Under phylogenetic taxonomy, dinosaurs are usually defined as the group consisting of Triceratops, Neornithes or modern birds, their most recent common ancestor, and all descendants. It has also been suggested that Dinosauria be defined with respect to the most recent common ancestor of Megalosaurus and Iguanodon, because these were two of the three genera cited by Richard Owen when he recognized the Dinosauria.
Both definitions result in the same set of animals being defined as dinosaurs: "Dinosauria = Ornithischia + Saurischia", encompassing theropods which are mostly bipedal carnivores and birds, ankylosaurians or armored herbivorous quadrupeds, stegosaurians or plated herbivorous quadrupeds, ceratopsians or herbivorous quadrupeds with horns and frills, ornithopods or bipedal or quadrupedal herbivores including "duck-bills", and, perhaps, sauropodomorphs which are mostly large herbivorous quadrupeds with long necks and tails.
Many paleontologists note that the point at which sauropodomorphs and theropods diverged may omit sauropodomorphs from the definition for both saurischians and dinosaurs. To avoid instability, Dinosauria can be more conservatively defined with respect to four anchoring nodes: Triceratops horridus, Saltasaurus loricatus, and Passer domesticus, their most recent common ancestor, and all descendants.