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

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 P-1 elements have been used 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 ( approximately equal to mass(0.75)) than the tooth area ( approximately equal to mass(0.67)). We reevaluate this assumption based on metabolic rates across animals and calculate the allometry in platform-bearing P-1 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 ectotherms. We also calculate the curvature of the P-1 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.

Automated summary

The drivers of the rapid morphological evolution of conodont elements are debated, with some arguing that the elements performed mechanical digestion based on positive allometric relationships. This study reevaluates the assumption that energy requirements and tooth area grew at different rates, finding support for a grasping-tooth hypothesis. It also suggests that conodont metabolic rates increased faster than mass, and food diversification may have been an important driver of the disparity in conodont elements.