Diet-induced tooth plasticity and implications for the inference of trophic interactions of vertebrates around the Permo-Triassic boundary – PLASTICiTEETH

Multicellular life was almost wiped out at the Permo-Triassic boundary (PTB), about 250 Myr ago. At the time, large volcanic degassing from the Siberian Trapps released huge amounts of CO2 in the atmosphere and initiated a series of spectacular fluctuations of the climate and chemistry of the oceans that lasted about 5 Myr. Most organisms were severely affected and more than 80 percent of all marine animal species went extinct. On the other hand, conodonts, an extinct group of small, eel-shaped, marine vertebrates thrived. Their abundant fossil record is arguably the best of all vertebrates for this time interval, and as a consequence, conodonts have become the best tool for both biochronology and geochemical reconstruction of the palaeoenvironments of the PTB interval.
The elucidation of past biotic crises has traditionally focused on abiotic factors such as climate changes, less so on biotic factors such as community structure, but the role of food-webs in either dampening or worsening climate change effects is now attracting more and more attention. Owing to their importance in the fossil record of the PTB interval, conodonts must have played a critical role in PTB trophic networks. Yet, conodonts’ dietary habits remain unknown, thereby hampering any accurate reconstruction of the evolution and resilience of foodwebs as a response to the crisis.
Conodonts used their pharyngeal P elements to process food. These P elements evolved very rapidly and displayed an amazing morphological variety. Since extant cyclostomes, presumably the closest extant relatives of conodonts, lack an equivalent of these pharyngeal ‘teeth’, functional hypotheses of P elements and trophic interpretations of conodonts have been very limited. Furthermore, these P elements are developmentally comparable to series of tooth-like organs (such as ganoid scales or lungfish tooth plates) rather than to single teeth, which suggests that diet proxies, such as dental topography, that are often used for the comparison of molars in mammals must be applied with caution.
Although the shapes of an animal’s teeth are thought to relate to its diet, the mechanisms leading to this presumed congruence between shape and function are still poorly understood. This has led people to develop morphology-independent proxies of diet such as microwear patterns and geochemistry, but also increasingly to test functional hypotheses using biomechanical models. Similarly, techniques such as dental topography, which may provide clade-independent proxies of diet have been widely used in extinct mammals.

The main objective of the proposed interdisciplinary research is to elucidate the ecological role of conodonts in ancient trophic webs, especially around the PTB. During the Early Triassic, conodonts display patterns of evolutionary radiations that suggest recurrent adaptation to distinct dietary niches. We propose to test this hypothesis. In order to do so, we plan to apply two independent proxies of diet, namely geochemistry (the ratio of stable calcium isotopes, d44/42Ca) and dental topography, to a large set of PTB conodont morphotypes. In order to constrain our interpretations of these measurements, shark teeth of the same age will be measured too. Applying dental topography metrics to conodonts and sharks also imply to extend their current range of application from single mammalian molars to partial or full fish dentitions. In order to weigh the relative role of each individual tooth within the dentition, we will use the diet-induced plasticity of pharyngeal teeth in cichlids as a model system, and explore its mechanisms in vivo and in silico via numerical models of the role of diet-induced mechanical stresses on the size and shape of a developing tooth. This project will have strong implications for our understanding of the role of development in the dynamics of evolutionary radiations in the direct aftermath of major biotic crises.