English: 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 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. 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. Plesiosauria is an order of Mesozoic marine reptiles. Plesiosaurs first appeared in the Early Jurassic Period and became especially common during the Jurassic Period, thriving until the Cretaceous-Paleogene extinction at the end of the Cretaceous Period. The typical plesiosaur had a broad body and a short tail. They retained their ancestral two pairs of limbs, which evolved into large flippers. All plesiosaurs had four paddle-shaped flipper limbs.