Article Outline
Some ornithischians quickly became quadrupedal (four-legged) and relied on body armor and other physical defenses rather than fleetness for protection. Plated dinosaurs, such as the massive Stegosaurus of the late Jurassic Period, bore a double row of triangular bony plates along their backs. These narrow plates contained tunnels through which blood vessels passed, allowing the animals to radiate excess body heat or to warm themselves in the sun. Many also bore a large spined plate over each shoulder. Stegosaurs resembled gigantic porcupines, and they probably defended themselves by turning their spined tails toward aggressors.
The armored ankylosaurs appeared during the Jurassic Period, but their early fossils are less common than those of stegosaurs. During the Cretaceous Period, stegosaurs were supplanted by armored dinosaurs such as Ankylosaurus. These animals were similar in size to stegosaurs but otherwise resembled giant horned toads. Some even possessed a bony plate in each eyelid as well as large tail clubs. Their necks were protected by heavy bony rings and spines, indicating that these areas needed protection from the attacks of carnivorous dinosaurs.
Ornithopods increased in variety from the middle Jurassic through the Cretaceous periods and became the most abundant plant-eating dinosaurs. They ranged in size from small runners that were 2 m (6 ft) long and weighed 15 kg (33 lb), such as Hypsilophodon, to elephantine cows that were 10 m (32 ft) long and weighed 4 metric tons, such as Edmontosaurus. These animals had flexible jaws and grinding teeth, which eventually surpassed those of modern cows in their suitability for chewing fibrous plants. The beaks of ornithopods became broader, earning them the name duck-billed dinosaur. Their tooth batteries became larger, their backs became stronger, and their forelimbs lengthened until their arms became elongated walking sticks, although ornithopods remained bipedal. The nose supported cartilaginous sacks or bony tubes, suggesting that these dinosaurs may have communicated by trumpeting. Fossil evidence from the late Cretaceous Period includes extensive accumulations of bones from ornithopods drowned in floods, indicating that duck-billed dinosaurs often migrated in herds of thousands. A number of spectacularly preserved fossils found in parts of North America preserve skin impressions, traces of muscles and other soft tissues, and possibly evidence of internal organs.
Pachycephalosaurs were small bipedal ornithischians with thickened skulls, flattened bodies, and tails surrounded by a latticework of bony rods. In many of these dinosaurs, such as the Pachycephalosaurus—a large specimen up to 8 m (26 ft) long—the skull was capped by a rounded dome of solid bone. Some paleontologists suggest that males may have borne the thickest domes and butted heads during mating contests. Eroded pachycephalosaur domes are often found in stream deposits from late in the Cretaceous Period.
The quadrupedal ceratopsians, or horned dinosaurs, typically bore horns over the nose and eyes, and had a saddle-shaped bony frill that extended from the skull over the neck. Some smaller early forms lacked the well-developed horns and frill, and were mainly bipedal, however. The bony frill was well developed in the late Cretaceous Triceratops, a dinosaur that could reach lengths of up to 8 m (26 ft) and weights of more than 12 metric tons. The frill served two purposes: It protected the vulnerable neck, and it contained a network of blood vessels on its undersurface to radiate excess heat. Large accumulations of fossil bones suggest that ceratopsians lived in herds. Ceratopsians found in Asia such as the 1.8-m (6-ft)-long Protoceratops did not reach the large size of North American forms.
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