Thescelosaurids Temporal range:Early–Late Cretaceous,130–66 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Mounted specimen ofThescelosaurus,Burpee Museum of Natural History
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Dinosauria
Clade:	†Ornithischia
Clade:	†Pyrodontia
Family:	†Thescelosauridae Sternberg,1937
Subfamilies[2]
†OrodrominaeBrownet al, 2013 †Albertadromeus †Koreanosaurus †Nevadadromeus? †Orodromeus †Oryctodromeus †Zephyrosaurus †ThescelosaurinaeSternberg, 1940 †Changchunsaurus? †Changmiania? †Fona[1] †Haya? †Jeholosaurus? †Nevadadromeus? †Oryctodromeus? †Parksosaurus †Thescelosaurus †Yueosaurus?
Synonyms[2]
ParksosauridaeBuchholtz, 2002 Jeholosauridae?Hanet al., 2012

Thescelosauridae is a clade ofneornithischians from theCretaceous ofEast Asia andNorth America. The group was originally used as a name byCharles M. Sternberg in1937, but was not formally defined until2013, where it was used by Brown and colleagues as the group unitingThescelosaurus andOrodromeus, based on their phylogenetic results.^[2][3] During a phylogenetic revision of neornithischians by Clint Boyd in2015, the authorship of Thescelosauridae was given to Brown and colleagues, which meant that the similar nameParksosauridae, informally defined in2002 by Buchholz, would have had priority over Thescelosauridae. The two clades had slightly different definitions, with Parksosauridae referring to all animals closer toParksosaurus thanHypsilophodon, but they contained the same taxa so Boyd used Parksosauridae under the assumption it had priority.^[4] However, in formalizing the clade following the regulations of thePhyloCode, Madzia, Boyd, and colleagues identified in2021 that Sternberg was the proper authority for Thescelosauridae, giving it priority over Parksosauridae. As well, they gave Thescelosauridae the definition of the largest clade containingThescelosaurus neglectus but notIguanodon bernissartensis, as long asHypsilophodon 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 ifThescelosaurus fell withinHypsilophodontidae, a family that has not been recently used but may be revived if the systematic position ofHypsilophodon was solidified at some point in the future.^[2] Madziaet al. identified the analysis of Madziaet al. in2018 as the reference analysis for the name Thescelosauridae, an analysis based on a revised version of the 2015 Boyd analysis.^[2][5]

AlthoughHypsilophodontidae was interpreted as a natural group in the early 1990s,^[6][7] 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 smallclades of closely related taxa.^{[8][9][10][11][12]} "Hypsilophodontidae" and "hypsilophodont" are better understood as informal terms for anevolutionary grade, not a true clade.Thescelosaurus has been regarded as both very basal^[7] and veryderived^[10] among the hypsilophodonts. One issue that has potentially interfered with classifyingThescelosaurus is that not all of the remains assigned toT. neglectus necessarily belong to it.^[13] Clint Boyd and colleagues found that while the cladeThescelosaurus included the genusBugenasaura and the species that had been assigned to that genus, there were at least two and possibly three species withinThescelosaurus, and several specimens previously assigned toT. neglectus could not yet be assigned to a species within the genus.^[8] It appears to be closely related toParksosaurus,^{[8][10][11][14][15]} although this relationship has been called into question.^[16]

The dissolution ofHypsilophodontidae was followed by the recognition of the distinct family Parksosauridae by Buchholz in 2002, defined as the most inclusive clade containingParksosaurus warreni, but notHypsilophodon foxii,Dryosaurus altus, orIguanodon bernissartensis.^[17][4] Boydet al. (2009) and Brownet al. (2011) found North American "hypsilophodonts" of Cretaceous age to sort into two related clusters, one (Orodrominae) consisting ofOrodromeus,Oryctodromeus, andZephyrosaurus, and the other (Thescelosaurinae) consisting ofParksosaurus andThescelosaurus.^[8][14] Brownet al. (2013) recovered similar results, with the addition of the new genusAlbertadromeus to theOrodromeus clade (calledOrodrominae) and several long-snouted Asian forms (previously described underJeholosauridae)^[9] to theThescelosaurus clade (calledThescelosaurinae).^[15]

Orodromines may haveburrowed, as is known forOryctodromeus which was found in burrows, based upon the packing of their bones in situations where they typically would have been scattered.^[12]

• Thescelosauridae
• Cretaceous dinosaurs
• Dinosaur families

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