Iguanodontia was first defined by Sereno (1998) to include all ornithopods closer to hadrosaurids than to hypsilophodontids. Since that time, some analyses have shown Hypsilophodontidae as paraphyletic, with some genera closer to hadrosaurids than others (e.g. Weishampel et al. 2003). To accommodae these alternative hypotheses while maintaining stability in taxonomic content, Thescelosaurus neglectus is added to the definition as an external specifier.
Gasparinisaura concosaltensis was originally described as an iguanodontian (Coria and Salgado 1996) but has since been positioned among hypsilophodontids (Weishampel et al. 2003). Gasparinisaura thus is not included as a specifier.