For most of this century, dinosaurs have been divided into Ornithischia and Saurischia either as independent groups or as subdivisions of Dinosauria. Dinosaurian monophyly was reestablished in the cladistic era with birds as descendants. The first definition for Dinosauria was proposed by Novas (1992:60) as “The common ancestor of Herrerasauridae plus Saurischia + Ornithischia, and all of its descendants.” Saurischia and Ornithischia were joined as Eudinosauria, a taxon that never gained currency.

Padian & May (1993:379) proposed a second-order revision, intentionally assigning Dinosauria to a less inclusive clade as “All descendants of the most recent common ancestor of birds and Triceratops.” This clade, equivalent to Novas’ Eudinosauria, is specified in all subsequent definitional variations for Dinosauria (Sereno 1998, Olshevsky 2000, Fraser et al. 2002). The active definition, a first-order revision of that given by Padian & May (1993), uses Triceratops horridus and Passer domesticus as specifiers.

It makes little sense to use the oldest genera or species as specifiers just because they were originally included by the nominal author of a taxon. Clarke (2004), for example, used the poorly understood genus Megalosaurus as a specifier for Dinosauria. Historic nominal authors often included taxa later found to have disparate relationships (e.g. Cope’s original Archosauria). The prevailing taxonomic content of a taxon over time is what constitutes prior use and is most important to recognize. Using Passer domesticus as a specifier for Dinosauria does mandate the inclusion of birds, something that Owen would not have been anticipated. Taxonomy, nonetheless, need not be shackled by the work of nineteenth century taxonomists, whose access to paleontologic and comparative information was far more limited.