Parksosauridae

Although Hypsilophodontidae was interpreted as a natural group in the early 1990s,[3][4] 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 (paraphyly), with various small clades of closely related taxa.[5][6][7][8][9] "Hypsilophodontidae" and "hypsilophodont" are better understood as informal terms for an evolutionary grade, not a true clade. Thescelosaurus has been regarded as both very basal[4] and very derived[7] among the hypsilophodonts. One issue that has potentially interfered with classifying Thescelosaurus is that not all of the remains assigned to T. neglectus necessarily belong to it.[10] Clint Boyd and colleagues found that while the clade Thescelosaurus included the genus Bugenasaura and the species that had been assigned to that genus, there were at least two and possibly three species within Thescelosaurus, and several specimens previously assigned to T. neglectus could not yet be assigned to a species within the genus.[5] It appears to be closely related to Parksosaurus,[5][7][8][11][12] although this relationship has been called into question.[13]

Head and arms of Thescelosaurus, Rocky Mountain Dinosaur Resource Center

The dissolution of Hypsilophodontidae has been 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.[14][2] Boyd et al. (2009) and Brown et al. (2011) found North American "hypsilophodonts" of Cretaceous age to sort into two related clusters, one (Orodrominae) consisting of Orodromeus, Oryctodromeus, and Zephyrosaurus, and the other (Thescelosaurinae) consisting of Parksosaurus and Thescelosaurus.[5][11] Brown et al. (2013) recovered similar results, with the addition of the new genus Albertadromeus to the Orodromeus clade (called Orodrominae) and several long-snouted Asian forms (previously described under Jeholosauridae)[6] to the Thescelosaurus clade (called Thescelosaurinae). They also formally defined Thescelosauridae (Thescelosaurus neglectus, Orodromeus makelai, their most recent common ancestor, and all descendants) and the smaller clades Orodrominae and Thescelosaurinae.[12]

A 2015 analysis by Clint Boyd recovered Parksosauridae outside Ornithopoda, as the sister taxon to Cerapoda.[2] However, a 2019 analysis by Herne et al., which placed Parksosaurus as the most basal member of Ornithopoda, would effectively restrict Parksosauridae to only having the one member, Parksosaurus, as well as reviving Thescelosauridae as a separate group.[13] Below is the cladogram by Herne et al. (2019):