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Traditionally, dinosaurs are divided into two groups, depending upon the structure of the femur bone. Those who had it similar to the femur bones of the reptiles, were put in the order “Saurischia”; in those days those who had femur bones similar to 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 (especially elders) 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 Ornithodira) – a group of 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 Dinosaurs type), which means dinosaurs and some related ancestral forms). But they – forms branches, from which al other types of Saurischia were developed, which were traditionally divided themselves into two groups, one is carnivorous and the other is herbivorous. First of them - Theropoda, a carnivorous bipedal dinosaurs, in similar with the bird’s legs and neck. Theropoda means “leg of a beast” 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 ancestors of the birds) as well as the big beasts of prey like Allosaurus and Tyrannosaurus. 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, Sauropodomorpha was herbivorous animals. There were two main sub groups – Sauropoda, (irrelevantly named as lizard’s foot), and their either ancestors or most primitives, but parallel relatives, Prosauropoda. However Prosauropoda were relatively small, more progressive type and all Sauropods – awkward giants with a small size heads, long necks and tails, massive bodies, and similar column legs. This group includes 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 more diversified groups of herbivorous animals. These were much smaller than 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 the fast legs and sharp sight and hearing. Ceratopsian dinosaurs (triceratops etc.) were the horned in the dinosaur world, their big horns, no less than, probably sufficient protective against even a huge and most cruel carnivorous animal. But there is an idea that these horns were more of 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 clubbed tail as a defensive and offensive weapon.
Ornithischia – in distinction from Sauraschian dinosaurs, reptiles and birds – possessed similar with the mammals cheek-muscles and pouches, which help during chewing. In this behalf they found something similar with the mammals. Of course but, many Ornithischia filled in the similar ecological niche as the modern ungulate animals.
Also among the dinosaurs we could see the tendency and to avimorphisation (the tendency to become a form of Theropod to all most a bird like figure), and to theromorphisation (towards mammals) in Ornithischia. Dinosaurs of the Mesozoic era in some way signified birds and mammals of Cenozoic era.
More complete information, regarding a more detailed classification, provisional and solid distribution of dinosaurs, you may find in other links.
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