Ichthyornis antecessor (Parris and Echols, 1992)

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 : 43-44

Ichthyornis antecessor

Plegadornis antecessor was named by Wetmore (1962) based on a distal humerus. Kashin (1972) noted that Plegadornis was preoccupied, and changed the name to Angelinornis antecessor . Olson (1975) used characters shared by Ichthyornis and Angelinornis antecessor to synonymize the latter genus and refer the specimen to Ichthyornis , but still recognized it as a valid species, Ichthyornis antecessor .

HOLOTYPE SPECIMEN: USNM 22820 is a left distal humerus and was illustrated in Olson (1975).

LOCALITY AND HORIZON: USNM 22820 is from the top of the Mooreville Formation just below the base of the Arcola Limestone Member (Selma Group; Wetmore, 1962) at Hewletts Farm, Greene County, Alabama ( Parris and Echols, 1992). The Mooreville Formation has been inferred to be approximately the same age as or slightly younger than the uppermost deposits represented in the Niobrara Formation ( Martin and Stewart, 1982) and early Campanian in age ( Parris and Echols, 1992).

DISCUSSION: Olson (1975) listed characters differentiating Ichthyornis antecessor relative to a specimen he referred to Ichthyornis dispar (YPM 1764) , pending a revision of Ichthyornis he mentioned as being undertak­ en by Pierce Brodkorb ( Olson, 1975). Such revision was never completed. The differentia from YPM 1764 consisted of the following: shaft of Ichthyornis antecessor not as heavy; brachial depression shallower and slightly more distal; dorsal supracondylar process more prominent and a pit at its base, shallower ( Olson, 1975). The attachment of the anterior articular ligament (lig. collaterale ventrale) was also, tentatively, considered to be different.

However, YPM 1764 clearly differs in size and several minor features of muscular attachment (like those mentioned in the differentia of Ichthyornis antecessor ) from the holotype of Ichthyornis dispar (YPM 1450) . Thus, it is a less than ideal proxy to have been used for the holotype of Ichthyornis dispar . The shaft of the holotype of Ichthyornis dispar is also more slender, or less ‘‘heavy’’, than YPM 1764. The distal width in YPM 1450 is 9.4 mm as opposed to the 10.5 mm of YPM 1764 and USNM 22820. The other characters noted in the differentia are very minor differences in muscular attachments that could be attributed to intraspecific variation. A pit at the base of the dorsal supracondylar process described of Ichthyornis antecessor is present in the Ichthyornis dispar holotype and other referred YPM Ichthyornis dispar specimens. Because the holotype of Ichthyornis dispar is from the Santonian, while that of Ichthyornis antecessor is from the Campanian, it is possible that slight differences in muscular attachment (if they can be confirmed to be other than individual variation) are due to anagenetic change within a single lineage. Ichthyornis antecessor is, therefore, considered a junior synonym of Ichthyornis dispar .

REFERRED MATERIAL: TMM 42522–1 is a distal humerus referred to Ichthyornis antecessor ( Parris and Echols, 1992) . It seems to have been referred to Ichthyornis antecessor because it was approximately the same age as the holotype (Campanian), but it was also judged to match closely the morphology of the holotype. However, on the points of morphological comparison noted, the specimen is equally like the holotype of Ichthyornis dispar and specimens referred to that species. TMM 42522–1 is smaller than the holotype of Ichthyornis antecessor ; the measure of the width of the distal end is 10.1 mm as opposed to 10.5 mm in the holotype. The same measure of the holotype of Ichthyornis dispar is 9.5 mm, making TMM 42522–1 intermediate in size between the holotype of Ichthyornis dispar and that of Ichthyornis antecessor .

ET 4396 (L85–6) is a proximal left carpometacarpus described by Parris and Echols (1992). As these authors mentioned, it cannot be compared to the holotype of Ichthyornis antecessor . It was referred to Ichthyornis antecessor because the humerus from the same formation was referred to that species and is from younger deposits than all named Ichthyornis species other than Ichthyornis antecessor ( Parris and Echols, 1992) . The fragment was also supposed to differ from other Ichthyornis material in several features of metacarpal I. However, nearly half of metacarpal I is missing and the mentioned attributes could not be confirmed. The carpometacarpus was also supposed to be more gracile than the Niobrara lchthyornis material to which it was compared (although this material in not identified). The other noted differences between the figured specimen and YPM specimens, here referred to Ichthyornis dispar , were not confirmed. The absence of a groove on the carpal trochlea and other figured morphologies correspond with those in YPM specimens referred to Ichthyornis by apomorphy, but it does not preserve any characters used in the diagnosis of Ichthyornis dispar .

OTHER VALID SPECIES NOT SUPPORTED AS

PART OF ICHTHYORNIS OR ICHTHYORNITHES Guildavis (new clade name)

DEFINITION: ‘‘ Guildavis ’’ is here defined as a stem­based name ( de Queiroz and Gauthier, 1992) for the clade including all taxa/specimens more closely related to YPM 1760 ( tener Marsh, 1880 , holotype specimen) than to Ichthyornis dispar or the following internal specifiers of Aves ( Gauthier and de Queiroz, 2001; see Introduction): Vultur gryphus , Struthio camelus Linnaeus, 1758 , Tetrao [ Tinamus ] major Gmelin, 1789, and Vultur gryphus Linnaeus, 1758 .

ETYMOLOGY: ‘‘ Guildavis ’’ is a combination of ‘‘Guild’’, for E. W. Guild, who col­

lected the holotype of tener and internal specifier, YPM 1760, and ‘‘ avis ’’, the Latin for bird.