Gauthier (1986) was first to propose a crown clade definition, later revised by Gauthier and de Queiroz (2001). In a particular sense (due to initial ignorance of any fossil record), they argued that this is the older tradition. Gauthier (1986) used Avialae to replace traditional Aves, although their phylogenetic definitions differ—one stem-based and the other node-based—so they are not synonyms. Gauthier (1986:36) explicitly defined Avialae as a stem-based taxon including all taxa closer to birds than to Deinonychosauria.
Subsequently, Gauthier and de Queiroz (2001:25-26) claimed that Gauthier (1986) really did not define Avialae. They proposed a form-qualified (“apomorphy-based”) definition based on the presence of “feathered wings . . . used for powered flight,” a definition that ranks among the most ambiguous. Gauthier (1986:36), nevertheless, was very explicit in formulating a stem-based definition for this taxon.
More recently, Clarke (2004) unwittingly proposed a new node-based definition for Avialae, attributing it in error to Gauthier (1986). Ironically, Clarke’s new definition now identifies the same node-based clade that many of us would like to continue to reserve for Aves.
If Aves is based on Archaeopteryx lithographica as the key internal specifier (as in this compilation), there is little sense to maintain another very similar taxon, Avialae, that currently, and in the forseeable future, has the same taxonomic content. Avialae is here regarded as inactive for this reason.