Neornithischia

Neornithischia ("new ornithischians") is a clade of the dinosaur order Ornithischia. It is the sister group of the Thyreophora within the clade Genasauria. Neornithischians are united by having a thicker layer of asymmetrical enamel on the inside of their lower teeth. The teeth wore unevenly with chewing and developed sharp ridges that allowed neornithischians to break down tougher plant food than other dinosaurs.

Neornithischians include a variety of basal forms historically known as "hypsilophodonts", including the Thescelosauridae. As these taxa do not all form a monophyletic clade, the term 'small-bodied early diverging ornithischian' (SBEDO) has been used to refer to these as a collective group. In addition, there are derived forms classified in the groups Marginocephalia and Ornithopoda. The former includes clades Pachycephalosauria and Ceratopsia, while the latter typically includes Hypsilophodon and the more derived Iguanodontia.