Thescelosaurus - Classification

Classification

Thescelosaurus has generally been allied to Hypsilophodon and other small ornithopods as a hypsilophodontid, although recognized as being distinct among them for its robust build, unusual hindlimbs, and, more recently, its unusually long skull. Peter Galton in 1974 presented one twist to the classic arrangement, suggesting that because of its hindlimb structure and heavy build (not cursorial, or built for running, by his definition), it should be included in the Iguanodontidae. This has not been followed, with Morris arguing strongly against Galton's classification scheme. At any rate, Galton's Iguanodontidae was not a natural group due to polyphyly, and so would not be recognized under modern cladistic usage.

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 (paraphyly), 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 specifically concerning Thescelosaurus 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.

With the dissolution of Hypsilophodontidae, Thescelosaurus has often been given its own family or subfamily, Thescelosauridae or Thescelosaurinae. Informal usage of these terms dates to the turn of the 21st century. 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. (2009) and Brown et al. (2011) found North American "hypsilophodonts" of Cretaceous age to sort into two related clusters, one consisting of Orodromeus, Oryctodromeus, and Zephyrosaurus, and the other consisting of Parksosaurus and Thescelosaurus. Brown et al. (2013) recovered similar results, with the addition of the new genus Albertadromeus to the Orodromeus clade and several long-snouted Asian forms (previously described under Jeholosauridae) to the Thescelosaurus clade. They also formally defined Thescelosauridae (Thescelosaurus neglectus, Orodromeus makelai, their most recent common ancestor, and all descendants) and the smaller clades Orodrominae and Thescelosaurinae.