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Traditionally, dinosaurs are divided into two groups, depending upon the structure of the femur bone. Those who had one similar to the femur bones of the reptiles, were put in the order “Saurischia”; in those days those who had femur bones similar to a bird’s femur bones were put in the order “Ornithischia”. (Paradoxical, as it is the Saurischia order who stood as the ancestors of the birds and not the Ornithischia). All the popular and academic publications still hold to this classification, but changed in a considerable degree for use of a different interpretation. The fact is that Linnaean and Cladistics has good and bad sides, and in some ideas one adds the other. The primary proto-dinosaurs (ancestors of the Ornithodira) – a group of a small early type of dinosaur’s Archeosaur, regarding which there was any discussion neither in the Saurischia nor in the Ornithischia. These living beings which were primarily considered as Ornithozoic thecodonts, even formally not considered as dinosaurs (although they may be called as “Dinosauromorphs” (or Dinosaur types, which means dinosaurs and some related ancestral forms). But they form branches, from which all other types of Saurischia were developed, which were traditionally divided themselves into two groups, one is carnivorous and the other is herbivorous. The first of them the Theropoda, was a carnivorous bipedal dinosaur, with the bird’s legs and neck. Theropoda means “leg of a beast” a satisfactorily inappropriate name; “leg of a bird” would have been better. In this group were included large and diversified forms of smaller ones (among them are the ancestors of the birds) as well as the big beasts of prey like the Allosaurus and Tyrannosaurus. The Theropoda is such a multifarious group of animals, that they must really consider the whole order and not the suborder (as in Linnean’s hierarchy).
Another group of Saurischia, the Sauropodomorpha was an herbivorous animal. There were two main sub groups – Sauropoda, (irrelevantly named as a lizard’s foot), and their either ancestors or most primitives, but parallel relatives, the Prosauropoda. However the Prosauropoda were relatively small, a more progressive type and all Sauropods – the awkward giants with small sized heads, long necks and tails, massive bodies, and similar column legs. This group includes the famous “Brontosaurus” (or Apatosaurus) and his relatives. As the modern elephants, they depended on their big size for protection against carnivorous animals.
Ornithischia (“Bird-hipped” dinosaurs), or Predentata (named because of the unique additional predentary bone in front of the jaw, which served it’s generation with the beak) existed a more diversified groups of herbivorous animals. These were much smaller than the Sauropod, but they survived because of the development of various means, not to become the food for those who eat their meat as the modern Theropods. Ornithopods for example, depended upon fast legs and sharp sight and hearing. Ceratopsian dinosaurs (triceratops etc.) were the horned ones in the dinosaur world, their big horns, probably were sufficiently protective against even a huge and most cruel carnivorous animal. But there is an idea that these horns were more for a decorative purpose than functional – however we don’t know the accurate and conclusive answer. And here are the Stegosaurus and Ankylosaurus which developed armor, spikes, and a clubbed tail as defensive and offensive weapons.
Ornithischia – in distinction from Sauraschian dinosaurs, reptiles and birds – possessed along with mammals cheek-muscles and pouches, which help during chewing. In this behalf they found something similar with the mammals. Of course, many Ornithischia filled in the similar ecological niche as the modern ungulate animals.
Also among the dinosaurs we could see the tendency to avimorphisation (the tendency to become a form of Theropod a bird like figure), and to theromorphisation (towards mammals) in Ornithischia. The dinosaurs of the Mesozoic era in some way signified the birds and mammals of the Cenozoic era.
More complete information, regarding a more detailed classification, the provisional and solid distribution of dinosaurs, you may find in other links.
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