Hesperornis had a sleek, feathered body and long legs with webbed feet. Its wings were small, used for steering when diving underwater. Its long jaws had many small sharp teeth. It was a marine predator, eating fish, ammonites and belemnites.
It lived in warm seas, coming ashore only to breed. Ungainly and vulnerable on land - Hesperornis couldn't walk - they crowded together in colonies for safety and chose inaccessible rocky outcrops.
Hesperornis spent most of its time floating on the sea surface. It travelled long distances by a combination of swimming and drifting. Hesperornis was a speedy swimmer, taking short dives to feed on shoals of fish or other passing food. Its legs could not support its weight so on land it had to push itself along on its belly. Unable to fly or walk, Hesperornis needed to be wary of predators: sharks and plesiosaurs at sea, dinosaurs and pterosaurs on land.