Millions of years ago, long before there were any people, there were dinosaurs. Dinosaurs were one of several kinds of prehistoric reptiles that lived during the Mesozoic Era, the “Age of Reptiles.”

Dinosaurs evolved from other reptiles (socket-toothed archosaurs) during the Triassic period, over 230 million years ago. Dinosaurs evolved soon after the Permian extinction, which was the biggest mass extinction that ever occured on Earth. During this time (the Triassic period), the mammals also evolved.
The dinosaurs dominated the Earth for over 165 million years during the Mesozoic Era, but mysteriously went extinct 65 million years ago. Paleontologists study their fossil remains to learn about the amazing prehistoric world of dinosaurs.

The largest dinosaurs were over 100 feet (30 m) long and up to 50 feet (15 m) tall (like Argentinosaurus, Seismosaurus, Ultrasauros, Brachiosaurus, and Supersaurus). The smallest dinosaurs, like Compsognathus, were about the size of a chicken. Most dinosaurs were in-between.
There were lots of different kinds of dinosaurs that lived at different times.
Some walked on two legs (they were bipedal), some walked on four (they were quadrupedal). Some could do both.
Some were speedy (like Velociraptor), and some were slow and lumbering (like Ankylosaurus).
Some were armor-plated, some had horns, crests, spikes, or frills.
Some had thick, bumpy skin, and some even had primitive feathers.

The Earliest Dinosaurs:
The first dinosaurs were small and lightly built, mostly about 10-15 feet long (3 to 4.5 m). They were bipedal carnivores or omnivores, and probably very agile and fast.
The world_s oldest-known dinosaurs have been found on Madagascar, an island off the coast of SE Africa. These dinosaur fossils date from about 230 million years ago during the Triassic period.
Until recently, the earliest-known dinosaur was Eoraptor lunensis (meaning “dawn raptor”) which lived about 228 million years ago. It was a small, primitive theropod (a bipedal meat-eater) about 3 feet (1 m) long. It lived in what is now Argentina, South America (fossils were found at the Ischigualasto Formation).

Some dinosaurs were carnivores (meat-eaters) but most were herbivores (plant-eaters). In any food chain, there have to be more organisms at the lower levels of the chain because the transfer of food energy is inefficient and much of the energy is lost at each stage of the process.
A large number of plants (called producers) can support a smaller number of plant-eaters (called primary consumers). These plant-eaters are eaten by a smaller number of carnivores (secondary consumers). As the number of carnivores in a community increases, they eat more and more of the herbivores, decreasing the herbivore population. It then becomes harder and harder for the carnivores to find herbivores to eat, and the population of carnivores decreases. In this way, the carnivores and herbivores stay in a relatively stable equilibrium, each limiting the others population. A similar equilibrium exists between plants and plant-eaters.
Plant-eaters (Herbivores) usually have blunt teeth that are good for stripping vegetation (leaves, twigs, etc.). Some also have flat teeth for grinding tough plant fibers. Many herbivores have cheek pouches in which they can store food for a while.
Plant-eaters (herbivores) usually have to eat a much larger volume of material than meat-eaters (carnivores) do in order to get the same amount of calories (this is because leaves, twigs, and roots are low in calories). Plant-eaters usually have larger digestive systems (than meat-eaters) that are needed to digest large amounts of tough plant fibers. Some dinosaurs swallowed rocks (called gastroliths) to help grind up the fibers in their guts. Some (like Ankylosaurus) even had fermentation chambers, where the plant fibers were dissolved.

Meat-eaters (Carnivores or theropods) need to have some way to get meat. Carnivorous dinosaurs usually had long, strong legs so thay they could run fast in order to catch their prey. They also needed big, strong jaws, sharp teeth and deadly claws that could kill and then tear apart the prey. Good eyesight, a keen sense of smell, and a large brain to plan hunting strategies are also very important for successful hunting. Many of the carnivores (like Deinonychus, Coelophysis and Velociraptor) may have hunted in packs, so social cooperation was necessary for a good hunt. Animals that are primarily scavengers (animals that eat meat that they did not kill themselves) need very sharp teeth and strong jaws for tearing into the meat and breaking the bones for nutritious bone marrow. Most carnivores are scavengers when given the opportunity. Some dinosaurs were fish eaters, including Baryonyx and Suchomimus. Some carnivores (like Coelophysis) have been found withs mall, fossilized animals within them. Some dinosaurs may have even been cannibals, eating their own kind. Some carnivorous dinosaurs include: Albertosaurus, Allosaurus, Coelophysis, Compsognathus, Deinonychus, Dilophosaurus, Eoraptor, Giganotosaurus, Megalosaurus, Suchomimus, Tyrannosaurus rex, Unenlagia, Utahraptor. Velociraptor, Yangchuanosaurus, and many others.
Only a few of the known dinosaurs were Omnivores (eating both plants and animals). Some examples of omnivores are Ornithomimus and Oviraptor, which ate plants, eggs, insects, etc. Also, most herbivores are “accidental omnivores” in that when they are eating plants, they also ingest a lot of insects and small animals.