decrease. Relative to the BCO cropping method, EEC causes RFI, SA, and size to increase, and is just as likely to cause DNE and OPCR to increase or decrease, although DNE is slightly more likely to increase. Topographic metrics are more sensitive to smoothing and cropping at low triangle counts compared to high. There is a positive, logarithmic correlation between DNE and OPCR and triangle count/resolution, and the slope/intersection of the regressions between the natural log of DNE/OPCR and triangle count/resolution is no better at predicting diet than topographic values at a constant triangle count/resolution. PCV tends to converge to a constant value or decrease with increases in triangle count/resolution, and RFI, SA, and size converge to a constant value as triangle count/resolution increases. When resolution is held constant, topographic variables are more correlated to each other than when triangle count is held constant. There appears to be no optimal triangle count or resolution for predicting diet, and diet appears to be more correlated to topographic metrics when triangle count, and not resolution, is held constant.