Ichthyornis dispar, Marsh, 1872 b

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 : 21-23

dispar Marsh, 1872b (converted species name)

DEFINITION: The species epithet of ‘‘ Ichthyornis dispar ’’ ( Marsh, 1872b) is convert­ ed here. The name ‘‘ dispar ’’ is defined as the species that includes YPM 1450. With its clade address, the name appears as ‘‘ Ichthyornis dispar ’’. The species names ‘‘ Ichthyornis anceps ’’ Marsh, 1872a ( Marsh, 1880), ‘‘ Ichthyornis agilis ’’ Marsh, 1873c ( Marsh, 1880), ‘‘ Ichthyornis victor ’’ Marsh, 1876, ‘‘ Ichthyornis validus ’’ Marsh, 1880, and ‘‘ Ichthyornis antecessor ’’ Wetmore, 1962 ( Olson, 1975) are not converted to clade and species names and are considered currently junior synonyms of ‘‘ Ichthyornis dispar ’’.

The name of the originally designated type species of the genus Ichthyornis ( Marsh, 1872b) is converted here as the name for the one currently recognized species of Ichthyornis (see further discussion below). This species, Ichthyornis dispar , is the valid type species of the genus Ichthyornis according to the ICZN (International Commission of Zoological Nomenclature, 1999: Article 67.2), as well as the appropriate internal specifier of the converted clade name ‘‘ Ichthyornis ’’ according to the current draft of the PhyloCode ( Cantino and de Queiroz, 2000: Article 11.8).

If a large species of Ichthyornis were to be recognized with a diagnostic feature being size, and since ‘‘ Ichthyornis anceps ’’ Marsh, 1872a ( Marsh, 1880) appears to be a valid name, then ‘‘ Ichthyornis anceps ’’ Marsh, 1872a, and not the more commonly discussed ‘‘ Ichthyornis victor ’’ Marsh, 1876 could have priority as a name for a single, larger taxon. The holotype of Ichthyornis anceps ( Marsh, 1872a) is from an individual the same size as that represented in the holotype of Ichthyornis victor (see section on Ichthyornis anceps , below).

HOLOTYPE SPECIMEN: YPM 1450 consists of portions of the skull, mandible, cervical, thoracic and sacral vertebrae, sternum, ribs, coracoid, humerus, ulna, radius, carpometacarpus, femur, tibiotarsus, and several vials of unidentifiable fragments (table 1). The holotype is described and figured in the Anatomical Description.

LOCALITY AND HORIZON: Marsh (1880: 197) identified YPM 1450 as collected by B. F. Mudge in 1872 ‘‘near the Solomon River’’. Brodkorb (1967) specified that locality as the Smoky Hill Chalk Member, Niobrara Formation, near the Solomon River in Section 1, Township 6, Range 19, in Rooks County. Although the source of this additional information was not cited ( Brodkorb, 1967: 177), Bardack (1965) appears to be the source followed. Bardack (1965) himself did not discuss how this more exact information on the locality was obtained. Stewart (1988) identified the locality of the holotype of Ichthyornis dispar as being from the combined zones of Cladoceramus undulatoplicatus and Platyceramus platinus that was taken to be most likely early Santonian in age (Stewart, 1990).

REFERRED MATERIAL: The 77 YPM specimens referred to Ichthyornis dispar (table 1) fall into two kinds: those which are referred based on the presence of one of the nine preserved autapomorphies used in the diagnosis of Ichthyornis dispar , and those referred based on correspondence of their morphologies with those of specimens referred on the basis of autapomorphy. The latter type of referral is an obviously weaker form of inference. Table 1 lists YPM specimens referred to Ichthyornis dispar , and the basis for their referral. For those specimens referred based on the preserved presence of one of the identified autapomorphic characters, numbers are given that correspond to the nine characters used in the Diagnosis (table 1). For those specimens that are referred on the basis of correspondence in every detailed aspect of their morphology, the specimen number of one specimen is given to which the element can be compared and which itself preserves one or more of the nine autapomorphies (table 1). Those specimens that are referred to Ichthyornis in this analysis, but differ in any attribute from the condition seen in the reference specimen listed for them in table 1, are discussed in the Taxonomic Revision below and are listed in table 4.

One partial postcranium from the BMNH (BMNH A905) and one partial skeleton from the SMM (SMM 2503) are referred to Ichthyornis dispar on the basis of apomorphy; in the case of BMNH A905, the specimen preserves characters 4 and 5 from the diagnosis below and in the case of SMM 2503, characters 2, 4, 5, 8, and 9. SMM 2503 must be the same as a specimen as that cited as SMM ‘‘13520’’ in Martin and Stewart (1977); the specimen, recently studied at the SMM (SMM 2503), was listed as collected in 1970 by J. D. Stewart and is a partial skeleton of a large Ichthyornis individual (matching the collection date, collector, and description of SMM ‘‘13520’’ in Martin and Stewart, 1977: 1331). These specimens are discussed because they preserve elements or morphologies not represented in the YPM material. One further specimen (SMM 2139) is discussed because it is the oldest Ichthyornis specimen from Kansas ( Martin and Stewart, 1982). SMM 2139 is a proximal carpometacarpus that can be referred only by morphological correspondence to Ichthyornis .

Several other elements previously referred to Ichthyornis are reviewed here as evaluated from their description and figures from the literature. This preliminary review is undertaken because the age range assessed for Ichthyornis is based on the referral of elements from as early as the Cenomanian, older than the Coniacian­Campanian YPM Smoky Hill Chalk Member material. A nearly complete referred humerus from the early Turonian (Kaskapau Formation) of Alberta ( Fox, 1984) may be referable to Ichthyornis dispar as it corresponds in all of its few figured morphologies with that taxon. However, the basis for this evaluation is weak: The diagnostic feature of the humerus of Ichthyornis dispar (see Diagnosis, character 5) is not visible or described.

Five partial coracoids and a radius from the late Cenomanian Belle Fourche Formation of Saskatchewan ( Tokaryk et al., 1997) were referred to Ichthyornis , with the referral of the coracoid based on the position of the scapular cotyla relative to the glenoid facet ( Tokaryk et al., 1997). In YPM specimens referred to Ichthyornis dispar by apomorphies this morphology is also seen (see Anatomical Description). However, this morphology is not found to be an apomorphy of Ichthyornis . Further study of the material is necessary to evaluate this referral. Several additional elements from the Campanian (Pfugerville Formation) and Coniacian (Gob­ er Formation) of Texas ( Parris and Echols, 1992) were referred to Ichthyornis dispar and Ichthyornis antecessor . Those referred to Ichthyornis antecessor are treated in the Taxonomic Revision. No morphologies were listed as the basis for the one complete humerus and one partial forelimb referred to Ichthyornis dispar . Again, no identified apomorphies of Ichthyornis are described or figured. A partial humerus from the Campanian Chico Formation of California ( Hilton et al., 1999) was referred to Ichthyornis , but no characters were cited for the basis of the referral. It corresponds in the few figured morphologies to those of Ichthyornis dispar in the short flexor process and the pronounced pits on the dorsal supracondylar process. However, none of these mentioned characters are apomorphies of Ichthyornis . A thoracic vertebra from the Campanian (Vermilion River Formation) of Manitoba ( Martin and Stewart, 1982) appears to have a centrally located parapophysis (L. Chiappe, personal commun.), a character synapomorphic of Enantiornithes , and is removed from Ichthyornis .