Thescelosauridae

Thescelosauridae is a group of small to medium-sized, two-legged neornithischian dinosaurs that lived during the Cretaceous in East Asia and North America. The name was first used by Charles M. Sternberg in 1937, but it wasn’t given a formal definition until 2013, when Brown and his team used it to describe the clade containing *Thescelosaurus* and *Orodromeus*, based on their phylogenetic work.

The family Hypsilophodontidae was once thought to be a natural group in the early 1990s, but that idea has since been abandoned. Instead, it turned out to be an unnatural collection of animals with varying degrees of closeness to Iguanodontia—essentially an evolutionary grade rather than a true clade. *Thescelosaurus* itself has been considered both very primitive and very advanced among these so-called hypsilophodonts. A complicating factor is that not all fossils assigned to *Thescelosaurus neglectus* necessarily belong to that species. Clint Boyd and colleagues found that the genus *Thescelosaurus* included *Bugenasaura* and its species, but there were at least two, possibly three, species within the genus, and several specimens once labeled *T. neglectus* couldn’t be tied to any specific species. *Thescelosaurus* seems closely related to *Parksosaurus*, though this link has been questioned.

After Hypsilophodontidae fell apart, Buchholz in 2002 recognized a distinct family called Parksosauridae, defined as the most inclusive clade containing *Parksosaurus warreni* but not *Hypsilophodon foxii*, *Dryosaurus altus*, or *Iguanodon bernissartensis*. Boyd et al. (2009) and Brown et al. (2011) sorted Cretaceous North American “hypsilophodonts” into two related groups: one (Orodrominae) with *Orodromeus*, *Oryctodromeus*, and *Zephyrosaurus*, and the other (Thescelosaurinae) with *Parksosaurus* and *Thescelosaurus*. Brown et al. (2013) got similar results, adding *Albertadromeus* to the Orodrominae group and several long-snouted Asian forms (previously placed in Jeholosauridae) to Thescelosaurinae.

In a 2015 revision of neornithischians, Clint Boyd attributed the name Thescelosauridae to Brown and colleagues. That meant Parksosauridae, informally defined in 2002 by Buchholz, would have had priority, since both clades contained the same taxa. Boyd therefore used Parksosauridae, assuming it came first.