The initial definition of Metornithes in Perle et al. (1993) as “a group within Aviale not including Archaeopteryx” is best interpreted as a stem-based definition, although not of the most typical form. It clearly referes to all members of Avialae except the basal member Archaeopteryx. Given that Avialae was defined using Archaeopteryx, the basal position of Archaeopteryx is assured by definition. Metornithes, so defined, is synonymous with Onithurae (sensu gauthier, 1986; Sereno, 1998). Ornithurae is a taxon of longstanding use and was defined years prior (Gauthier 1986).
Without comment, Chiappe (1995) proposed a node-based definition for Metornithes using Mononykus and Neornithes as specifiers. Later authors have positioned Mononykus and alvarezsaurids in various positions outside Aves (= Aviale), such that Chiappe’s definition would overlap with the definitions proposed later for several taxa in widespread use. Sereno (1997, 2001) positioned Mononykus outside ornithomimids, and so the definition would overlap with that for Maniraptoriformes. Others have positioned alvarezsaurids within Maniraptora.
Using Mononykus or another alvarezsaurid as a specifier for a large clade is inadvisable, given disagreement over the position of alvarezsaurids among coelurosaurians. Metornithes is here considered inactive.