Stenopterygius

Stenopterygius is an extinct genus of thunnosaur ichthyosaur known from Europe (England, France, Germany, Luxembourg and Switzerland).[1][2] Maximum length was 4 meters (13 feet).

Description
Stenopterygius was physically similar to the better known Ichthyosaurus, but had a smaller skull and narrower flippers. Beautifully preserved fossils of Stenopterygius have been found in Germany.[5] Its skull was extended into a kind of a beak and was armed with a quantity of large teeth. The limbs had been transformed to fin-like structures. The tail terminated in a large, semicircular, leathery, vertical caudal fin and even a triangular dorsal fin was present.

Paleobiology
The habits of Stenopterygius were similar to those of present-day dolphins. It spent most of its life in the open sea, where it hunted fish, cephalopods and other animals. The abdominal cavity of skeletons of this ichthyosaur often contains the remains of such food.[6][7]

One famous fossil is that of a mother and baby that died in childbirth (ichthyosaurs were viviparous). This proved that ichthyosaur infants were born tail-first, just like cetaceans, to prevent them from drowning before fully clearing the birth canal.[8]

Stenopterygius was a very fast swimmer, with a cruising speed similar to that of tuna, which is among the fastest of all living fishes.