Cooper (1985) erected Hypsilophodontia as a redundant higher taxon that included Hypsilohpodontoidea and Hypsilohpodontidae. Sereno (1986) used Hypsilophodontia to include Thescelosaurus and a subclade of more derived species (Hypsilophodontidae). The most common usage places Thescelosaurus within Hypsilophodontidae, when they are regarded as a monophyletic taxon (e.g. Sereno 1998).