[4] However, in formalizing the clade following the regulations of the PhyloCode, Madzia, Boyd, and colleagues identified in 2021 that Sternberg was the proper authority for Thescelosauridae, giving it priority over Parksosauridae.

As well, they gave Thescelosauridae the definition of the largest clade containing Thescelosaurus neglectus but not Iguanodon bernissartensis, as long as Hypsilophodon foxii was not in the group, modifying previous definitions for Thescelosauridae in order to maintain its modern use, so that the clade was not applied if Thescelosaurus fell within Hypsilophodontidae, a family that has not been recently used but may be revived if the systematic position of Hypsilophodon was solidified at some point in the future.

[2][5] Lesothosaurus Agilisaurus Hexinlusaurus Yandusaurus Kulindadromeus Jeholosaurus Yueosaurus Nanosaurus Hypsilophodon Cerapoda Notohypsilophodon Parksosaurus Thescelosaurus assiniboiensis Thescelosaurus garbanii Thescelosaurus neglectus Haya Changchunsaurus Koreanosaurus Oryctodromeus Albertadromeus Kaiparowits orodromine Orodromeus Zephyrosaurus Although Hypsilophodontidae was interpreted as a natural group in the early 1990s,[6][7] this hypothesis has fallen out of favor and Hypsilophodontidae has been found to be an unnatural family composed of a variety of animals more or less closely related to Iguanodontia (paraphyletic), with various small clades of closely related taxa.

One issue that has potentially interfered with classifying Thescelosaurus is that not all of the remains assigned to T. neglectus necessarily belong to it.

[16] The dissolution of Hypsilophodontidae was followed by the recognition of the distinct family Parksosauridae by Buchholz in 2002, defined as the most inclusive clade containing Parksosaurus warreni, but not Hypsilophodon foxii, Dryosaurus altus, or Iguanodon bernissartensis.