Ichthyosaurs were one of the first major groups of reptiles to become fully adapted to life in the ocean. Their evolutionary history begins all the way back during the Early Triassic shortly after the end-Permian mass extinction. The terrestrial ancestors of this group gradually acquired a range of aquatic adaptations, first becoming semi-aquatic and then eventually fully marine animals. Throughout the middle and late Triassic period ichthyosaurs diversified dramatically. They developed their dolphin-like streamlined bodies and became important predators in marine ecosystems. Some species, including Shonisaurus, evolved enormous sizes and ranked among the largest marine reptiles of the era, growing to be larger than some of today’s sperm whales. Some of these ancient giants may have even evolved specialized jaw structures used for filter feeding. Ichthyosaur sizes reached their peak towards the end of the Triassic period but most of the larger groups of ichthyosaurs died out during the end-Triassic extinction event. 

The animals who made it through to the Jurassic period were generally smaller and more heavily built than the large, slender ichthyosaurs of the late Triassic period. They remained a key part of the ocean food web as generalists and even large macropredators all the way through the Jurassic, although they faced increased competition from other types of marine reptiles such as plesiosaurs. Although ichthyosaur diversity declined during the Cretaceous, the group continued to be represented by advanced members of the subfamily Platypterygiinae before they disappeared completely during the early part of the late Cretaceous.