Hesperornis is an extinct species of flightless aquatic bird that lived during the Coniacian to Maastrichtian sub-epochs of the belatedly Cretaceous (89-65 million years past).
Hesperornis were very big birds, reaching up to 2 metres (6.5 feet) in length. They had nearly no wings and hunted in the waters of the North American Inland Sea, swimming with influential hind legs.
Their feet were almost certainly lobed rather than being webbed as in today's grebes.
Approximating other Mesozoic birds such as Ichthyornis, Hesperornis had teeth in its beak which were used to hold prey (most probable fish), although in the hesperornithiform lineage they were of a dissimilar arrangement than in any other known bird with the teeth sitting in a longitudinal furrow rather than in individual sockets.
On land, Hesperornis may or may not have been capable to walk. Indeed, the leg bones 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 substance of much speculation.
Some have even pointed out that it cannot be entirely ruled out that these birds were ovoviviparous in its place of incubating their eggs. In any case, young Hesperornis grew fairly rapidly and continuously to adulthood, as is the case in new birds, but not Enantiornithes.
Hesperornis were preyed upon by big marine carnivores. A Tylosaurus specimen found contained the bones of a Hesperornis in its belly.