Yes, as the reptiles spread out over the land, some of them returned to the water. Over a period of time, they evolved and adapted to the water again. Their legs gradually evolved back into fins and flippers; eyes adapted to seeing underwater; and bodies became streamlined for better speed in the water. In addition, they could no longer lay their eggs on land. Thus, they evolved a way of producing living young within their bodies, a process called ovoviviparous. The Ichthyosaurs, or “fish lizards,” were the most fishlike true reptiles.
Though it looks much like modern fish do today, the Ichthyosaur was still a reptile, and one of the first true reptiles to live in the water exclusively (iStock).