Austinornis lentus, Marsh, 1877 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 : 52-54

lentus Marsh, 1877b

(converted species name)

DEFINITION: The name ‘‘ lentus ’’ Marsh, 1877b is converted and defined as the species that includes YPM 1796 (see the Introduction of the Taxonomic Revision for the definition of ‘‘species’’ used). With its clade address (sensu Cantino et al., 1999), the converted name of this species is ‘‘ Austinornis lentus ’’. The name was originally published in the combination ‘‘ Graculavus lentus ’’ ( Marsh, 1877b) and later in the combination ‘‘ Ichthyornis lentus ’’ ( Marsh, 1880).

HOLOTYPE SPECIMEN: YPM 1796 is a wellpreserved distal left tarsometatarsus (fig. 17).

LOCALITY AND HORIZON: Marsh (1880: 198) described the holotype specimen as collected by B. F. Mudge near Fort McKinney, Texas, in 1876. Brodkorb (1967: 178) specified that it was from the Austin Chalk from ‘‘Colling’’ County (misprint of Collin County; Parris and Echols, 1992) near Fort Mc­ Kinney. There is no more precise stratigraphic or locality data available. Its provenance should be further investigated.

DISCUSSION: Graculavus (Ichthyornis) lentus Marsh (1877b) is removed from Ichthyornis . It is placed phylogenetically as part of Aves rather than part of Ichthyornis (Part II, Results). Removal from Ichthyornis was suggested by Martin and Stewart (1982: 325), although no evidence was given as a basis for this recommendation. Shufeldt (1915: 76) also remarked in passing that Graculavus lentus ( Ichthyornis lentus of Marsh, 1880) was ‘‘some tetraonine species’’ ( Galliformes , Tetraonidae : Grouse) which is consistent with the identification made here.

The holotype differs in nearly all anatomical features that could be compared between it and Ichthyornis dispar (as represented by referred YPM tarsometatarsi, i.e., YPM 1456, YPM 1464, YPM 1739, YPM 1771; see table 1 for the basis of referral). A tarsometatarsus is not preserved in the Ichthyornis dispar holotype. Unlike other specimens referred to Ichthyornis dispar , a slight groove extends proximally from the juncture of metatarsals II and III on the dorsal surface of the tarsometatarsus. Also on the dorsal surface, a distinct groove extends proximally from the distal vascular foramen. This vascular foramen also has a much more solid distal enclosure than in Ichthyornis dispar . Metatarsal II approaches IV in distal extent as opposed to being much shorter than IV in Ichthyornis dispar . The trochlea of metatarsal II is strongly rounded as opposed to ginglymoid in Ichthyornis dispar . The metatarsal I fossa is slightly more proximally situated than in Ichthyornis dispar and better developed. On the distal plantar surface, intermuscular lines are well developed while these are not marked in Ichthyornis dispar . One is located along the lateral edge of the shaft and a second slightly more medially positioned, parallel to the first, and extending toward metatarsal IV. Finally, the trochlear surface of metatarsal III is asymmetrically developed with the lateral edge of this trochlea extending distinctively proximal to the medial; in Ichthyornis dispar the edges of the trochlea are symmetrical.

The last mentioned morphology was re­ cently described for a taxon (i.e., Paraortygoides messelensis, Mayr, 2000 ) identified as most closely related to crown clade Galliformes and suggested to be a derived character of that taxon + crown clade galliforms ( Mayr, 2000). The other morphologies described in the differentia (e.g., the strength of the fossa metatarsi I and plantar intermuscular lines) are consistent with the development of these features in other galliforms surveyed (e.g., Alectura lathami , Meleagris gallopavo ), but their distribution needs to be further evaluated.

REFERRED SPECIMENS: No specimens have been referred to this taxon.