The evolutionary history of Ichthyosaurus is not entirely known, but enough details have been discovered to compile a good amount of evidence of the lineage of Ichthyosaurus. Generically, because we are not too worried with exact organisms in the family line right this second, it is understood that in the late Triassic a terrestrial reptile line adapted to a semi-aquatic lifestyle over time. This semi-aquatic lifestyle slowly adapted to becoming more and more aquatic as time went by. Eventually, so well adapted to the water that terrestrial life was abandoned completely, the forebears of the Ichthyosaurus genus were fine tuning their adaptations and developing their niche in the environment. Air breathing was not abandoned, but, unlike modern marine mammals, a more fish-like propelling apparatus was formed in the Ichthyosaur tail. This probably developed from side to side tail propulsion by the ancestor reptiles. As the ability to come out on land was lost the ability to lay reptilian eggs on dry land was changed over millennia. The end result, in most marine reptiles, except the giant marine turtles, was for the amniotic egg to change and adjust over that time into an amniotic gestation within the parent which is certainly not unique in the animal kingdom either then or now; some sharks, extant reptiles, other extinct marine reptiles, and of course mammals carry fetuses internally and give birth to live young. An adaptation we have not discussed, however, is found in the ear bones of Ichthyosaurs. Many marine reptiles developed ear bones allowing for highly fluid motion which were incorporated into the skull so as to not disorient the animal underwater. However, fusion into the skull is thought to have suggested low detection rates of fish and other marine animals. Ichthyosaurus, however, had large and thick ear bones that, while suited to swimming with great agility, also allowed the Ichthyosaurs to detect prey at much longer ranges than many other marine reptiles. Their large eyes- Ophthalmosaurus, a different genus of Ichthyosaur that had probably the largest eyes of any Ichthyosaur genus- were also well suited to prey detection, and threat avoidance. Everything about Ichthyosaurs points toward, as we can see, a highly successful predator that maintained a dominance over their ecological niche for a very long time.