Hesperornis is an extinct genus of flightless aquatic birds that lived during the Santonian to Campanian sub-epochs of the Late Cretaceous (89-65 mya). One of the lesser known discoveries of Othniel Charles Marsh in the late 19th century Bone Wars, it was an important early find in the history of avian paleontology. Famous locations for Hesperornis are the Late Cretaceous marine limestones from Kansas and the marine shales from Canada, but the genus had probably a Holarctic distribution.
Hesperornis were very large birds, reaching up to 2 meters (6.5 feet) in length. They had virtually no wings, swimming with powerful hind legs. Their toes were probably lobed rather than being webbed, as in today's grebes; like in these, the toes could rotate well, which is necessary to decrease drag in lobed feet but not in webbed ones such as in loons, where the toes are simply folded together.
Like many other Mesozoic birds such as Ichthyornis, Hesperornis had teeth in its beak which were used to hold prey (most likely fish). In the hesperornithiform lineage they were of a different arrangement than in any other known bird (or in non-avian theropod dinosaurs), with the teeth sitting in a longitudinal groove rather than in individual sockets, in a notable case of convergent evolution with mosasaurs.
Of the many species described in this genus - such as the famous H. regalis or the huge H. rossicus -, not all may be valid. Some, such as H. macdonaldi, are known from very few or even a single bone and cannot be properly compared against the more plentiful (but also incomplete) remains of other similar-sized and contemporary taxa. Coniornis altus (which includes H. montana) may also belong into this genus. In addition, there are some unassigned remains, such as SGU 3442 Ve02 and LO 9067t and bones of an undetermined species from Tzimlyanskoe Reservoir near Rostov. The small form, initially called H. gracilis, was later moved into a monotypic genus Hargeria, and ultimately placed in Parahesperornis.
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.
On land, Hesperornis may or may not have been able to walk. They certainly were not able to stand upright like penguins as in the early reconstructions. Their legs attached far at the back and sideways, with even the lower leg being tightly attached to the body (see photo of skeleton). Thus, they were limited to a clumsy hobble at best on land and would indeed have been more nimble if they moved by sliding on their belly or galumphing. Indeed, the leg skeleton of the hesperornithids was so much adapted to diving that their mode of locomotion while ashore, as well as where it laid its eggs and how it cared for its young is a matter of much speculation.
Some have even pointed out that it cannot be completely ruled out that these birds were ovoviviparous instead of incubating their eggs. In any case, young Hesperornis grew fairly quickly and continuously to adulthood, as is the case in modern birds, but not Enantiornithes. More young birds are known from the fossil record of the more northernly sites than from locations further south. This suggests that at least some species were migratory like today's penguins which swim polewards in the summer.
Hesperornis were preyed upon by large marine carnivores. Tylosaurus proriger specimen SDSMT 10439 contains the bones of a Hesperornis in its gut, for example.