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Ornithurae Haeckel 1866
A subclass of Aves, the definition of Ornithurae is in fact extremely contentious. Traditionally, the group has been viewed in terms of a stem-taxon, as all birds extant or extinct closer to Neornithes than to Archaeopteryx, a definition advanced in both cladistic (e.g., Gauthier 1986) and non-cladistic analyses (e.g., Martin 1983). Chiappe (1991) redefined Ornithurae as a far more restrictive node-based taxon including only the common ancestor of Hesperornis and Carinatae and its descendants, defending this redefinition of the taxon on grounds of clarity and historical usage. Needless to say, the munificence of these arguments is not readily apparent, least of all the latter as the historial usage of Ornithurae has consistently adhered to the earlier stem-based interpretation favored by earlier authors. Sereno et al. (2002) advocated continued use of the traditional definition of Ornithurae over that followed by Chiappe in his 1991 analysis and this is by far the most defensible approach, concurred with by the author. This definition of Ornithurae has been adhered to regardless of the view of basal avian phylogeny (e.g., contrast Hou et al. 1996 and Chiappe 2002) and thus has the greatest utility in discussing the early evolution of birds.
Hou et al. (1996) assigned nine characters as synapomorphies of Ornithurae, while Chiappe (2002) appears to have assigned as many as 20. For simplicity and regard to space, the author will follow Hou et al. (1996) in listing characters synapomorphic of Ornithurae, and merely refer the reader to Chiappe's (2002) exhaustive cladistic analysis for a more thorough review of the characters underwriting the monophyly of Ornithurae:
- Furcula laterally compressible
- Scapular facet of coracoid rounded
- Procoracoidal process distinct, and straplike
- Carina of sternum approaches the cranial margin of sternum
- Sternum with coracoidal sulci
- Uncinate processes present
- Tarsometatarsal fusion disto-proximal
- Outermost metacarpals fused distally
- Tarsal cap of tarsometatarsus present
Hou et al. (1996) appear to have considered the following traits further synapomorphic of Ornithurae, but they belong to as yet unnamed groups:
- Chiappe, L. 1991. Cretaceous avian remains from Patagonia shed new light on the early radiation of birds. Alcheringa 15: 333-338.
- Chiappe, L. 2002. Basal bird phylogeny: problems and solutions. In: Chiappe, L. & Witmer, L. (eds.), Mesozoic Birds: Above the Heads of Dinosaurs, 448-472.
- Gauthier, J. 1986. Saurischian monophyly and the origin of birds. In: Padian, K. (ed.), The Origin of Birds and the Evolution of Flight, 1-55.
- Hou, L., Martin, L. D., Zhou, Z. & Feduccia, A. 1996. Early adaptive radiation of birds: evidence from fossils from northeastern China. Science 274: 1164-1167.
- Martin, L. 1983. The origin and early radiation of birds. In: Brush, A. H. & Clark Jr., G. A., (eds.), Perspectives in Ornithology, 291-338.
- Sereno, P., Rao, C., Li, J. 2002. Sinornis santensis (Aves: Enantiornithes) from the Early Cretaceous of Northeastern China. In: Chiappe, L. & Witmer, L. (eds.), Mesozoic Birds: Above the Heads of Dinosaurs, 184-208.