Thescelosauridae

Thescelosauridae is a clade of neornithischians from the Cretaceous of Asia, North America and possibly South America. The group was originally used as a name by Charles M. Sternberg in 1937, but was not formally defined until 2013, where it was used by Brown and colleagues as the group uniting ''Thescelosaurus'' and ''Orodromeus'', based on their phylogenetic results. During a phylogenetic revision of neornithischians by Clint Boyd in 2015, the authorship of Thescelosauridae was given to Brown and colleagues, which meant that the similar name Parksosauridae, informally defined in 2002 by Buchholz, would have had priority over Thescelosauridae. The two clades had slightly different definitions, with Parksosauridae referring to all animals closer to ''Parksosaurus'' than ''Hypsilophodon'', but they contained the same taxa so Boyd used Parksosauridae under the assumption it had priority. 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. Madzia ''et al.'' identified the analysis of Madzia ''et al.'' in 2018 as the reference analysis for the name Thescelosauridae, an analysis based on a revised version of the 2015 Boyd analysis.

History

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 (paraphyletic), 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 classifying ''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'', although this relationship has been called into question.

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''. 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''. 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) to the ''Thescelosaurus'' clade (called Thescelosaurinae).

References

{{Taxonbar, from=Q13574271

Ornithischians
Cretaceous dinosaurs
Maastrichtian extinctions
Prehistoric dinosaur families