Sereno (1998) initially proposed a node-based definition for Ornithopoda and constructed a node-stem triplet with its two subclades, stem-based Heterodontosauridae and Euornithopoda. Without referencing this initial definition or providing further explanation, Norman et al. (2004) proposed a stem-based definition for Ornithopoda.
The active definition is a first-order revision of Sereno (1998) that also includes several negative taxon qualifiers, so that the taxon Ornithopoda will not apply to a much larger clade within Ornithischia, should heterodontosaurids prove more closely related to any other clade besides Euornithopoda. If the analysis of Sereno (1999) is confirmed by later studies, the taxon is available and would correspond fairly closely to the content with which it has been associated since the mid 1980’s.
Recently Butler (2005) has suggested that most ornithopods (i.e. what may be regarded as euornithopods) share a closer relationship with Marginocephalia than with heterodontosauirds. If this is confirmed by later studies, then the taxon Ornithopoda as here defined would not be applicable.