Ichthyornis is a genus of seabird from the Late Cretaceous of North America. Its fossil remains are known from the (Turonian-Middle Campanian, 93.5-75 mya) chalks of Alberta, Alabama, Kansas, New Mexico, Saskatchewan, and Texas, in strata that were laid down in the Western Interior Seaway; some fossils from other locations like Argentina and Central Asia are sometimes referred to this taxon.
Although the wings and breastbone are very modern in appearance (suggesting strong flight ability), the jaws retained numerous small, sharp teeth. Unlike earlier birds such as Enantiornithes, it appears to have matured to adulthood in a rather short, continuous process.
The fossil material is known to contain bones of immature individuals. It has not been thoroughly analyzed according to ontogenetical size differences, as for this, destruction of the fossils would be necessary.
Ichthyornis is close to the ancestry of modern birds, the Neornithes, but represents an independent lineage. It was long believed that it was closely related to some other Cretaceous taxa known from very fragmentary remains — Ambiortus, Apatornis (including the now separated Iaceornis) and Guildavis — but these seem to be closer still to the ancestors of modern birds. New data on the radiation of the latter, which is now known to have started already in the Cretaceous (see Vegavis), could shed more light on the exact relationship of these taxa. To agree with the 2004 review, the former order Ichthyornithiformes and the family Ichthyornithidae are now superseded by the subclass Ichthyornithes, which in the paper was also defined according to phylogenetic taxonomy.
There has been considerable confusion about the attribution of the fossil material. The similarity of the lower jaw and teeth to those of mosasaurs (fish-eating marine lizards) is so great that as late as 1952, it was argued that it really belonged to a diminutive species or young individual of or related to the genus Clidastes (Gregory, 1952); it was described as Colonosaurus mudgei.
Of the several described species, only one, Ichthyornis dispar, is currently recognized, following the seminal review by Julia Clarke. It is, however, entirely possible that several species have once existed, given the large spatial and temporal range over which fossils have been found. Many bird species show little difference in their skeletons, and the Ichthyornis material comes from a wide range of strata, so possibly more than one species existed, the bauplan being so successful as to change little in morphology.
The presumed "Ichthyornis" lentus actually belongs into the early galliform genus Austinornis. "Ichthyornis" minusculus from the Bissekty Formation (Late Cretaceous) of Kyzyl Kum, Uzbekistan is probably an enantiornithine.