Dinosaurs are a type of reptile that existed millions of years ago. They thrived for nearly 165 million years. The last dinosaur died off nearly 65 million years ago. The only evidence that these majestic creatures existed are fossil remains. Based on the study of these fossils, Richard Owen proposed the taxonomic category Dinosauria in 1842. In 1887, Harry Seeley used the fossil remains to divide this category of reptiles into two orders: Saurischian and Ornithischia.
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The Ornithischia order included a wide range of plant-eating dinosaurs. These dinosaurs were plated, horned and armored. The Stegosaurus is one of the more famous of this order. Originally, Seeley used the hip structure as a means of separating the dinosaurs into two groups. Ornithischia fossils exhibit a pelvis bone structure similar to birds. As a result, this was the key characteristic to separate fossils into dinosaurs orders. With increased numbers of fossil finds, improved technology and clearer understanding of the relationship of structure and function, other characteristics of Ornithischia were identified. These common characteristics not only involve the hip but also the jaw, skull, teeth and sacral vertebrae structure.
1) “Bird-hips:” The 3 bones that make up the reptile pelvis are arranged in such away that makes it appear similar to the hip of a bird. It is this characteristic that was first used to divide dinosaurs into two groups. Since 1887, the number of fossil remains have created a clearer picture. Seeley’s premise that this group of dinosaurs developed into birds was incorrect but the use if the hip structure has proved a valid means of identifying fossils as Orithischia.
2) Predentary: This is a scoop-like bone that extends in front of the teeth of the lower jaw. It is an extra bone not found in any other dinosaur. Together with the front area of the upper jaw, it provides an excellent tool for tearing off pieces of vegetation. The tip of its snout lack teeth. The predentary bone is has become the key feature in identifying Orithischia. Some modern day taxonomist use the term Prendentia instead of Orithischia to name this group of dinosaurs.
3) The antorbital fenestrae are opening or gaps that are found on the skull just in front of the eyes. These are believed to act as air sinuses. The Ornithischia have very small or completely closed antorbital fenestrae. The lack of antorbital fenestrae is open to question.
4) The jaw joint is located below the line of upper teeth. The jaw angle is straight forming a pincher jaw shape as opposed to the L-shaped jaws seen in mammals.
5) Molars have low crowns with three pyramid shaped mounts. The molars are the teeth that provide a grinding surface. Molar with three points on top of tooth may increase the efficiency of the grinding surface.
6) The vertebrate column is made of a series of bones (vertebrae) that are held together by soft tissue. The last several vertebrate are fused into a plate in most land animals. The amount of fusing is a distinguishing characteristic . The Orithischia have at lease 5 sacral vertebrae that are not fused into a plate.
7) Orithischia have ossified tendons above sacral region. This remains a mystery but it may serve as a means of strengthening the lower back, especially the tail. This would provide an aid to the ability to swim.
Once believed to be the ancestor of birds, Orithischia most probably represented a dead end in evolution. This group developed into a wide and successful group of vegetarian dinosaurs.