Stenopterygius (Jurassic)

Stenopterygius is a small 3-metre ichthyosaur which inhabited Europe salt-water basins 183-179 million years ago. The name means "narrow fin". Their diet included fish and mussels.
The Stenopterygius looked similar to sharks and dolphins, had a beaked skull and jaws packed with sharp teeth. Limbs which over time evolved into fins, one caudal and the other dorsal triangle. David Martill in 1987 questioned the existence of a dorsal diving organ.
One of the earliest recovered Stenopterygius fossils had a fin like imprint on its 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 a dorsal fin as a rudder.
Being a viviparous reptile, it delivered its offspring into the water and, like cetaceans, a tail came first for spawns did not drown.