Ichthyornis

Ichthyornis (meaning "fish bird", after its fish-like vertebrae) was a genus of toothed seabirds from the Late Cretaceous of North America. Its fossil remains are known from the chalks of Alberta, Alabama, Kansas, New Mexico, Saskatchewan, and Texas, in strata that were laid down in the Western Interior Seaway during the Turonian-Campanian ages, about 95-85 mya. Ichthyornis is a common component of the Niobrara Formation fauna, and numerous specimens of both adult and immature individuals have been found.[1]

Ichthyornis has been historically important in shedding light on bird evolution. It was the first discovered prehistoric bird preserved with teeth, and Charles Darwin noted its significance during the early years of the theory of evolution. Ichthyornis remains important today as it is one of the few Mesozoic era birds known from more than a few specimens.

Description
It is thought that Ichthyornis was the Cretaceous ecological equivalent of modern seabirds such as gulls, petrels, and skimmers. An average specimen was the size of a pigeon, 24 centimetres (9.4 in) long, with a skeletal wingspan (not taking feathers into account) of around 43 centimetres (17 in),[3] though there is considerable size variation among known specimens, with some smaller and some much larger than the type specimen of I. dispar.[2]

Ichthyornis is notable primarily for its combination of vertebrae which are concave both in front and back (similar to some fish, which is where the bird gets its name) and several more subtle features of its skeleton which set it apart from its close relatives. Ichthyornis is perhaps most well known for its teeth. The teeth were present only in the middle portion of the upper and lower jaws. The jaw tips had no teeth and were covered in a beak. The beak of Ichthyornis, like the hesperornithids and other primitive birds, was compound and made up of several distinct plates, similar to the beak of an albatross, rather than a single sheet of keratin as in most modern birds.[4] The teeth were more flattened than the rounded teeth found in crocodilians, though they became wider towards the base of the crown. The tips of the teeth were curved backward and lacked any serrations.[2]

Although the wings and breastbone were very modern in appearance (suggesting strong flight ability and placing it with modern birds in the advanced group Carinatae), the jaws had numerous small, sharp teeth. Unlike earlier birds such as the enantiornithines, it appears to have matured to adulthood in a rather short, continuous process.

Timespan and evolution
Ichthyornis fossils have been found in almost all levels of the Niobrara Chalk, from beds dating to the late Coniacian age (about 89 million years ago) to the Campanian age (about 85 million years ago).[5] Even earlier remains attributed to Ichthyornis have been found in the Greenhorn Formation of Kansas, dating to the early Turonian age (about 93 million years ago),[3] and the Cenomanian of Saskatchewan, dating to about 95 million years ago.