Ichthyosaurus ( from Greek words fish and lizard ) are marine reptiles, that lived in Mesozoic Era, almost in all its time span 250 – 90 millions years ago. Ichthyosaurus reached in length 2 – 20 meters ( members of their family was the biggest marine reptiles, that lived on Earth to date ); their skin was scaleless; they were warm-blooded; and had very big eyes, this says, that ichthyosaurus hunted at night time or dived deep in Mesozoic seas. As all reptiles, they had lungs and had need to swim to surface to make air breathes. They were unable to lay eggs in seas and their body was not adapted for land walking, so they developed viviparity, like modern dolphins have. Ichthyosaurus are very similar to modern whales and dolphins in their shape and way of life, this is the result of convergent evolution, when animals of different species develop similar shapes and way of life in close living conditions. Ancestors of whales and dolphins made the same way, as ancestors of ichthyosaurus did 200 millions years before them.
For now no transitional forms between land reptiles and ichthyosaurus are found : the earliest of know already lived in seas. Previously it was thought, that they originated straight from amphibia, but later studies show, that they originated from reptiles, whose ancestors were amphibia. Ichthyosaurus was the most diverse in late Triassic Period, they occupied lots of ecological niches : from biggest marine predators to small fish hunters. In Jurassic their diversity was still high, but there were no such big species, like in previous period. Probably, this was caused by increased competition from sharks and first plesiosaurs. Previously it was thought, that in early Cretaceous Period amount of ichthyosaurus species abruptly decreased, but new studies show, that they were still diverse at that time. And only at the middle of Cretaceous they went extinct from Earth’s surface.
Ichthyosaurus had 4 limbs with big amount of phalanges, that were completely transformed into fins. Many species had back and tail fins, that were not present in any form in their ancestors – reptiles. Earlier scientists reconstructed ichthyosaurus without back and tail spins, because there are no bones, that support them. But later fossils were found, which contain prints of fins and animals were reconstructed from that time with these parts of bodies. Ichthyosaurus had circle of bone plates in each eye, that protected eyes from high pressure of deep waters. They had the biggest eyes from all vertebrates in both relative and absolute meanings. Their hearing was weakly developed, but smelling was advanced and, probably, they had some kind of bio electricity sensors on surface of their skin. Earlier it was thought, that their skin had colors, like modern dolphins have : dark on top and light at bottom to camouflage them closer to water surface; but new studies show, that they had dark pigment all around surface of their skin. This camouflaged them in deep waters and helped to perform thermoregulation. First fossilized bones of ichthyosaurus were found at the end of 18th century and during 19th and 20th centuries amount of found fossilized bones increased each year.