A clade that appeared around 230 million years ago. Its distinguishing features: an upright posture with the limbs held under the body, a perforate hip joint (an open acetabulum) and a femur whose head is directed inwards.
It is traditionally divided into Saurischia and Ornithischia according to the arrangement of the hips. Recent revisions have proposed alternative groupings, but the classical classification remains the one in widest use.
Bipedal carnivores, from the small Compsognathus (a metre long) to the colossal Tyrannosaurus and Spinosaurus (over 12 metres).
Feathers evolve among the coelurosaur theropods, first for thermoregulation and display, later adapted for flight. Modern birds descend from small maniraptorans, like Velociraptor but smaller.
Their features: hollow bones even in the giants, three working fingers on the hand, serrated teeth, sometimes replaced by beaks.
They take in the Triassic prosauropods, bipedal or facultatively quadrupedal, and the sauropods proper of the Jurassic and the Cretaceous.
The sauropods are the largest land animals that have ever lived. Patagotitan measured about 37 metres; the published estimates of its mass run from a little over 40 to a little over 80 tonnes, and the spread is that wide because none of these skeletons is complete and the weight is extrapolated from the circumference of the limb bones. A very long neck, a counterbalancing tail, pencil-shaped or spoon-shaped teeth for stripping vegetation.
To carry that mass, pneumatised bones, hollow and light, and a respiratory system like that of birds, with air sacs.
Herbivores, bipedal or facultatively quadrupedal. They take in the iguanodonts and the hadrosaurs.
The duck-billed hadrosaurs of the late Cretaceous develop complex dental batteries, with hundreds of teeth replacing one another continuously. They were probably gregarious animals, living in herds.
Parasaurolophus had a hollow crest on its skull that could have worked as a resonating chamber for low-frequency calls: the acoustic model is built on the shape of the bone, and nobody has ever recorded that sound.
A group taking in the ceratopsians and the pachycephalosaurs, brought together by bony expansions at the back of the skull.
The ceratopsians (Triceratops, Styracosaurus) have a bony frill, facial horns and a stout beak. They were probably gregarious, and there is evidence of fights between males of the same species.
The pachycephalosaurs have skull domes of compact bone which, according to mechanical models, could take a head-on blow, perhaps used in ritual head-butting contests, though the exact mechanics remain under debate.
Armoured herbivores. They take in the stegosaurs, of the Jurassic, and the ankylosaurs, mostly of the Cretaceous.
Stegosaurs such as Stegosaurus have plates along the back, probably serving for thermoregulation and display, and a tail armed with four spikes, the thagomizer, an offensive weapon against predators.
The ankylosaurs are living tanks: armour of osteoderms over the whole back, bony eyelids and, in some cases (Ankylosaurus, Euoplocephalus), a bony club at the end of the tail.