Amazingly enough, dinosaurs are divided into two orders:  Saurischia and Ornithischia.  Saurischia (lizard-hipped) are characterized as such because of their forward jutting pubic hipbones and their elongated neck bones.  They are divided into two infraorders:  carnivorous (meat-eating) bipedal (walking on two legs) theropods and herbivorous (plant-eating) quadrupedal (walking on four legs) sauropodomorphs.  Ornithischia (bird-hipped) are characterized as such because their pubis (hipbone) is slanted back parallel to another hipbone and they have a predentary bone at the tip of their toothless lower jaw.  They are plant eaters.  There are three infraorders:  the birdlike ornithpods, the armored thyreophorans, and the horned or thick skulled marginocephalians. 

Thyreophora belong to the Ornithischia order.  Thyreophora (shield bearers) are characterized as armored with rows of protective bony studs, spikes or plates arranged down their backs, small cheekbones and heavy limbs.  They were quadrupedal and believed to have been herbivores (plant eaters).  Thyreophora are divided into two infraorders:  Stegosauria and Ankylosauria.

Stegosauria is the first infraorder.  They were slow moving creatures ranging in size from medium to large.  They were armed with two rows of bony plates or spikes on their backs that were attached to their skin rather than their skeletons.  They also had tail spines.  Two known families are:

Huayangosauridae- which had deep, short snouts and teeth in its beak.  It grew to be approximately thirteen feet in length.  Fossils were located in China.

Stegosauridae (roof lizard) were the largest memeber of its family weighing approximately three tons and growing in lengths of approximately thirty feet.  It had a small head.  Its hind legs were much larger than its front legs.  Fossils were located in North America and Europe.  One family member is the Kentrosaurus (spiky lizard).  They were one of Africa’s best-known stegosaurs though they were smaller than the stegosaurus.  They grew to be sixteen feet in length.  They had plates on their backs and spiny horns approximately two feet long that ran from the top of the back to the tail.  It is believed that they swallowed stones to help digest their food.  Fossils were found in Africa.  Another family member is the Tuojiangosaurus (Two River lizard).  They had plates that were V-shaped running along their backs.  The largest ones were on the back and decreased in sized on both the tail and the neck.  There were two pairs of long spiky horns on the tail.  They grew to be approximately seven feet high and twenty-three feet long.  Fossils were located in China.

Ankylosauria is the second infraorder.  They were believed to have heavy bodies supported by four sturdy legs and a long, muscular tail.  They were armored with bony studs for protection.  Two known families are:

Nodosaurisae (lumpy lizard) which had an arched back with bony plates.  Some of them had long sharp spines on their shoulders.  They did not have a tail club.  They grew to be approximately thirty feet in length and they were believed to have traveled in herds.  Fossils were found in North America.  One family member was the Edmontonia.  They were one of the largest Nodosaurs.  They were well armored.  They were built solid with bony plates running down their backs to their tails.  There were extra plates over the neck and skull and they had two vertebrae that joined together at the head.  This made it difficult for them to bend their necks.  They grew up to twenty-three feet long.  Fossils were found in North America.

Ankylosauridae were the largest of the true ankylosaurs.  They are believed to have been thirty-three feet in length and weighed up to four tons.  The tail was a heavy bony club that weighed around one hundred and ten pounds.  They were able to smash an enemy’s skull or face.  It is believed they were herding animals.  Fossils were located in North America.  One family member is the Euoplocephalus.  They were well armored.  They were covered with spikes starting at their eyelids and continuing down the length of their backs and tail, which was shaped like a club.  They grew to be twenty-three feet long and weighed up to two tons.  Fossils were located in North America.