Early evolution of trophic disparity in Pachycormidae (Actinopterygii)

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Pachycormidae is a lineage of bony fishes best known for the evolution of gigantic suspension feeders, but also comprising small- to mid-sized fish- and squid-eating species. First recorded from the Early Jurassic of Europe, pachycormids are characterized by diversity in body shape, size, and diet, with analyses recovering two key evolutionary radiations: a group comprising the giant suspension-feeders, and a macrocarnivorous (‘toothed’) lineage. The proposed project aims to examine the generation of species and ecological (dietary) diversity early in the evolutionary history of Pachycormidae. The proposed project will seek to answer the following question: Is the early burst of species-level diversity in Pachycormidae associated with concomitant dietary diversity? The question will be addressed from two complementary angles. (1) An ecological perspective (a detailed study of dietary divergence in the earliest pachycormids); and (2) an evolutionary perspective (diversity in tooth- and jaw shape over deep time in an explicitly evolutionary context). The combination of these two perspectives will address the relationship between diversity in species and diet over both geologically short and long timescales to give a refined picture of the scale on which the results apply. Pachycormids are already rather diverse by their first appear in the fossil record, in the Early Jurassic Posidonienschiefer Formation of Germany. However, dietary divergence in these early forms is less well-characterized, and at the most basic level, the relationship between tooth- and jaw morphology and dietary ecology has not been robustly demonstrated. The first project objective will be to test the assumption that morphology relates to diet by comparing shape with preserved gastrointestinal contents. Secondly, the project will address how variation in tooth and jaw shape changes over a geologically short interval by analyzing diversity in shape. This will be executed at the specimen level, to minimize potentially confounding influences of shape and dietary variation within species vs. between species. The project will go on to examine the evolutionary implications of the relationship between anatomy and diet. To do so will involve the generation of a robust species-level phylogenetic hypothesis for Pachycormidae. This more detailed phylogenetic hypothesis will provide a framework with which to analyze evolutionary rates and evolutionary divergence in tooth and jaw shape over time in Pachycormidae. The objective is to test whether rates of morphological evolution are highest early in the history of the group, and to assess whether ecomorphological divergence follows a similarly explosive trajectory or increases gradually over time.