The major feature that made dinosaurs different from other reptiles was the position of their limbs. Reptiles normally have legs that support from the sides like an alligator, leaving the animal hugging the ground. Dinosaurs on the other hand had legs that allowed them to walk fully erect like most mammals.

Dinosaurs are divided into two groups, "Saurischia" -  the lizard hipped dinosaurs and "Ornithischia" - the bird hipped dinosaurs . Two other groups are often associated with them but are not truly dinosaurs. They are "Pterosauia" - the winged reptiles and the "Crocodilia" - which even includes modern crocodiles.

The  "Saurischia" group includes such dinosaurs as Tyrannosaurus Rex and Apatosaurus. Like the hips of other reptiles they have one pelvic bone that  points forward and one that points backward.

The "Ornithischia" group includes dinosaurs such as Stegasaurus and Ankylosaurus. These dinosaurs have hips similar to a bird's. Both of the pelvic bones point backward in the same direction.  There is also a difference in the jawbone of the two groups. A "Saurischian" has a jawbone that holds the teeth called a "dentary". An "Ornithischian" also has a similar jawbone but also has an extra beaklike bone in the front of it's teeth. This is called a "predentary" and is only found among the "Ornithischian" dinosaurs.

The "Saurischians are divided into two suborders -  "Sauropoda" and "Theropoda. The "Sauropods" are composed of the gigantic dinosaurs with long necks and tails like "Diplodocus" and "Apatosaurus". The "Saurapods" were primarily plant eaters.

The "Theropods" were quite different. Smaller than the "Sauropods", they were bipeds, meaning they walked on two legs. Some examples of "Theropods" are "Deinonychus" and "Tyrannosaurus Rex".  After Adam sinned these dinosaurs became meat eaters.

There are different kinds of "Ornithischian" dinosaurs too. However, they are divided into four suborders - "Ornithopoda", "Ceratopsia", "Stegosauria", and "Ankylosauria". 


The "Ornithopods" had two strong rear legs, but sometimes walked on all four legs. The "Hadrosaurs" or duck-billed dinosaurs were also "Ornithopods". They had a flat duck like beak, webbed feet, and strong tails. Many of the duckbills had a bony crest on top of their heads.

The "Ceratopsians" all had tough skin and a "frill", a large plate that extends up from the back of the head like a shield. They also had horns and walked on all four legs. The "Ceratopsians" were all plant eaters like the "Triceratops".

 
The "Stegosaurians" were a small suborder. They were the plated dinosaurs like "Stegosaurus" and "Kentrosaurus".

 
The "Ankylosaurians" are known as the armored dinosaurs. Their bodies were covered with bony  armor and spikes. Their tails had big hard balls of bone and spikes on them and were especially dangerous. Examples are "Scolosaurus" and "Ankylosaurus"