There is a very obvious division into two types of feeding: ,Carnivory meat eating and Herbivory, plant eating. The greatest diversity of dinosaurs is to be found among the herbivores, which include representatives from both the major groups of dinosaurs, the Saurischians and Ornithischians. The only carnivores were the theropds saurischians and, although they were relatively conservative in body design they were far from conservative in the ways they went about obtaining their food.
Most theropods were meat-eaters. These share the same kind of teeth. All are narrow, sharply pointed and curved with the back edge, and in some cases the front edge as well, marked by fine serations much like a horse or a serrated kitchen knife. Such teeth are ideal for penetrating the flesh of their prey to bring about a speedy kill.
Extremely small theropods may well have fed upon insects, in which case their teeth would have tended to be smaller and more spiky. Being able to rip into flesh would not have been as important as being able to crack open the tough exoskeletons of insects such as beatles and dragonflies, small, spiky teeth are ideal for this purpose.
Extremely large theropods, such as Tyrannosaurs may have fed in a slightly different way from the majority. As the top predators of the Late Cretaceous, they probably fed on the numerous herding species of Hadrosaurs and Ceratopians which lived at the time.
Once its victim was safely subduced, the Tyrannosaurs could have consumed it at its leisure, using its sharp teeth to slice off sizeable chunks of meat. The jaws of these tremendous creatures were slightly flexible so that large pieces of meat could be swallowed.
The bone in the skulls of most theropods are surprisingly thin and light, enclosing large spaces in the side of the face. This combination enables the skull to be both light and maneuverable as well as very flexible and strong. Large openings at the front for the nostrils and the middle for the eyes are expected in all skulls, but here other openings are found in front and behind the eyes which provide areas for the attachments of powerful jaw muscles.
New discoveries in theropods seem to break the rules. For example, as well as the conventionally toothed theropods there was a variety of toothless types.
The Ornithomimosaurs("bird-imitating Lizards") such as Ornithomimus, Struthiomimus and Gallumimus are so similar in their appearance to living ostriches that it would seem very probable that they lived and fed in similar ways. The beak would have allowed them to feed on pratically anything, provided it was small enough to be swallowed: small animals such as lizards, amphibians, and mammals; or perhaps fruit and berries.
The Oviraptorosaurs ("egg-snatching lizards") such as Oviraptor and Caenagnathus may well have had feeding habits broadly similar to those of the Ornithomimosaurs, but it is possible that they were specialist feeders for example egg predators. The short, thick and heavy jaws may well have been adapted, specially for cracking open large thick-shelled eggs of dinosaurs.
In general, plants are much more difficult to feed on than meat, while meat is readily cut up and easily digested, plant tissues are much tougher and need to be chipped and ground down to release their goodness. What is more, plants are made largely of material known as cellulose. Unfortunately, no animals have ever developed digestive enzymes capable of breaking down cellulose; this presents herbivores with even greater difficulties should they want to extract the maximum amount of nutritious material from the plants that they eat.
Herbivores, therefore all tend to show various adaptations associated with breaking up tough plant food, and providing space in their guts for microbes to live and do their important jobs.
The early Sauropodomorphs of the late Triassic and early Jurassic, known as "prosauropods" and including Plateosaurs and Anchisaurus, were the first tall herbivores ever to appear on land. Before this time, the highest that any plant eater could reach must have been about 4ft. (1.2m). The animals themselves had relatively small heads and long necks and bodies which could be tilted up, so that they could stand on their back legs with ease, giving them a vertical reach of about 20ft. or more in some larger varieties.
The jaws were lined with quite small, diamond-shaped teeth with very coarsely roughened edges, which seem well suited to tearing off pieces of vegetation, though they were clearly not specifically adapted for grinding or chewing up this kind of food. Grinding of the plant food seems to have taken place in a special part of the gut, similar to the gizzard of a modern bird. The gizzard was a powerful muscular bag, lined with pebbles known as gastroliths ("stomach-stones") which the animal had swallowed deliberately.
Evidence that this actually happened comes from the discovery of piles of angular, polished stones within the rib cages of some early sauropodomorphs such as Massopondylus and showing the approximate position of the muscular stone-lined gizzard. Crocodiles today have a powerful muscular stomach, and they too swallow stones which not only act as a ballast, but also help to crush up the bones of their prey.
The later Jurassic was the time of appearance of the greatest plant eaters that have ever walked the earth.
From the outset ornithischians appear to have taken a rather different approach to feeding on plants. All have a sharp horny beak at the front of the mouth, which would have been very effective at cropping plants. There was probably some partitioning between these dinosaurs of the types of plants upon which they fed, based upon the shape of their beak. Narrow, pointed beaks were clearly suitable for specialist feeders capable of picking off fruit or tender shoots individually. Ornithischians with broader muzzles probably fed in a more rough and ready manner taking mixtures of leaves, twigs, bark and anything else caught between the guillotine-like edges of the beak. Behind the beak the jaws were equipped with varying arays of teeth, depending upon the toughness of the food.
The most simple type of ornithischian tooth was approximately diamond-shaped with coarsely serrated edges and rather similar to those of sauropodomorphs. Such teeth, as we have seen are well designed for tearing plants, but not for grinding or chewing plant material up in the mouth.
Ceratopians had skulls constructed very differently from those of the ornithopods. The bones were very rigidly held together, with no possibility of movement or hinging of the upper jaw. The beak was extremely narrow, consisting of sharply hooked upper and lower parts arranged and looking rather like those of a parrot. This type of beak was clearly designed specifically for cutting, rather than for cropping or browsing as are the vast majority of ornithischian beaks.
Behind the narrow beak the jaws were lined with hundreds of teeth. Again these teeth form interlocking sets of batteries as seen in larger ornithopods, but instead of forming pavements of teeth which act like millstones, the teeth of the ceratopians are very sharp and locked together in such a way that they form a guillotine-like blade. They must have been used to slice the plants into short sections by being passed alternately in and out past the teeth.
When the plant food had been sliced sufficienly finely it would have been swallowed and processed in the stomach. It seems likely that these ceratopians may have also used a gizzard to pulp the sliced food more thoroughly before digestion. Although no firm evidence exists for gastroliths inside ceratopians skeletons.
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