HOW TO TUNA FISH: DRIVERS OF DIVERSITY IN PELAGIARIA (TUNAS, MACKERELS AND THEIR KIN)

Teleost fishes are the dominant group of living vertebrates, with over 30,000 species assigned to more than 50 orders, displaying an astounding diversity of body shapes, skull and jaw morphologies. Key innovations in these anatomical systems allowed teleosts to successfully diversify to inhabit all types of aquatic environments. Understanding how this diversity evolved from the first appearance of teleosts in the geological record (Triassic, ~250 million years ago) is problematic, in part because of the almost unmanageable scope of this question, involving what constitutes half of all living vertebrates. Modularity, or the division of complex structures such as the skull into a smaller number of integrated, independent units known as modules, provides a way forward.

Modularity has been well studied in many tetrapods, particularly in mammals but also birds, squamates (amniotes), and amphibians. Several studies have identified that tetrapod skulls are highly modular, although there are differences among the major clades in both the precise organisation of modules and their relationship to macroevolutionary patterns. However, whether the division of skull variation into distinct modules is characteristic of all vertebrates, and whether cranial modularity is an evolutionary driver of vertebrate diversity in general, is unclear. This is because modularity in the fish skull has received little attention, with most previous studies limited to the neurocranium or overall body form, limiting the comparison to tetrapods. This is despite the considerable range in morphologies exhibited by teleosts, which provides an ideal system for testing the association between cranial modularity and morphological variation. Our goal is to investigate skull modularity in the morphologically diverse but numerically tractable teleost group Pelagiaria (16 families, 80 genera, including the elongate cutlassfish, plate-shaped pomfrets and the black swallower, which can consume fishes larger than itself), for comparison to tetrapod patterns. Our project represents an important first step towards broader investigations of teleost modularity, leading to a more comprehensive understanding of drivers of vertebrate biodiversity.