"S. popularis" was about 15 metres (50 ft) long. A longer specimen is now placed in "Shastasaurus" rather than "Shonisaurus". "Shonisaurus" had a long pointed mouth that contained teeth only at the front end. It had a large whale-like body, a long dolphin-like snout, 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.