Artwork of an ichthyosaur hunting ammonites. Ichthyosaurs, such as ichthyosaursus, shastasaurus and ophthalmosaurus, were among the earliest marine reptiles, first appearing around 250 million years ago and living throughout most of the Mesozoic era. They coexisted with ammonites, which appeared even earlier, and may have preyed upon them. Both groups died out at or towards the end of the Cretaceous period.