Growth allometry and dental topography in Upper Triassic conodonts supports trophic differentiation and molar-like element function

Abstract

AbstractConodont dental elements are distinguished by their high disparity and rapid morphological evolution. P1 elements located in the pharynx are the most rapidly evolving, but their function in the animal has been only investigated in a handful of taxa and proposed to be analogous to mammal molars. This hypothesis predicts that their surface area should show positive allometry with respect to element length, as has been previously identified in 2D projections in two Carboniferous taxa. Here we apply the same method to test this hypothesis in 3D models of platform-bearing P1 elements of two common Late Triassic taxa, Metapolygnathus communisti and Epigondolella rigoi. We further hypothesise that these commonly co-occurring taxa differed in their growth allometry, reflecting their different trophic niches. Platform length grew isometrically with respect to element length, whereas log-transformed platform area showed positive allometry with respect to element length, with slopes equal 3.86 in M. communisti and 4.16 in E. rigoi, supporting a function of the platform analogous to molars and trophic differentiation. We cross-tested the latter interpretation by dental topographic analysis using Dirichlet Normal Energy (DNE). Specimens of the adult growth stage of E. rigoi showed higher DNE values than specimens of the same growth stage in M. communisti, consistent with stronger positive allometry of platform surface and with a higher demand for energy in this species. DNE values of platform surface increased linearly in function of element length and log-transformed platform area, indicating no ontogenetic changes. Based on DNE values available for primates, those of the adult growth stages were similar to those reported for insectivores or folivores in the case of E. rigoi and for folivores or omnivores in the case of M. communisti. Previous studies applying morphological and ultrastructural proxies for the dietary position of conodonts addressed mostly stratigraphically older conodont taxa, but our results indicate that Late Triassic species occupied the predator/scavenger niche in spite of the highly developed diversity of gnathostomes in this niche. We also show that within this broad niche, co-occurring taxa differed in their diets, which supports trophic diversification as an important driver of the remarkable disparity of their elements.