A large, flightless bird of the Cretaceous period that swam in oceans and caught fish with a tooth-lined bill. It had small wings close to the body; other than helping the bird navigate the water, they were of little use. Hesperornis relied more on powerful hind legs and webbed feet to chase prey and evade predators. It's believed that its flattened tail helped it change depth and direction underwater. It had a shape similar to that of modern grebes.
Although graceful at sea, the bird was clumsy on land and apparently only ventured there to breed and lay eggs.