Playing by their own rules: clade-specific trends in mandibular form and function disrupt current paradigms of early tetrapod evolution

Abstract

Quantitative research into the evolution of vertebrate feeding has largely focused on the mandible, using morphofunctional changes in the lower jaw as an indication of broader shifts in overall feeding mode. In the context of major ecomorphological transformations, such as the origin of tetrapods, the paradigm of a trade-off between "fast-jawed", aquatic suction-feeders versus "strong-jawed", terrestrial biters is particularly prominent. However, the assumption underlying this perspective--that the mandible constitutes a reliable proxy for overall feeding mechanisms--has yet to be rigorously validated. Here, we test this widespread notion by examining both morphology (via geometric morphometrics) and function (via commonly-used functional metrics) across a broad dataset of extant and extinct vertebrate mandibles. We find that extant clades exhibit phylogenetically-structured morphofunctional solutions to the challenge of feeding in different environments, distinct from the trends exhibited by extinct taxa, thus highlighting the need for caution when relying on mandibles for deep-time reconstructions of feeding mode. By providing the first large-scale validation of functional trait differences across vertebrate feeding mechanisms, our research offers the quantitative context needed to decipher broad macroevolutionary trends in vertebrate feeding, thus better informing events such as the tetrapod water-to-land transition.