Hypsilophodont

Hypsilophodonts (named after the Hypsilophus genus of iguana lizard, literally meaning "high-crested tooth") were small ornithopod dinosaurs, regarded as fast, herbivorous bipeds on the order of 1–2 meters long (3.3-6.6 feet). They are known from Asia, Australia, Europe, New Zealand, North America, and South America, from rocks of Middle Jurassic to late Cretaceous age. The group traditionally has included almost all bipedal bird-hipped dinosaurs other than iguanodonts, and some early phylogenetic analyses found it to be a natural group,^[1][2] but recent studies have found that the group is mostly paraphyletic and the taxa within represent a series leading up to Iguanodontia.^{[3][4][5][6][7][8][9]} Some of these studies have moved some traditional hypsilophodonts out of the Ornithopoda.^[10][11] Thus, the only certain member at this time is Hypsilophodon. This area of the dinosaur family tree is complicated by a lack of research.

Contents 1 Relationships 1.1 Alternate versions and new data 2 Paleobiology 3 References

Relationships

The following cladogram of hypsilophodont relationships depicts the paraphyletic hypotheses; the "natural Hypsilophodontidae" hypothesis has been falling out of favor since the mid-late 1990s. It is after Boyd et al. (2009), the most recent analysis of hypsilophodonts.^[9] Ornithischia, Ornithopoda, and Iguanodontia were not designated in their result, and so are left out here. Additional ornithopods beyond Tenontosaurus are omitted. Dinosaurs traditionally described as hypsilophodonts are found from Agilisaurus or Hexinlusaurus to Hypsilophodon or Gasparinisaura.

unnamed	Scutellosaurus unnamed Lesothosaurus unnamed Heterodontosaurus unnamed Agilisaurus unnamed Hexinlusaurus unnamed Othnielosaurus unnamed unnamed unnamed unnamed Oryctodromeus unnamed Orodromeus Zephyrosaurus unnamed Parksosaurus Thescelosaurus unnamed Hypsilophodon unnamed Gasparinisaura unnamed Tenontosaurus Other ornithopods

In the most recent review of hypsilophodonts, by Norman et al. (2004),^[7] the following genera were known well enough to include in a phylogenetic analysis:

• Agilisaurus
• A. multidens (now Hexinlusaurus)^[12]
• Bugenasaura (now regarded as a junior synonym of Thescelosaurus)^[9]
• Gasparinisaura
• Hypsilophodon
• Orodromeus
• Othnielia (now Othnielosaurus)^[13]
• Koreanosaurus^[14]
• Parksosaurus
• Thescelosaurus
• Zephyrosaurus

The following genera were regarded as valid, but weren't classified:^[7]

• Anabisetia
• Atlascopcosaurus
• Drinker
• "Gongbusaurus" wucaiwanensis (= "Eugongbusaurus")
• Fulgurotherium
• Jeholosaurus
• Leaellynasaura
• Notohypsilophodon
• Qantassaurus
• Yandusaurus
• "Proctor Lake hypsilophodont"

Several other genera belong here somewhere, but are very poorly known or outright dubious:^[7]

• Laosaurus
• Nanosaurus
• Phyllodon
• Siluosaurus

The only new hypsilophodont genera named since this publication are Changchunsaurus^[15] and Oryctodromeus^[8]

Alternate versions and new data

Several recent studies on the base of Ornithischia, such as Butler (2005),^[16] Barrett et al. (2005),^[17] Xu et al. (2006),^[18] and Butler et al. (2007)^[10] have found different arrangements of basal Ornithischia and basal ornithopods that have some bearing on the tree, but each has found different relationships. Given the lack of work on this area since the 1980s, this is not unexpected. One aspect is the reclassification of several genera outside of Ornithopoda, such as Agilisaurus and Othnielosaurus (=Othnielia).^[8][11]

Varricchio et al. (2007) found that their new genus, Oryctodromeus, forms a clade with two other genera from Montana, Orodromeus and Zephyrosaurus.^[8] This relationship was also found by Boyd et al. (2009), who also recovered a Parksosaurus–Thescelosaurus clade more closely related to it than to other ornithopods, which would unite all North American Cretaceous hypsilophodonts in one clade.^[9]

Paleobiology

Hypsilophodonts were small (often 1–2 m [3.28 to 6.56 ft (2.00 m)] long), bipedal, cursorial herbivores.^[7] The typical informal comparison has been to gazelles. However, there is evidence that at least some of them made burrows as places to rear their young.^[8] While so far only known for Oryctodromeus, additional possible hypsilophodont burrows have been found in slightly older rocks in Victoria, Australia.^[19]

References