The clade Ornithischia is defined by several anatomical traits shared by all its members. The clade is considered a natural group of dinosaurs sharing a common ancestor. All ornithischians were herbivorous; some were bipedal and others quadrupedal. They ranged widely in size, from small bipeds such as Heterodontosaurus (Early Jurassic, Africa) and Hypsilophodon (Early Cretaceous, United States, England, and Portugal) that were no more than 3 or 4 feet (1 m) long to the tank-sized horned dinosaurs such as Triceratops (Late Cretaceous, western North America) and bipedal "duck-bill" dinosaurs such as Edmontosaurus (Late Cretaceous, western North America) that may have been more than 40 feet (12 m) long and weighed more than 3 tons (2.7 metric tons). The ornithischians' diversity grew until, by the Cretaceous Period, they were the most common group of plant-eating dinosaurs—quite a feat for a clade that barely held its own during the rapid rise of the Saurischia.
Despite the seemingly vast differences in body size and anatomy of the ornithischians, they are joined by several key traits.
Ornithischian pelvis. The basis for the name of this clade was the presence of a pelvic girdle with a backward-pointing pubis bone.
Predentary bone. Found in all ornithischians, the predentary was a single, scoop-shaped bone that capped the front of the bottom jaw.
Toothless beak. The front tip of the snout (upper jaw) was a toothless beak with a roughened texture.
Leaf-shaped cheek teeth. All ornithischians had leaf-shaped teeth with a triangular crown and narrow roots. Tooth rows were set back from the front of the mouth, in the cheek region. Some later ornithischians developed highly complex batteries of these teeth, specialized for grinding tough plants.
Reduced antorbital fenestrae. The hole, or window, in the skull in front of the eye was reduced in size in the ornithischians.
Eyelid bone. The so-called eyelid or palpebral bone was a narrow bone across the outside of the eye socket, or orbit.
Five or more sacral vertebrae. The number of fused sacral vertebrae added to the strength of the ornithischian frame and aided the animals' mobility.
Reduced fifth toe. The fifth toe of the ornithischian was reduced to a small stub with no joints.
Ossified tendons. Ossified tendons (tendons that turned into bone) located around the sacral region of the vertebral column and, often, the tail provided stiffening for balance.
This list represents the traits that were found in all ornithischi-ans. This set of features also defines a common point of departure from which evolution took off to deliver some of the most unusual, specialized, and ornamented tetrapods ever to walk the planet.
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