Large aquatic reptile that lived in Mesozoic times and was particularly abundant in the Jurassic period. Fishlike in appearance, averaging 2–3 m/6.5–10 ft in length, with a large caudal (tail) fin, its limbs were highly adapted for use as steering paddles. Ichthyosaurs were carnivorous, air-breathing, and apparently viviparous (the young developed in the mother before birth) as the skeletons of young have been discovered inside the body of a large fossilized individual.
The vertebrae were short and deeply biconcave, making the backbone very flexible. The caudal vertebrae continued into the lower lobe of the sharklike double tail-fin. The skull had a long beak bearing numerous conical teeth, the external nostrils were far back, and the large eyes were ringed with sclerotic plates. Earlier Triassic ichthyosaurs were more primitive, with a straighter, smaller tail, less specialized paddles, and a shorter snout.
A 200-million-year old fossil comprising the end of the body and tail was found 1995 in Lyme Regis, England. Unusually, it contained petrified traces of soft tissues, including the outline of the tail fin. This discovery enabled palaeontologists to confirm that early in the Jurassic period ichthyosaurs had developed a fishlike tail, with many extra finger-joints and sometimes extra digits.
In 2004 Norwegian scientists discovered the fossil of a new species of ichthyosaur. The 5-m/16-ft-long fossilized remains of an unknown species of ichthyosaur were found among a group of remains dating back to the Jurassic period on the island of Svalbard. No previous remains of an ichthyosaur had ever been found in this region or dated from this part of the Jurassic period.