Ichthyornis - Classification

Classification

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 In Clarke's 2004 review, the former order Ichthyornithiformes and the family Ichthyornithidae are now superseded by the subclass Ichthyornithes, which in the paper was also defined according to phylogenetic taxonomy as all descendants of the most recent common ancestor of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

Of the several described species, only one, Ichthyornis dispar, is currently recognized, following the seminal review by Julia Clarke. Marsh had previously named a specimen now attributed to I. dispar as Graculavus anceps. Clarke argued that because the rules for naming animals laid out by the ICZN state that a type species for a genus must have originally been included in that genus, Ichthyornis anceps is ineligible to replace I. dispar as the type species and so must be considered a junior synonym even though it was named first. However, Michael Mortimer pointed out that this is incorrect; while I. anceps cannot become the type species of Ichthyornis, the ICZN does not preclude it from becoming the senior synonym of the type species I. dispar. Therefore, I. ancesps should have been considered the correct name for the only recognized Ichthyornis species.

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.

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.