A new stem-based taxon, Ornithomimiformes, is proposed to accommodate clades, such as Alvarezsauridae, that may well be closer to ornithomimosaurs than to birds. If no clades are positioned as sister taxa to Ornithomimosauria, then the definitional type and content of Ornithomimiformes would be the same as Ornithomimosauria, which would have priority. The reasons for erecting this taxon (and another in similar circumstances—Oviraptoriformes) come from the negative reaction to broadening the taxonomic content of existing taxa (Ornithomimosauria, Oviraptorosauria) as explained below.
Osmólska (1997) was first to define Ornithomimosauria using a stem-based definition. This definition, however, was constructed with the expectation that troodontids constitute the immediate sister taxon (following Holtz 1994), a relationship that has not been supported in subsequent analyses. Sereno therefore modified Osmólska’s stem-based definition as “All maniraptoriforms closer to Ornithomimus than to Neornithes” (Sereno 1998:65). Although not altogether that different from Osmólska’s initial definition, several authors have objected to the possible inclusion of alvarezsaurids or therizinosaurids within Ornithomimosauria when they are positioned as the closest relatives of ornithomimids (e. g. Padian et al. 1999, Kobayashi and Lü 2003, Makovicky et al. 2004). Sereno’s definition was an attempt to make use of an existing name (Ornithomimosauria) that was originally coined in redundancy with Ornithomimidae. Although Ornithomimidae has gradually assumed a less inclusive meaning with the discovery of new basal members, the family has been used in recent literature to circumscribe the entire clade (e. g. Galton and Smith 1990). Nonetheless, it is true that the taxonomic content of Ornithomimosauria as originally conceived by Barsbold (1976) would be markedly altered if other clades were included as a result of Sereno’s definition.
Padian et al. (1999) proposed an independent solution to the problem presented by Osmólska’s original stem-based definition by presenting a node-based definition based on Pelecanimimus and Ornithomimus. This definition has the advantage of overlapping completely with its traditional taxonomic content. It has disadvantage that any newly discovered, closely-related form positioned just outside the named clade will be excluded. The history of Ornithomimosauria, like many clades, is that the younger, more diverse taxa are found first, with ever more basal forms trickling in as the decades pass. Osmolska’s definition, doubtless, was proposed to include newly found basal forms. After all, had Pelecanimimus been discovered today rather than a decade ago (Perez-Moreno et al.1994), it would have been excluded from Ornithomimosauria. As it stands, there is little evidence to position the recently discovered Shenzhousaurus (Ji et al. 2003) within a clade so defined that is sometimes shown without basal resolution (Norell et al. 2001, Xu et al. 2002). The authors preferring a node-based definition for Ornithomimosauria fail to mentionOsmólska’s original stem-based definition (Padian et al. 1999, Kobayashi and Lü 2003, Makovicky et al. 2004).
Given the drawbacks of both stem- and node-based definitions of Ornithomimosauria reviewed above, a more fruitful option followed here respects Osmólska’s original definitional type and taxonomic content, maintains current content while allowing the incorporation of newfound basal forms. The active definition of Ornithomimosauria is stem-based using the eponymous Ornithomimus edmontonicus as an internal specifier but excludes all clades that have ever been proposed to have a close relationship with ornithomimosaurs. A new stem-based taxon, Ornithomimiformes, is erected to accommodate clades such as Alvarezsauridae that may be closer to ornithomimids than to birds. The principal drawback here is that Holtz (1996, 2000) has redefined Arctometatarsalia from a form-qualified (“apomorphy-based”) definition to one that overlaps with that given here for Ornithomimiformes. This new version of Arctometatarsalia differs so markedly from its original definitional type and taxonomic content (Holtz 1994) that priority in this particular case is set aside.