Ichthyornithes

Extinct clade of dinosaurs
Automatic taxobox
| name = Ichthyornitheans
| fossil range = Late Cretaceous,
| image = Ichthyornis Clean. png
| image caption = Cast of an Ichthyornis dispar skeleton, Rocky Mountain Dinosaur Resource Center
| taxon = Ichthyornithes
| authority = Marsh, 1873
| subdivision ranks = Subgroups
| subdivision ref =
And see text
| synonyms = Ichthyornithiformes Furbringer, 1888

A study on an Ichthyornis endocast reveals that it had a relatively "primitive" brain compared to modern birds, similar to that of Archaeopteryx and other non avian theropods. Conversely, it had a palate remarkably convergent with that of modern neognaths.

Ichthyornitheans were close to the ancestry of modern birds, the crown group Aves, but represents an independent lineage of toothed seabird like animals. 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. However, these seem to have been more closely related to modern birds than to Ichthyornis dispar. 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.

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. Another unnamed species similar to Ichthyornis is known from isolated remains found in Campanian age rocks from Alberta. Two more species also represented only by fossil shoulder bones are known from the Cenomanian age Ashville Formation near Carrot River, Saskatchewan. Though originally thought to be species of Ichthyornis, they probably represent one or more new genera.

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. The clade names are positioned based on their definitions.