Ornithischia Seeley, 1887

Included taxa. Lesothosaurus, Technosaurus, Pisanosaurus, Thyreophora, Ornithopoda, and Marginocephalia, and all dinosaurs that are more closely related to these taxa than to saurischians.

Temporal range. Camian-Maastrichtian.

Distribution. Global.

Diagnosis. Ossified palpebral over the orbit; presence of an unpaired predentary; maxillary and dentary tooth crowns triangular in lateral view; ossified epaxial tendons along the neural spines; ilium with elongate, dorsoventrally low preacetabular process; opisthopubic pelvis; pubic shafts slender, rod-like.

Remarks. Seeley (1887, 1888) first noted the fundamental differences in the pelvic girdles of different taxa of dinosaurs, and referred all the opisthopubic members then known to a clade he named Orn ithischia, in reference to the opisthopubic condition of the pelvis in birds. Whereas the monophyly of the other major group of dinosaurs, the Saurischia, has repeatedly been doubted (e.g. Charig et al. 1965; Bakker and Galton 1974; Bakker 1986), the Orn ithischia were generally accepted as a monophyletic group, even after the discovery of nonavian theropod dinosaurs with an opisthopubic pelvis (Barsbold 1979).

The monophyly of the Omithischia is supported by a large number of synapomorphies (Sereno 1986), many of which are in some way related to the herbivorous diet of all known members of this clade. Following Sereno (1986, 1991 b, 1997), Lesothosaurus (Text-fig. 4b) and basal Thyreophora (Scutellosaurus, Scelidosaurus, Emausaurus) are regarded here as some of the most basal taxa of o rn ithischians, and most character codings are based on these animals.