Ichthyosaurs were giant marine reptiles that resembled dolphins in a classic example of convergent evolution. They thrived during much of the Mesozoic era. During the middle Triassic Period, they evolved from as yet unidentified land reptiles that moved back into the water, in a development parallel to that of the ancestors of modern-day dolphins and whales.They were particularly abundant in the Jurassic Period, until they were replaced as the top aquatic predators by another reptilian order named plesiosaurs in the Cretaceous Period. They averaged 7-13 feet in length, with a porpoise-like head and a long, toothed snout. It has been estimated that ichthyosaurs could swim at speeds up to 25 mph. They most likely fed on fish, and a few of the larger species had heavy jaws and teeth that indicated they fed on smaller reptiles. They ranged so widely in size, and survived for so long, that they are likely to have had a wide range of prey. Although ichthyosaurs looked like fish, they were not.