Ichthyornis Marsh, 1872b

CLARKE, JULIA A., 2004, Morphology, Phylogenetic Taxonomy, And Systematics Of Ichthyornis And Apatornis (Avialae: Ornithurae), Bulletin of the American Museum of Natural History 2004 (286), pp. 1-179 : 20-21

Ichthyornis Marsh, 1872b

(converted clade name)

DEFINITION: ‘‘ Ichthyornis ’’ is defined as a modified apomorphy­based name for the clade stemming from an ancestor that possessed all of the morphologies described by apomorphies 2, 5, 6, 7, 8 (in the Ichthyornis dispar Diagnosis) homologous (sensu Patterson, 1982) with those in YPM 1450 (holotype of Ichthyornis dispar Marsh, 1872b ) and more closely related to YPM 1450 than to Aves (internal specifiers of Gauthier and de Queiroz, 2001; see Introduction). YPM 1450 is the holotype of ‘‘ Ichthyornis dispar ’’, which is the valid type species of the genus name ‘‘ Ichthyornis ’’ as specified by Marsh (1872b) and under the ICZN ( International Commission on Zoological Nomenclature, 1999).

While nine characters are currently found to be autapomorphies of Ichthyornis dispar , only those characters preserved in YPM 1450 are used in the definition of the name. Thus, even if some or all of the other characters used in the diagnosis of dispar (e.g., 1, 3, 4, 9) are found to be absent in the species of which YPM 1450 is a part and instead diagnostic of another species of Ichthyornis , the name ‘‘ Ichthyornis ’’ will still apply to a clade including YPM 1450.

Further, if some of the characters used in the definition of the name ‘‘ Ichthyornis ’’ are discovered to be plesiomorphies of a more inclusive clade of which Aves is a part, the definition would remain valid and apply to the portion of the stemclade intended by the definition (i.e., a subclade of the YPM 1450 stem clade). The name ‘‘ Ichthyornis ’’ could not refer to a clade including Aves, as mentioned, because the modifier of this apomorphy­based name used specifies only that specimens more closely related to YPM 1450 than to Aves may be part of a clade named ‘‘ Ichthyornis ’’.

However, if all five of the autapomorphies used in the definition were optimized as plesiomorphies of a more inclusive clade of which Aves was a part, ‘‘ Ichthyornis ’’ would apply to the total dispar stem clade and become a heterodefinitional synonym of ‘‘ Ichthyornithes ’’ (defined earlier in the document and presumably to be converted before it; Cantino and de Queiroz, 2000: Article 14). The definition would remain valid as the characters currently found to be autapomorphies would remain homologous as plesiomorphies of a more inclusive clade. The name ‘‘ Ichthyornis ’’, in this instance (i.e., as a heterodefinitional synonym), would not be reavailable for redefinition. In the unlikely event that all five characters are so optimized, it is mandated here as one additional formal modifier of the definition that the given definition be invalidated if the intent of the definition (i.e., to name a subclade of the total stem) is violated. Such an occurrence is considered highly unlikely, however.

SPECIFIER: The internal specifier of the name ‘‘ Ichthyornis ’’ is YPM 1450, the holotype of Ichthyornis dispar (described in Marsh, 1872a, 1872b, indicated by description and specimen number as ‘‘YPM 1450’’ by Marsh, 1880). Following Article 11.8 of the current draft of the PhyloCode ( Cantino and de Queiroz, 2000), the internal specifier of the clade name ‘‘ Ichthyornis ’’ as convert­ ed from a genus name should use the type of the genus as the internal specifier. As noted below, although the species name ‘‘ anceps ’’ was coined before ‘‘ dispar ’’ for a specimen now supported as part of Ichthyornis ( Marsh, 1872a vs. Marsh, 1872b), it was not one of the original nominal species of the genus Ichthyornis and cannot, according to the ICZN ( International Commission on Zoological Nomenclature, 1999: Article 67.2) be the type of the genus Ichthyornis . Thus, given this provision of the ICZN as well as Article 11.8 of the PhyloCode, the appropriate internal specifier for the converted clade name ‘‘ Ichthyornis ’’ is the first named nominal species (and type) of the genus Ichthyornis , which is dispar .