Hesperornithiformes Sharpe, 1899[3] Hesperornithes is an extinct and highly specialized group of aquatic avialans closely related to the ancestors of modern birds.

They inhabited both marine and freshwater habitats in the Northern Hemisphere, and include genera such as Hesperornis, Parahesperornis, Baptornis, Enaliornis, and Potamornis, all strong-swimming, predatory divers.

Although some of the smaller and more basal species, like those belonging to the subgroups Enaliornithidae and Brodavidae, might have been able to fly, the larger hesperornithids like Hesperornis and Baptornis had only vestigial wings.

As in the case of modern foot-propelled diving birds, the femur and metatarsus of these animals were short, whereas the tibia was long.

[6] Some researchers think that on land they had to slide on their bellies and push with their legs; the hip and knee joints were shaped such that these species could not move them dorsoventrally, and in a resting position the feet projected sideways from the body, which would have prevented them from walking upright.

Small hesperornithean bones are known from the freshwater deposits of the Late Cretaceous of the Judith River Group as well as the Hell Creek and Lance Formations, and in several Eurasian sites.

The group was often considered to be related to loons and grebes,[12] or to the Paleognathae (based on perceived similarities in the bony palate).