 Thus, they suggested that the diversification of ichthyosauriforms was partially driven by the evolution of hard-shelled prey.However, hupehsuchians and nasorostrans ultimately went extinct at the boundary between the Early and Middle Triassic, producing a "taxonomic bottleneck".At the boundary, sea level changes and volcanism led to poorly oxygenated oceans, producing a characteristic carbon isotope signature from decaying organic material in rock strata at the boundary.