Ichthyozaur

Ichthyosaurs are marine diapsid vertebrates, sometimes very large, which resemble current dolphins. They appeared 250 million years ago, slightly before the dinosaurs. They measured between one and ten meters in length and could reach 40 km/h without doping.
against diets of all kinds, its skin rests on a layer of insulating fat which improves its hydrodynamics and buoyancy, and therefore makes it easier to sleep in!!