Cymbospondylus is a primitive Triassic ichthyosaur of enormous length, approximately ten meters. It is also, due to its geneology a giant mesosaur. All ichthyosaurs and thalattosaurs are derived from mesosaurs. Cymbospondylus is one of the few ichthyosaurs to retain the long, sinuous body shape of mesosaurs.
Despite the Size Difference
Actually an order or two of magnitude larger in size, the giant ichthyosaur Cymbospondylus (Leidy 1868) kept the basic proportions of Stereosternum, but with a shorter neck and limbs transformed into flippers.
A comparison of skulls helps make the point.
The long premaxilla, the posteriorly shifted nares, the size of the supratemporal are obvious shared traits with Wumengosaurus acting as a transitional taxon. Details at reptileevolution.com.
No other taxa are closer in the large reptile tree to thalattosaurs and ichthyosaurs than mesosaurs.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Leidy J 1868. Notice of some reptilian remains from Nevada: Proceedings of the American Philosophical Society, v. 20, p. 177-178.