Hesperornis (from Ancient Greek ἕσπερος (hésperos), meaning "western", and ὄρνις (órnis), meaning "bird") is a genus of cormorant-like ornithuran that spanned throughout the Campanian age, and possibly even up to the early Maastrichtian age, of the Late Cretaceous period. It had virtually no wings, and swam with its powerful hind legs. Studies on the feet initially indicated that Hesperornis and kin had lobed toes similar to modern-day grebes, as opposed to webbed toes as seen in most aquatic birds such as loons. More recent work looking at the morphometrics of the feet in hesperornithiformes and modern sea birds has thrown this interpretation into question, making webbed toes equally as likely as lobed toes for this group.

Like many other Mesozoic birds such as Ichthyornis, Hesperornis had teeth as well as a beak. 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. The team headed to Kansas where Marsh had dug before. 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". The specimen was large, wingless, and had strong legs—Marsh considered it a diving species. Unfortunately, the specimen lacked a head. Marsh named the find Hesperornis regalis, or "regal western bird".

Marsh headed back west with a smaller party the following year. In western Kansas, one of Marsh's four students, Thomas H. Russell, discovered a "nearly perfect skeleton" of Hesperornis. This specimen had enough of its head intact that Marsh could see that the creature's jaws had been lined with teeth. Marsh saw important evolutionary implications of this find, along with Benjamin Mudge's find of the toothed bird Ichthyornis. 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". By 1873 their friendship dissolved into open hostility, helping to spark the Bone Wars. While Marsh would rarely go into the field after 1873, the collectors he paid continued to send him a stream of fossils. 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.

Classification and Species

thumb|upright|H. regalis specimen at the AMNH

Many species have been described in this genus, though some are known from very few bones or even a single bone and cannot be properly compared with the more plentiful (but also incomplete) remains of other similar-sized taxa. In many cases, species have been separated by provenance, having been found in strata of different ages or in different locations, or by differences in size.

The first species to be described, the type species, is Hesperornis regalis. 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 70 million years ago).

In 2002, Martin and Lim formally recognized several new species for remains that had previously been unstudied or referred without consideration to previously named North American hesperornithines. 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.

Paleobiology

thumb|H. regalis skeleton in swimming pose; note feet pointing sideways

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, 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. This would have prevented them from bringing the legs underneath the body to stand, or under the center of gravity to walk. Instead, they likely moved on land by pushing themselves along on their bellies, like modern seals.

Young Hesperornis grew fairly quickly and continuously to adulthood, as is the case in modern birds, but not Enantiornithes.

References

Sources

Further reading

  • Everhart, M.J. (2012): Oceans of Kansas: Hesperornis regalis Marsh 1872 – Toothed marine birds of the Late Cretaceous seas . Version of 2012-September-19.
  • Everhart, M.J. 2011. Rediscovery of the Hesperornis regalis Marsh 1871 holotype locality indicates an earlier stratigraphic occurrence. Kansas Academy of Science, Transactions 114(1–2):59–68.
  • Eastern Kentucky University: Closeup of Hesperornis skull . Retrieved 2007-NOV-04.
  • savageancientseas.com: Forward view of H. regalis skeleton in diving pose. Note realistic position of legs and toe rotation. Retrieved 2007-NOV-04.
  • UC Davis: Moveable 3D rendering of patella (kneecap) of Hesperornis sp.. Digitized from KUVP PU17193. Requires Java, IFC or 3DC plugin. Retrieved 2007-NOV-04.