General build Ichthyosaur

cgi restoration of ichthyosaurus communis

while earliest known members of ichthyosaur lineage more eel-like in build, later ichthyosaurs resembled more typical fishes or dolphins, having porpoise-like head short neck , long snout. ichthyosaur fore- , hindlimbs had been transformed flippers. species had fin on backs , more or less vertical fin @ rear of rather short tail. although ichthyosaurs looked fish, not.

examples of distinct features shared both dolphins , derived ichthyopterygians

evolutionary biologist stephen jay gould said ichthyosaur favourite example of convergent evolution, similarities of structure analogous, not homologous, not caused common descent, similar adaptation identical environment:

sea-going reptile terrestrial ancestors converged on fishes evolved dorsal fin , tail in right place , right hydrological design. these structures more remarkable because evolved nothing—the ancestral terrestrial reptile had no hump on or blade on tail serve precursor.