Shonisaurus lived during the late Triassic period and was found in British Columbia. Its diet consisted mainly of fish and cephalopods. It had a long pointed mouth that contained teeth only at the front end. It had a whale-like body, and its flippers were much longer, and narrower than in other ichthyosaurs. All of these features suggest that Shonisaurus may be a relatively specialised offshoot of the main ichthyosaur evolutionary line.

It is the largest genus of ichthyosaur that has yet been found. The better known S. popularis species was around 15 metres (49 ft) long, but more recently discovered species, S. sikkanniensis, was longer still, at 21 metres (69 ft).