What is Neornithischia?

All genasaurians more closely related to Parasaurolophus walkeri, than to Ankylosaurus magniventris or Stegosaurus stenops. Neornithischia is the great branch of ornithischian dinosaurs that includes all the familiar beaked plant-eaters except the armoured ankylosaurs and stegosaurs. These dinosaurs inherited the basic feeding toolkit of Genasauria—most notably, teeth sat slightly inset from the outer edge of the jaws, creating space for cheeks that helped keep food in the mouth while chewing. But they also added an important new innovation of their own: a thick layer of enamel on the inner surfaces of their lower teeth, which wore out unevenly, creating sharp ridges that acted like natural serrated blades, allowing neornithischians to slice and grind tougher, more fibrous plants than earlier dinosaurs could manage. That improvement in dentition opened the door to a remarkable evolutionary radiation, which eventually gave rise to Marginocephalia and Ornithopoda—the latter of which would take oral processing to the next level.

Neornithischians experimented with a variety of anatomical and ecological strategies while remaining relatively small and agile during the early stages of their history. In the skeleton, the upper arm (humerus) became noticeably longer than the sshoulder blade (scapula), a departure from more primitive ornithischian proportions. Exceptional fossils of Kulindadromeus, a probable close relative of Cerapoda, reveal that early neornithischians possessed a surprisingly complex body covering, including simple scales, plate-like scales, filamentous structures, and unique scale-like plates from which filaments sprouted, showing that complex body coverings were not restricted to theropods.

Neornithischians became one of the dominant herbivorous components of Jurassic and Cretaceous ecosystems. Through their two major descendant branches, Thescelosauridae and Cerapoda, they occupied an extraordinary variety of niches and evolved some of the most sophisticated plant-processing adaptations in dinosaur history. They also provide some of the best evidence for the diversity of dinosaur skin coverings, social behaviour, and ecological specialisation. Their descendants survived until the end-Cretaceous extinction, when all non-avian dinosaurs disappeared, leaving behind a fossil record that continues to reshape our understanding of dinosaur biology.

Neornithischia and Cerapoda, as eventually defined, included pretty much the same critters. Technically, the latter should've been sunk into the former as a junior synonym because it was named a year later and, in fact, it was... by its own author Paul Sereno! However, after a little rejig, Neornithischia now houses Cerapoda (Marginocephalia and Ornithopoda) plus a small handful of non-cerapod neornithischians (some of which are grouped as Jeholosauridae and Thescelosauridae).

Click here For DinoChecker's A-Z list of Neornithischians.