Ichthyosaurus. Artwork of Ichthyosaurs swimming underwater. This extinct animal was a type of aquatic dinosaur. Like dolphins, they had streamlined bodies, long snouts, powerful tails and large eyes. Ichthyosaurs emerged 240 million years ago in the Triassic era, and became extinct 90 million years ago. Similar to modern aquatic mammals, they were thought to have evolved from terrestrial animals moving back into the water. They lived entirely in the water and were viviparous (gave birth to live young).