Aves Linnaeus, 1758

Rauhut, Oliver W. M., 2003, The interrelationships and evolution of basal theropod dinosaurs, Special papers in palaeontology 69, pp. 1-213 : 23

Aves Linnaeus, 1758

Definition. Following Padian and Chiappe (1998), and in contrast to Gauthier (1986), Aves are defined here with their fossil stem-group representatives included. Thus, Aves may be defined as Archaeopteryx and Neornithes, and all descendants of their most recent common ancestor.

Temporal range. Tithonian-Recent.

Distribution. Global.

Diagnosis. Asymmetric, aerodynamic contour feathers; humerus longer than femur; radius longer than humerus.

Remarks. Birds are the only living group of dinosaurs. More than 9000 extant species of birds are known, and our knowledge of their fossil history is increasing rapidly. In 1960, Brodkorb estimated the total number of bird species that have existed as more than 1-5 million; of course, such estimates are rather speculative, but they might give an idea of the disparity in taxonomic diversity between this OTU and others.

Unfortunately, improved knowledge of the anatomy of advanced theropods and the discovery of connecting links has made a formal diagnosis of Aves increasingly difficult, since the acquisition of avian characters in theropod evolution was gradual. The matter is further complicated by the high diversity of birds; the characters listed in the diagnosis above are lost in many avian lineages, and the first reversals might have occurred soon after the origin of this group (see Chiappe 1995; Chiappe et al. 1996; Padian and Chiappe 1998). However, it is assumed here that flight arose only once in birds, and it is the main diagnostic feature of this group, as expressed by aerodynamic feathers and forelimb proportions. Following Chiappe et al. (1996) and Novas (1996Z?), alvarezsaurids are regarded here as basal birds (see Sereno, 1999, for an alternative view) and are thus not treated as a separate OTU.

Since Archaeopteryx ( Text-fig. 5f ) is the most basal bird ( Chiappe 1995), character codings are mainly based on this taxon; only if character states cannot be determined in Archaeopteryx is information from other basal birds used, mainly Rahonavis (UA 8656; Forster et al. 1998), Confuciusomis (GPI, JM, three unnumbered specimens), Hesperomis, and Ichthyornis ( Marsh 1880; Elzanowski 1991).

Figures

text-fig. 5. Skull reconstructions of representatives of Jurassic OTUs in left lateral view, a, Dilophosaurus wetherilli, Early Jurassic (Sinemurian-Pliensbachian), Kayenta Formation, Arizona, USA; based on UCMP V 4214 and V 6468. B, Syntarsus rhodesiensis, Early Jurassic (Hettangian-Sinemurian), Forest Sandstone, Zimbabwe; composite reconstruction based on many isolated skull elements from the National Museum of Natural History in Harare (see Appendix), c, Magnosaurus oxoniensis, Middle Jurassic (Callovian), Oxford Clay, England; based on OUM J 13558, unpreserved elements shaded. D, Monolophosaurus jiangi, Middle Jurassic, Wucaiwan Formation, China; redrawn from Zhao and Currie (1993b). E, Allosaurus fragilis, Late Jurassic (Kimmeridgian-Tithonian), Morrison Formation, USA; based on MOR 693. f, basal bird Archaeopteryx sp., Late Jurassic (Tithonian), lithographic limestones of Solnhofen, Germany; based on Wellnhofer (1974), Elzanowski and Wellnhofer (1996), and the Berlin, Eichstätt, and Munich specimens. G, Ceratosaurus sp., Late Jurassic (Kimmeridgian-Tithonian), Morrison Formation, USA; based on USNM 4735 and UMNH VP 5278. H, Ornitholestes hermanni, Late Jurassic (Kimmeridgian-Tithonian), Morrison Formation, USA, based on AMNH 619. Abbreviations as in Text-figure 4, and: If, lacrimal fenestra; mf, maxillary fenestra; nf, nasal foramen; pmf, promaxillary fenestra. Scale bars represent 10 mm (b, f, h), 50 mm (c) and 100 mm (a, d, e, g).