"Ichthyornis" is close to the ancestry of modern birds, the Aves, 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", "Iaceornis" and "Guildavis" — but these seem to be closer to the ancestors of modern birds than to "Ichthyornis dispar". In Clarke's 2004 review, the former order Ichthyornithiformes and the family Ichthyornithidae are now superseded by the clade Ichthyornithes, which in the paper was also defined according to phylogenetic taxonomy as all descendants of the most recent common ancestor of "Ichthyornis dispar" and modern birds.