Ichthyosaurs are the name of the Mesozoic marine reptiles which superficially resembles dolphins. However, unlike dolphins ichthyosaurs had four flippers where the hind pair was reduced in size, and their tail was directed vertically and were heterocercal but opposite to that in sharks. Ichthyosaurs originated in the Early Triassic and by the Late Triassic they dominated the worlds' oceans. This was also the time of the largest of them all; with its up to 23 meters Shonisaurus was the time equivalent of todays' large whales. In fact, adult members of the Shonisaurus genera have been shown to be lacking teeth, but if they ever evolved baleens, like some whales, is not known. In any case, most other ichthyosaurs had a diet consisting mainly of different kind of squid and fish. This is known because one sometimes can find fossilised remains of the animals' last meals in its stomach region. Other very well preserved fossils have also given us a peek into ichthyosaur reproduction, showing us that they in fact gave birth to live young and had thus made the final step in becoming totally aquatic. By the Early Jurassic ichthyosaurs had begun to diminish, and only one variety of body shape remained. And in the Cretaceous only one genus were still roaming the oceans ( Platypterygius ). After being around for an impressive 150 million years (240-90 mya) the ichthyosaurs went extinct, leaving no known living relatives.