[1][2] One of the lesser-known discoveries of the paleontologist O. C. Marsh in the late 19th century Bone Wars, it was an early find in the history of avian paleontology.

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.

Studies of the bone surface show that at least the tips of the jaws supported a hard, keratinous beak similar to that found in modern birds.

[10] The palate (mouth roof) contained small pits that allowed the lower teeth to lock into place when the jaws were closed.

Aside from finding more bones belonging to the flying reptile Pteranodon, Marsh discovered the skeleton of a "large fossil bird, at least five feet in height".

[17] In an 1873 paper Marsh declared that "the fortunate discovery of these interesting fossils does much to break down the old distinction between Birds and Reptiles".

He eventually received parts of 50 specimens of Hesperornis, which allowed him to make a much stronger demonstration of an evolutionary link between reptiles and birds than had been possible before.

H. regalis is also the best known species, and dozens of specimens (from fragments to more complete skeletons) have been recovered, all from the Smoky Hill Chalk Member of the Niobrara Formation (dating to the early Campanian age, between 90 and 60 million years ago).

H. crassipes was larger than H. regalis, had five ribs as opposed to four in the first species, and differed in aspects of the bone sculpturing on the breastbone and lower leg.

[21] Marsh explicitly named his second species of Hesperornis in 1876 for an incomplete metatarsus recovered from the same layers of the Niobrara chalk as H. regalis.

This mistake was rectified by later authors, who sank Hargeria back into Hesperornis and renamed the more distinctive specimen Parahesperornis.

These include the very small H. mengeli and H. macdonaldi, the slightly larger H. bairdi, and the very large H. chowi, all from the Sharon Springs member of the Pierre Shale Formation in South Dakota and Alberta, 80.5 million years ago.

[28] 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.

Baptornis advenus Brodavis varneri Brodavis baileyi Fumicollis hoffmani Parahesperornis alexi Hesperornis Hesperornis was primarily marine, and lived in the waters of such contemporary shallow shelf seas as the Western Interior Seaway, the Turgai Strait, and the North Sea,[29][2] which then were subtropical to tropical waters, much warmer than today.

[31] Traditionally, Hesperornis is depicted with a mode of locomotion similar to modern loons or grebes, and study of their limb proportions and hip structure has borne out this comparison.

[33] Like loons, the legs were probably encased inside the body wall up to the ankle, causing the feet to jut out to the sides near the tail.

[36] A Hesperornis leg bone uncovered in the 1960s was examined by David Burnham, Bruce Rothschild et al. and was found to bear bite marks from a young polycotylid plesiosaur (possibly a Dolichorhynchops or something similar).

The Hesperornis's bone, specifically the condyle, shows signs of infection, indicating the bird survived the initial attack and escaped the predator.