The active definition is a first-order revision of the original and only previous definition (Sereno 1998) that includes additional external specifiers to ensure stability of taxonomic content under alternative hypotheses of the higher-level phylogeny of Ornithischia.
Recently Butler (2005) proposed an alternative cladogram disbanding taxa central to this clade. His strict consensus tree (Butler 2005:fig. 25) does not show any special relationship between taxa regarded as hypsilophodontids and Iguanodontia. This unusual result, which differs from Sereno (1986, 1999) and all previous quantitative analyses, would appropriately eliminate use of the taxon Euornithopoda, as here defined and as used in the literature since the mid 1980’s. Butler’s result, however, is unconventional and may well not survive future cladistic tests. The point here, nonetheless, is that the active definition restricts use of Euornithopoda to clades that maintain taxonomic content similar to its previous usage in the literature.