The nearly 400 species of ornithischians were entirely herbivorous. This is evident from their leaf-shaped teeth, which bear small denticles.
They also evolved an entirely new bone at the front of the lower jaw (the predentary). The name Ornithischia ("bird hip") refers to the fact that the pubis is
turned backward in the pelvis. Many forms also had a basket-like mesh of bony tendons along the backbone and tail. Ornithischians first appeared in the Late
Triassic Period, but the earliest forms (such as Pisanosaurus and Galtonia) were quite rare despite being widespread.
By the Early Jurassic Period, ornithischians had begun to diversify. This diversity included the armored dinosaurs, thyreophorans
(“shield bearers”). The first thyreophorans (Scutellosaurus) were small bipeds, but soon the group evolved into large quadrupeds either
with plates and spikes (stegosaurs, “covered lizards”) or with a dense covering of bony scutes and shields
(ankylosaurs, “curved lizards”). Some ankylosaurs had a club of bone at the end of the tail, as well as bony eyelids. Both
stegosaurs and ankylosaurs were large, reaching a few tons in weight and eight meters in length. Stegosaurs appeared in the Middle Jurassic but went extinct
during the Cretaceous, whereas ankylosaurs lasted from the Middle Jurassic through to the end of the Cretaceous.
A second major group of ornithischians were
the marginocephalians (“margin heads”). These dinosaurs developed a ridge of knobby bone around the back edge of the skull.
Among them, pachycephalosaurs (“thick head lizards”) thickened the bony roof of the skull over the brain into a
“dome” several centimeters thick (as in Pachycephalosaurus). Much of the rest of the skull was covered in knobs and spikes. These
bipedal ornithischians were relatively small (a few meters long) and lived from the Early Cretaceous (or perhaps Late Jurassic) to the end of the Cretaceous.
The other type of marginocephalian, ceratopsians (“horn faces”), are best known for having prominent horns over the eye and
nose, as well as an extensive bony neck frill. In addition, they evolved another new skull bone (the rostral) at the very tip of the snout, and they had a
battery of cheek teeth for chewing plants. Early ceratopsians (such as Psittacosaurus) were bipeds, but most were quadrupedal. They reached nearly
10 meters in length and could weigh several tons. Ceratopsians lived throughout the Cretaceous Period.
The most diverse group of ornithischians were the ornithopods (“bird feet”), known from the Jurassic and Cretaceous Periods.
Early ornithopods (such as heterodontosaurs) were small and bipedal, often with long limbs.
As the group evolved, its members became progressively larger and tended toward quadrupedalism, although all retained the ability to walk on two legs. Well-known
ornithopods include forms such as Camptosaurus, Iguanodon, and the hadrosaurs (or duck-billed dinosaurs). Advanced ornithopods developed a closely
packed battery of teeth for chewing vegetation.