Temnodontosaurus ichthyosaurs, illustration. Ichthyosaurs were marine reptiles that lived from 248 to 90 million years ago, during the Triassic, Jurassic and Cretaceous. They were carnivorous predators, and being streamlined and swift were extremely well adapted to their marine habitat. They inhabited an ecological niche that was similar to that of the present day porpoises (marine mammals). Temnodontosaurus ichthyosaurs existed around 200 to 175 million years ago in the Early Jurassic.