Neoselachian (“modern type”) sharks and rays radiated rapidly through the Jurassic and Cretaceous, with no extant clades known from the Triassic but all modern clades being present by the end of the Cretaceous. Within this period of radiation, sharks and rays adopted a wide diversity of body forms, diets and autecology. This study will investigate the role played by niche separation between taxa, and dispersal into new niches, in aiding this dramatic radiation. The study will focus on two time periods where fossils are abundant and widespread; the Middle-Late Jurassic and the “mid” Cretaceous. Data will be obtained from museum collections and also from extensive sampling of rocks in the UK and Morocco. Environmental data and species occurrence will allow ecological specificity of taxa to be assessed, whilst studying taxa from different water masses will allow constraints due to water temperature and nutrient levels to be investigated, as well as the possible diachroneity of expansion into different palaeoenvironments in different water masses. Rocks known to represent brackish and freshwater environments will be sampled to assess ecological expansion into non marine environments.