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Dinosaurs, just like today's mammals, were divided into three groups: herbivores (plant-eaters), carnivores (meat-eaters), and omnivores (plant and meat-eaters). Herbivores outnumbered the other two groups. From looking at the dinosaur classification tables 65% of all dinosaurs were plant-eaters, and only 35% were carnivores and omnivores.

The reasons for such proportions lay in the biological equilibrium that is present in nature, called a food chain. At the bottom of the chain are plants. They are called producers (autotrophs), because they get their energy from the sun and convert it into chemical energy (during photosynthesis) that animals need. Primary consumers of plants were plant-eating dinosaurs.

A large population of herbivores could be supported only by an even larger number of plants. Then the herbivores were eaten by carnivores, or secondary consumers. Therefore, there needed to be more herbivores than carnivores. This food chain creates biological balance between each group. For example when the number of herbivores increased, the number of plants available decreased. In the effect, some of the herbivores couldn't find enough food and died.

Once the population of plant-eaters decreased, the number of plants could increase again. Therefore, availability and nature of plants dictated the fate of herbivores and indirectly, also the fate of carnivores. Throughout the Mesozoic Era, when dinosaurs lived, due to changes in climate and environment plants evolved along with animals and often caused extinction of some dinosaurs and the rise of other kinds of dinosaurs.

There are several ways to study dinosaurs' diets. Some of them include: studying fossilized droppings (coprolites), fossilized stomach contents, teeth, gastroliths (rocks swallowed to help digestion) found in some dinosaur fossils, stomach size (girth).

HERBIVORES were recognized by their blunt teeth that helped to strip leaves off of branches and some flat teeth to grind plants. They also had much larger stomachs than carnivores, because they had to eat a lot more food to sustain the same amount of energy (plants are much lower in calories). The only way to find out what kind of plants they consumed is to study their fossilized stomach remains and their coprolites.

Fossilized stomach contents are very rare and coprolites are not easy to match with certain dinosaurs. Because of the limited findings, scientists can guess dinosaurs' diet only based on the kind of teeth they had, where they lived and what kinds of plants were available at that time.

During the Triassic period of the Mesozoic Era (about 228 million years ago), when dinosaurs evolved, all the continents were joined together as one continent, Pangaea. The center was a dry and hot land mass, while the polar regions were milder and wet. During that Era, Pangaea started to break apart and the weather started to change. Along with it changed the plant life and the dinosaurs. There were several kinds of plants that dominated the landscape and were important food for plant-eating dinosaurs:

- Conifers included redwood, yews, pines, cypress, Araucaria (also called Norfolk Pine or monkey puzzle tree). During the Jurassic period, flowering plants started to dominate and began to overtake conifers.

- Cycadophytes included Cycads and Cycadeoids, plants with woody stems and tough leaves. They grew during Triassic and Jurassic periods, but started to die out during he Cretaceous period. Cycadeoids became extinct.

- Gingkos deciduous trees with soft leaves that fall during cold weather grew during the Jurassic and Cretaceous periods and are still around today.

- Pteridophytes primitive vascular plants like club mosses, horsetails and ferns. They lived only in moist areas where they reproduced through spores and rhizomes (underground stems) and were a source of food for dinosaurs that lived in those areas.

- Pteridosperms seed ferns, which looked like spore ferns. They grew in southern area of the continent Pangaea during the Triassic period and became extinct at the end of it.

- Angiosperms flowering plants included magnolia, laurel, barberry, sycamores, and palms. They filled the landscape during late Jurassic period. They were fast growing, very adaptable and were the reason for the dramatic increase in dinosaurs' numbers and kinds. Most dinosaur fossils that have been found are dated from the Cretaceous period, when the plants were widely available.

Some of the plant eating dinosaurs were: Ankylosaurus, Diplodocus, Iguanodon, Kentosaurus, Protoceratops, Stegosaurus, Triceratops, and many others.

CARNIVORES had long legs that would let them run fast and easily chase down the prey. They had sharp teeth and claws that made the killing, breaking bones, and tearing flesh easy. Scientists say that in order to be effective hunters they must have had good eyesight, strong sense of smell, and a large brain to plan hunting strategies. Some carnivores like Velociraptor and Coelophysis might have hunted in groups, which required cooperation. Many meat-eaters were also scavengers (ate the remains of a killed animal) and there were still others that ate fish, like Baryonyx and Suchomimus.

Some of the carnivores were: Albertosaurus, Coelophysis, Compsognathus, Eoraptor, Gigantosaurus, Suchomimus, Tyrannosaurus rex, Velociraptor, and others.

OMNIVORES ate plants, eggs, insects, and animals. There are only a few known, like Ornithomimus, which had strong, long legs, a hard beak, but no teeth. It is said that also many herbivores were 'accidental omnivores' as they sometimes would eat insects and small animals along with plants.

There is still little known about dinosaurs' diet. Fossils leading to expanding that knowledge are very rare. The research continues as scientists want to find out what for sure caused the mass extinction of dinosaurs 65 million years ago. There are some speculations that changes in dinosaurs' diet might have been part of the reason.

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