Ichthyornis agilis (YPM, 1209)

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 : 40-41

Ichthyornis agilis

Graculavus agilis ( Marsh, 1873c) was named shortly after Ichthyornis dispar ( Marsh, 1872b) and was subsequently referred to Ichthyornis ( Marsh, 1880) . It was described in two short sentences, and a holotype specimen was not named in the publication. The holotype specimen was not mentioned to be a proximal carpometacarpus, and the differentiation provided of Graculavus agilis from Graculavus anceps ( Marsh, 1873c) is unsupported, primarily because the two holotypes are nonoverlapping parts of a left carpometacarpus that cannot be directly compared.

HOLOTYPE SPECIMEN: YPM 1209 , a proximal left carpometacarpus lacking metacarpal I (fig. 10B).

LOCALITY AND HORIZON: Marsh (1880) specified that he collected YPM 1208 from Butte Creek in 1872. Butte Creek, also known as Twin Butte Creek, is a well­known locality of what is now Logan County, Kansas, in the Smoky Hill Chalk Member, Niobrara Formation ( Stewart et al., 1990), representing beds between Marker Units 15 and 19 ( Bennett, 1990) of Hattin (1982). Stewart (1990) estimated that the interval containing Hattin’s (1982) Marker Units 8–10 to be upper Santonian in age. Thus, because Marker Units 15–19 are above these units, they must be from, at the earliest, later in the upper Santonian (than Marker Units 8–10) or from the lower Campanian. The top of the Smoky Hill Chalk Member is early Campanian in age ( Stewart et al., 1990).

DISCUSSION: Marsh (1873c: 230) originally distinguished Graculavus agilis as a somewhat smaller species than Graculavus anceps with more ‘‘slender’’ metacarpals and lacking a ‘‘carpal fossa’’. The holotype, however, does not appear to be smaller or more delicately built than the holotype of Ichthyornis anceps , so far as the widths of the metacarpals can be compared. As mentioned above, however, no part of the holotype of Ichthyornis agilis can be directly compared with the holotype of Ichthyornis anceps , which is a distal, as opposed to a proximal, carpometacarpus.

There are no identified diagnostic characters from the proximal end of the carpometacarpus identified for Ichthyornis dispar ; thus, as indicated in table 1, YPM 1209 is referred by morphological correspondence. The morphologies of the pisiform process, as well as the configuration of muscle scars on the posterior surface of metacarpal II and anteroventral surface of metacarpal III, correspond with those preserved in the other carpometacarpi referred to Ichthyornis dispar (see table 1). The carpal trochlea superficially appears shorter anteroposteriorly and wid­ er than in the best preserved referred carpometacarpus, YPM 1724. However, the proximal end of YPM 1724 is crushed dorsoventrally, while the Ichthyornis agilis holotype is crushed in the opposite direction, as betrayed by a prominent ridge visible crossing the carpal trochlea dorsoventrally. The Ichthyornis dispar holotype itself does not preserve the proximal end of the carpometacarpus.

The Ichthyornis agilis specimen differs in size from Ichthyornis dispar . It is approximately the size of specimens referred to Ichthyornis dispar by apomorphy that are, by comparison, as large as the individual represented by the holotype of the synonymized Ichthyornis victor . For example, the dorsoventral width of the shaft of metacarpal II is 3.9 mm both for the holotype of Ichthyornis agilis and for a specimen here referred by apomorphy to Ichthyornis dispar (but previously referred to Ichthyornis victor ; YPM 1724). However, as discussed above, size is not considered to diagnose distinct species of Ichthyornis without further information on, for example, the ontogenetic stage and stratigraphic position of the preserved individuals. If, in the future, Ichthyornis victor were recognized as a valid taxon, with larger size as an accessory diagnostic character, the holotypes of Ichthyornis victor and Ichthyornis agilis would both appear to be specifiers of the same species (distinct from Ichthyornis dispar ). In that case, the following question would have to be resolved: the name ‘‘ Ichthyornis agilis ’’ ( Marsh, 1873c) was coined prior to ‘‘ Ichthyornis victor ’’ (1876), but may be nomenum dubium because of a lack of diagnostic features and the invalid differentia used at its naming ( Marsh, 1873c).

REFERRED SPECIMENS: Marsh (1880) referred only an isolated ulna (YPM 1453) to this taxon. As this specimen cannot be compared to the holotype, there is no evidence to support this referral.