"Hesperornis" hunted in the waters of such contemporary shelf seas as the North American Inland Sea, the Turgai Strait and the prehistoric North Sea, which then were subtropical to tropical waters, much warmer than today. They probably fed mainly on fish, maybe also crustaceans, cephalopods and mollusks, as do the diving seabirds of today. Their teeth were helpful in dealing with slippery or hard-shelled prey.