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Dinosaurs are a varied group of animals from taxonomic, morphological and ecological standpoints.

Clockwise from top left: Microraptor gui (a winged theropod), Apatosaurus louisae (a giant sauropod), Edmontosaurus regalis (a duck-billed ornithopod), Triceratops horridus (a horned ceratopsian), Stegosaurus stenops (a plated stegosaur), Pinacosaurus grangeri (an armored ankylosaur) They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event 201 million years ago.

Their dominance continued through the Jurassic and Cretaceous periods and ended when the Cretaceous–Paleogene extinction event led to the extinction of most dinosaur groups 66 million years ago.

Such modifications, originating in the most recent common ancestor of a certain taxonomic group, are called the synapomorphies of such a group.

Nesbitt found a number of further potential synapomorphies, and discounted a number of synapomorphies previously suggested.

It has also been suggested that Dinosauria be defined with respect to the MRCA of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria.