Ornithurae has had an unfortunately complex definitional history that has been usefully summarized as three interpretations—stem, node and apomorphy (Gauthier and de Queiroz 2001). Ornithurae (“bird tail”), as originally coined by Haeckel (1866), included all species closer to extant birds (Neornithes) than to Archaeopteryx lithographica, the sole occupant of Haekel’s Sauriurae (“reptile tail”). During the last 25 years, Martin (1983) and colleagues have attempted to ally Enantiornithes and other basal birds with Archaeopteryx in Sauriurae—a configuration that has not survived a reasonable cladistic test. Martin’s use of Ornithurae, nonetheless, was consistent with its traditional use in that it included all species closer to extant birds than to Archaeopteryx lithographica (Sereno 1999). This same interpretation was codified in Gauthier’s (1986) initial stem-based definition of Ornithurae. The active definition in this compilation uses species as specifiers and constitutes a first-order revision of Gauthier’s (1986) historically-informed, clear initial definition of Ornithurae.

Chiappe (1995:350), in contrast, redefined Ornithurae as a less inclusive, node-based taxon: “All birds derived from the common ancestor of Hesperornithiformes and Neornithes.” This matches the taxonomic content of Martin’s Ornithurae, which is presumably the raison d’etre for the redefinition. It conflicts with a historical interpretation of Ornithurae as a receptacle for taxa closer to extant birds, Gauthier’s original definition, Martin’s stated intentions, and apomorphies suggested by the etymology of the taxon. Although the redefinition has been questioned for the aforementioned reasons (Sereno 1998, 1999b), there has been no defense. Use of Ornithurae sensu Chiappe was simply designated the interpretation of choice for contributors to a recent volume on early bird evolution (Chiappe and Witmer 2001; see Sereno 2001).

In the same year, Gauthier and de Queiroz (2001:27-29) also chose to redefine Ornithurae, this time as an “apomorphy-based” taxon: The clade stemming from the first panavian with a “bird tail.” For some, the presence of a pygostyle—most simply understood as a drastically shortened tail with fused distal caudal vertebrae—is the essence of a “bird tail” and would justify placing the taxon exactly as in Gauthier (1986), namely, for all birds more advanced that Archaeopteryx all of which still exhibit this morphology. While admitting that this etymological interpretation of Ornithurae has a pedigree dating back to the nineteenth century, Gauthier and de Queiroz (2001:27-29) decided to take a more rarefied view—suggesting that Haeckel’s (1866) intention was the identification of a clade based on the “characteristic tail morphology of all modern birds.” They used this to justify listing many additional apomorphies under the concept “bird tail,” while encouraging future taxonomists “describing new taxa that bridge current morphological gaps” to “refine the definitions of these characters as necessary.” The many problems with a definition of this sort are discussed elsewhere (Sereno 2005a).