Stenopterygius is a small 3-metre ichthyosaur inhabited Europe salt-water basins 183-179 million years ago. The name means "narrow fin". The feed included fish and mussels.

The Stenopterygius looked similar to sharks and dolphins, had a beaked skull and jaws packed with sharp teeth. Limbs with the time evolved to fins, one caudal and the other dorsal triangle. David Martill in 1987 questioned the dorsal diving organ existence. One of the earlies recovered Stenopterygius fossils got a fin like imprint on a back. Martill assumed the imprint to be simply a torn skin flap. As the reptile had hind limbs, the scientist stated that the dinosaur used hind limbs, instead of dorsal fin as a rudder.

Being a viviparous reptile delivered an offspring in the water and, like cetaceans, a tail came first for spawns not to drown.