Parksosauridae is a clade or family of small ornithischians which have previously been generally allied to hypsilophodontids. Parksosauridae is often considered a synonym of Thescelosauridae, but the two groups cannot be synonyms because Parksosauridae is defined as a stem, while Thescelosauridae is defined as a node.
Classification
Although Hypsilophodontidae was interpreted as a natural group in the early 1990s, 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, with various small clades of closely related taxa. "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 and very derived among the hypsilophodonts. One issue that has potentially interfered with classifyingThescelosaurus is that not all of the remains assigned to T. neglectus necessarily belong to it. 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. It appears to be closely related to Parksosaurus. The dissolution of Hypsilophodontidae has been followed by the recognition of the distinct family Parksosauridae. This area of the dinosaur family tree has historically been complicated by a lack of research, but papers by Clint Boyd and colleagues and Caleb Brown and colleagues have specifically addressed these dinosaurs. Boyd et al. and Brownet al. found North American "hypsilophodonts" of Cretaceous age to sort into two related clusters, one consisting ofOrodromeus, Oryctodromeus, and Zephyrosaurus, and the other consisting of Parksosaurus and Thescelosaurus. Brown et al. recovered similar results, with the addition of the new genus Albertadromeus to the Orodromeus clade and several long-snouted Asian forms to the Thescelosaurus clade. They also formally defined Parksosauridae and the smaller clades Orodrominae and Thescelosaurinae. A 2015 analysis by Clint Boyd recovered Parksosaurids outside Ornithopoda, as the sister taxon to Cerapoda.
