Ichthyopterygia ("fish flippers") was a designation introduced by Sir Richard Owen in 1840 to designate the Jurassic ichthyosaurs that were known at the time, but the term is now used more often for both true Ichthyosauria and their more primitive early and middle Triassic ancestors.
Basal ichthyopterygians (prior to and ancestral to true Ichthyosauria) were mostly small (a meter or less in length) with elongated bodies and long, spool-shaped vertebrae, indicating that they swam in a sinuous, eel-like manner. This allowed for quick movements and maneuverability that were advantages in shallow-water hunting. Even at this early stage, they were already very specialised animals with proper flippers, and would have been incapable of movement on land.
These animals seem to have been widely distributed around the coast of the northern half of Pangea, as they are known the Late Olenekian and Early Anisian (early part of the Triassic period) of Japan, China, Canada, and Spitsbergen (Norway). By the later part of the Middle Triassic, they were extinct, having been replaced by their descendants, the true ichthyosaurs.
Fossil remains of derived marine ichthyopterygians are known from Early Triassic rocks of Spitsbergen, Norway. These rocks are dated to just 2 million years after the Permian-Triassic extinction event, indicating that ichthyopterygians at the very least originated very early in the Triassic, before the Late Smithian crisis (a widespread ocean anoxic event that may have allowed ichthyopterygians to dominate deeper waters and temnospondyls to dominate shallow waters) and that ichthyosauromorphs as a whole originated during the Permian and were survivors of the Permian-Triassic mass extinction.
Taxonomy
Superorder Ichthyopterygia
? Genus Isfjordosaurus
Family Parvinatatoridae
Family Thaisauridae
Family Utatsusauridae
Eoichthyosauria
Order Grippidia
Order Ichthyosauria
Phylogeny
Below is a cladogram modified from Cuthbertson et al., 2013.
References
General references
Ellis, Richard, (2003) Sea Dragons - Predators of the Prehistoric Oceans. University Press of Kansas McGowan, C & Motani, R. (2003) Ichthyopterygia, Handbook of Paleoherpetology, Part 8, Verlag Dr. Friedrich Pfeil