Name: Hesperornis (Western bird).
Phonetic: Hess-per-or-niss.
Named By: Othniel Charles Marsh - 1872.
Synonyms: Conirnis, Hargeria, Lestornis.
Classification: Chordata, Aves, Hesperornithiformes, Hesperornithidae.
Species: H. regalis (type), H. altus, H. bairdi, H. chowi, H. crassipes, H. gracilis, H. macdonaldi, H. mengeli, H. montana, H. rossicus.
Type: Piscivore.
Size: Up to 1.8 meters long for the largest species.
Known locations: Canada, Russia, Sweden, USA.
Time period: Campanian of the Cretaceous.
Fossil representation: Numerous remains representing multiple species.

       Hesperornis seems to have been a prehistoric cross between a penguin and grebe. Although there were many flight capable birds during the Campanian, Hesperornis seems to have become secondarily flightless. This is indicated by the greatly reduced wings that were little more than stubs. However, although they were useless for flight, they would greatly enhance the birds streamlining as it dived underwater.
       When underwater, Hesperornis was probably a very graceful and agile swimmer, its feet pushing it down under the water, while the long neck would easily allow Hesperornis to pluck fish from their hiding places. But because the legs and feet were angled to provide maximum propulsion in the water, Hesperornis would have been very cumbersome on the land, perhaps only being able to shuffle along.
       The beak of Hesperornis is well adapted for catching fish, with sharp teeth running the length of the lower jaw, and at the back of the upper. A good example of convergent evolution is the fact that the teeth are not in sockets like their dinosaur ancestors, but in a longitudinal groove that ran down the beak. Although not related to Hesperornis, or even birds in general, this same tooth holding groove is also seen in the mosasaurs that swam the Cretaceous seas. On a quick side note, Hesperornis were occasional prey to mosasaurs, as evidenced by Hesperornis fossils found within what was once the gut of a Tylosaurus. Another adaptation for the beak of Hesperornis are small holes in the palate that allow the lower teeth to slot into them when the beak is closed.
       Although the majority of Hesperornis remains are known from seas such as the Western Interior Seaway, some have also come from freshwater deposits, possibly following river systems as they searched for fish.

Further reading
- Preliminary description of Hesperornis regalis, with notices of four other new species of Cretaceous birds - The American Journal of Science and Arts, series 3 3(13-18):360-365 - Othniel Charles Marsh - 1872.
- On the Odontornithes, or birds with teeth. - American Journal of Science 10:403-408 - Othniel Charles Marsh - 1875.
- Notice of new Odontornithes - The American Journal of Science and Arts 11: 509–511. - Othniel Charles Marsh - 1876.
- A new Cretaceous bird allied to Hesperornis. - The American Journal of Science and Arts, series 3 45(265):81-82 - Othniel Charles Marsh - 1893.
- The fossil remains of a species of Hesperornis found in Montana - The Auk 32 (3): 290–294. - R. W. Shufeldt - 1915.
- A middle Campanian, nonmarine occurrence of the Cretaceous toothed bird Hesperornis Marsh - Canadian Journal of Earth Sciences 11 (9): 1335–1338 - R. C. Fox - 1974.
- A new hesperornithid and the relationships of the Mesozoic birds - Transactions of the Kansas Academy of Science 87(3/4):141-150 - L. D. Martin - 1984.
- New observations on the skull of Hesperornis with reconstructions of the bony palate and otic region - Postilla 207: 1–20. - A. Elzanowski - 1991.
- Gesperornisy v Rossii [Hesperornithiforms in Russia]. - Russian Journal of Ornithology 2(1):37-54 - L. A. Nessov & A. A. Yarkov - 1933.
- Bone microstructure of the diving Hesperornis and the volant Ichthyornis from the Niobrara Chalk of western Kansas - Cretaceous Research 19 (2): 225–235. - A. Chinsamy, L. D. Martin & P. Dobson - 1998.
- Hesperornis (Aves) from Ellesmere Island and palynological correlation of known Canadian localities - Canadian Journal of Earth Sciences 36 (9): 1583–1588. - L. V. Hills, E. L. Nicholls, L. Nez-Betelu, M. Koldo D. J. & McIntyre - 1999.
- Aquatic birds from the Upper Cretaceous (Lower Campanian) of Sweden and the biology and distribution of hesperornithiforms - Palaeontology 48 (6): 1321–1329 - Jan Rees & Johan Lindgren - 2005.
- Hind limb and pelvis proportions of Hesperornis regalis: A comparison with extant diving birds - Journal of Vertebrate Paleontology 26 (3): 115A. - F. Reynaud - 2006.
- A new evolutionary lineage of diving birds from the Late Cretaceous of North America and Asia - Palaeoworld 21 (1): 59–63. - Larry D. Martin, Evgeny N. Kurochkin & Tim T. Tokaryk - 2012.


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