The only group of dinosaurs which survived the end-Cretaceous extinction were the theropods - as birds. All were bipedal, and many Mesozoic forms were carnivorous and bore sharp teeth in their jaws (like Tyrannosaurus and Deinonychus). However, therizinosaurs seem to have been herbivorous, and other groups either reduced (oviraptorosaurs) or lost (ornithomimids, modern birds) their teeth.
Sauropodomorphs include primitive forms ("prosauropods") and true sauropods, which include the largest terrestrial animals of all time. Aptly described by Ann Elk in a Monty Python sketch as being "thin at one end, much, much thicker in the middle, and then thin again at the far end", sauropodomorphs were all herbivorous. All forms had small heads relative to body size, and jaws which lacked chewing teeth.
One of the first dinosaurs ever described was an ornithopod, the thumb spike-wielding Iguanodon. However, the most easily recognisable ornithopods are the duck-billed hadrosaurs, some of which had elaborate head crests. Ornithopods, which were exclusively herbivorous, varied greatly in size and shape from the diminutive, bipedal and fleet-footed Hypsilophodon to the ponderous, hulking Shantungosaurus.
Most of the thyreophorans, or "shield bearers", can be placed in one of two main groups: the plated and spiked stegosaurs; and the armoured (and sometimes club-tailed) ankylosaurs. All thyreophorans, except the most primitive forms, were probably only able to walk on all fours, and all were herbivorous. They were the best-armoured dinosaurs to evolve, and their remains are known from every continent.
The "fringe-headed" Marginocephalia includes two main groups: the bipedal, thick-skulled and often dome-headed pachycephalosaurs; and the frilled (and often horned) ceratopsians. Early ceratopsians were probably equally able to walk bipedally or quadrupedally, whereas the later ceratopsids were exclusively quadrupedal. All marginocephalians were herbivorous, and most (if not all) hail from the Northern Hemisphere.