And see text Ichthyornithiformes Furbringer, 1888[2] Ichthyornithes is an extinct group of toothed avialan dinosaurs very closely related to the common ancestor of all modern birds.

It may be that while the flightless, aquatic hesperornitheans were much more sensitive to changes in shoreline extent and sea level during that time, driving their evolution and adaptation into more specialized forms, the flying, tern-like ichthyornitheans were not as dependent on specific coastal or sea-level conditions, and they were able to inhabit basically the same ecological niche for a very long period.

It was long believed that several similar Cretaceous species known from fragmentary remains, including Ambiortus, Apatornis, Iaceornis and Guildavis, were members of the Ichthyornithes in addition to the group's namesake, Ichthyornis.

In Julia Clarke's 2004 review of Ichthyornis-like fossils, the former order Ichthyornithiformes and the family Ichthyornithidae were considered synonyms of the clade Ichthyornithes, which in that paper was defined according to phylogenetic taxonomy as all descendants of the most recent common ancestor of Ichthyornis dispar and modern birds.

[7] Ichthyornis dispar from North America and Janavis finalidens from Belgium are currently the only valid species of ichthyornitheans well-supported enough by distinctive fossil evidence to have received scientific names.

[8] The cladogram below is the result of a 2014 analysis by Michael Lee and colleagues that expanded on data from an earlier study by O’Connor & Zhou in 2012, showing the relationship of Ichthyornis to other ornithurines.