An extinct group of Cretaceous toothed birds. All members were foot-propelled, seagoing birds that could reach the size of a modern Emperor penguin. The largest known hesperornithiform reached a maximum length of over 5 ft (1.5 m). All of the known fossils are from marine rocks that are Cretaceous in age (about 100–65 million years old); they are most abundant from North America, especially Kansas. Although these birds are thought to be closely related to living birds (the group or clade Neornithes)—along with Ichthyornithiformes—the evolutionary interrelationships among Hesperornithiformes are not well understood by paleontologists.