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Dinosauria itself was re-defined as the last common ancestor of Triceratops horridus, Passer domesticus, Diplodocus carnegii, and all of its descendants, to ensure that sauropods and kin remain included as dinosaurs.
Many prehistoric animal groups are popularly conceived of as dinosaurs, such as ichthyosaurs, mosasaurs, plesiosaurs, pterosaurs, and pelycosaurs (especially Dimetrodon), but are not classified scientifically as dinosaurs, and none had the erect hind limb posture characteristic of true dinosaurs.
Dinosaurs have always been an extremely varied group of animals; according to a 2006 study, over 500 non-avian dinosaur genera have been identified with certainty so far, and the total number of genera preserved in the fossil record has been estimated at around 1850, nearly 75% of which remain to be discovered.
Some are herbivorous, others carnivorous, including seed-eaters, fish-eaters, insectivores, and omnivores.
Birds are now recognized as being the sole surviving lineage of theropod dinosaurs.A variety of other skeletal features are shared by dinosaurs.However, because they are either common to other groups of archosaurs or were not present in all early dinosaurs, these features are not considered to be synapomorphies.Nesbitt found a number of further potential synapomorphies, and discounted a number of synapomorphies previously suggested.Some of these are also present in silesaurids, which Nesbitt recovered as a sister group to Dinosauria, including a large anterior trochanter, metatarsals II and IV of subequal length, reduced contact between ischium and pubis, the presence of a cnemial crest on the tibia and of an ascending process on the astragalus, and many others.