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, 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 analyses found it to be a natural group, but recent studies have found that the group is mostly paraphyletic and the taxa within represent a series leading up to Iguanodontia. Some of these studies have moved some traditional hypsilophodonts out of the Ornithopoda. Thus, the only certain member at this time is Hypsilophodon. This area of the dinosaur family tree is complicated by a lack of research.
}} }} }} This cladogram is based on Norman et al. (2004), the most recent review, with the results of the very similar cladogram from Weishampel et al. (2003) used to clarify the position of Iguanodontia, which was left out of Norman et al. Thescelosaurinae is currently used informally.
The following genera were regarded as valid, but weren't classified:
Several other genera belong here somewhere, but are very poorly known or outright dubious:
Several recent studies on the base of Ornithischia, such as Butler (2005), Barrett et al. (2005), Xu et al. (2006), and Butler et al. (2007) 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).