Sauropterygians were a highly diversified lineage of marine reptiles contemporary with the ichthyosaurs and later with the mosasaurs.  Early sauropterygians first appear at the Olenekian-Anisian boundary some 245 million years ago, about the same time as the ichthyosaurs' sudden origin.  At their first appearance, the sauropterygians were already diversified into several genera, including Cymatosaurus, Dactylosaurus, Proneusticosaurus, Nothosaurus, Placodus, Hemilopas, Saurosphargis, as well as indeterminate members of the family Pachypleurosauridae.[1]  Both they and the ichthyosaurs are "very distinct from any putative ancestor when they first appear in the fossil record."[2] 

     They continued to gradually diverge into several families, genera, and species.[3]  In one example, the divergence of the long-necked plesiosaurs and the short-necked pliosaurs is evidenced by intermediate teeth from the Lyme Regis formation, which suggest that a gradual divergence between the two forms occurred in the early Jurassic.[4]  It also appears that primitive sauropterygians, unlike the ichthyosaurs, were not capable of trans-Pacific migration until sometime after the Carnian, at least 30 million years or more after their debut.[5]  Some of the sauropterygians, such as the nothosaurs, make good intermediaries between land and sea creatures, for they possessed longer limbs with foot-like paddles.

     A common myth of evolution is that every organism is "perfectly adapted to its environment."  The truth is, not every organism is so perfectly adapted.  In the case of the sauropterygians, a critical adaptive deficiency existed, which they never evolved to overcome, despite their long tenure of 180 million years on the planet.  They suffered from decompression syndrome, which is the deterioration of bones due to excess nitrogen in the blood caused by diving deep under water.  Unlike sauropterygians, whales have evolved cardiovascular adaptations to successfully avoid this problem.[6]  Whales have accomplished this in less than 50 million years.  Yet the sauropterygians could not accomplish a similar adaption, even though they lived in the ocean more than three times longer than the whales.  It is a curious feature of evolution that these great beasts of the deep were able to develop all the necessary equipment for life in the water within a brief period, yet were unable to find a solution to decompression syndrome in 180 million years.  Apparently, there are limits to how far natural selection can take a form.