Though scientists haven't yet found a true dinosaur that lived in the water, there were aquatic animals that lived at the time of the dinosaurs and are considered dinosaur-like. Some of their names are Plesiosaurus, Elasmosaurus, Macroplata, Kronosaurus and Peloneustes. These animals lived in the Mesozoic era. Other water-dwelling faux-dinosaurs were Ichthyosaurus, Temnodontosaurus, Stenopterygius and Ophthalmosaurus. These lived from the middle Triassic period to the early Cretaceous period.
Ichthyosaurus is one of the best known of these animals because archaeologists found so many of its well-preserved fossilized remains. It looked like a dolphin with a long snout full of fine teeth and was about the size of a dolphin. But though it had to breathe air and gave birth to live young, Ichthyosaurus wasn't a dolphin but a reptile. Though its hearing appeared to be well developed, this animal probably hunted by sight, for its eyes were huge.
A somewhat earlier marine reptile was Kronosaurus, which could grow to 42 feet long and had a 9 foot long skull. The skull of this animal was longer than the skull of Tyrannosaurus rex. Kronosaurus used the bite force of its jaws to overcome prey like sharks, large squid and even smaller members of its species.