How do palaeontological data refine our understanding of adaptive radiation and the evolution of modern biodiversity?

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Swordfishes, needle-nosed predators of the high seas; flounders, gastronomically familiar and bizarrely asymmetrical bottom dwellers; remoras, literal hangers-on that hitch rides on sharks using a suction cup on their heads: few fishes, or vertebrates, show more strikingly different anatomies or modes of life. Divergent as they are, genetic studies indicate that these fishes, as well as several others with equally curious traits, are all closely related, forming a bough in the tree of life called Carangimorpha. Cases like this, where organisms with shared ancestry branch out over time to assume divergent bodyplans and lifestyles, are known as adaptive radiations. Previous research on this topic has focused on living groups with poor fossil records, like anole lizards or cichlid fishes. However, fossils are the only direct means of timing evolutionary events, and yield unique evidence of anatomies pruned from the tree of life by extinction; as such, they are critical in understanding how modern biodiversity was assembled. Our project seeks to use this exceptional group of fishes as a laboratory to not only understand how their specialisations arose, but also explore the ways in which fossils can be especially useful for answering these questions. Specifically, we will ask: (1) what are the steps leading to the origin of peculiar carangimorph body 'designs'; (2) when in geological time did these changes occur?; and (3) what do fossils tell us about the speed and manner in which these changes took place? Fossils are critical in answering these questions. For instance, recent discoveries revealed how flatfishes evolved to have both eyes on one side of their head. Finding such transitional forms is typically rare, but the spectacular array of living carangimorphs is complemented by a trove of complete fossils. Modern preparation (involving chemical treatment or methods akin to sandblasting) and imaging (CT scanning) techniques can be used to extract fossils from surrounding rock. We will uncover details of exceptional fossils that document the early stages in the evolution of adaptations of living carangimorphs, including the rapier-like snout of billfishes and the suction disc of remoras. Fossils cannot speak for themselves and we cannot simply trace evolution by peeling back rock layers. We must discover the relationships of fossils to living species. We will combine palaeontological data with anatomy and DNA data from modern fishes to place fossils in a tree alongside living relatives, allowing us reconstruct the sequence of changes leading to specialized modern bodyplans. Including both fossils and living species is also important because fossils can influence estimated relationships among living species and vice versa. To build a timeline for major events in carangimorph evolution, we need to find out when in Earth's history each branch in its family tree split off. Fossilization is a rare event, and so even the oldest fossil of a particular branch might be a relatively late arrival. We therefore need to combine our fossil data with an indirect approach known as the molecular clock. If we know the rate at which genetic mutations build up, we can estimate how long ago living species split from each other and produce a 'time tree': a family tree with an absolute time scale. With all of these components in place, we can address important questions about how fossils impact our understanding of adaptive radiation and the evolution of biodiversity. Our time tree allows us to apply statistical tools for determining the rate at which anatomical features changed over time. This can be used to see whether change was rapid early in evolutionary history or whether it was slow and gradual, and whether rates of evolution varied between marine and freshwater environments. Critically, we will conduct our analyses with and without fossils, allowing us to decide whether studies based only on modern data might be misleading.