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

Abstract

Conodont elements have high rates of morphological evolution, but the drivers of this disparity are debated. Positive allometric relationships between dimensions of food-processing surfaces and entire P₁ elements have been used in to argue that these elements performed mechanical digestion. If involved in food processing, the surface of the element should grow at a rate proportional to the increase in energy requirements of the animal. This inference of function relies on the assumption that the energy requirements of the animal grew faster (≅ mass^0.75) than the tooth area (≅ mass^0.67). We re-evaluate this assumption based on metabolic rates across animals, and calculate the allometry in platform-bearing P₁ elements of Late Triassic co-occurring taxa, Metapolygnathus communisti and Epigondolella rigoi, using 3D models of ontogenetic series. Positive allometry is found in platform and element dimensions in both species, supporting a grasping-tooth hypothesis, based on the assumption that metabolic rate in conodonts scaled with body mass similarly to that in fish and ectoterms. We also calculate the curvature of P₁ platform surface using the Dirichlet Normal Energy (DNE) as a proxy for diet.DNE values increase with body mass, supporting the assumption that conodont metabolic rates increased faster than mass^0.67. We finally find that adults in both taxa differ in their food bases, which supports trophic diversification as an important driver of the remarkable disparity of conodont elements.