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. anceps" should have been considered the correct name for the only recognized "Ichthyornis" species. All other supposed species of "Ichthyornis" have not been supported as valid. The presumed ""Ichthyornis" lentos", for example, actually belongs into the early galliform genus "Austinornis". ""Ichthyornis" minusculus" from the Bissekty Formation (Late Cretaceous) of Kyzyl Kum, Uzbekistan, is probably an enantiornithine. All other "Ichthyornis" species are synonymous with "I. dispar".