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What is a Dinosaur?

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 were reptiles and most hatched from eggs. No dinosaurs could fly and none lived in the water.

Plant-eating sauropods were the largest animals to ever walk on Earth - but blue whales are more massive than any of the dinosaurs were! 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. It is very difficult to figure out how the dinosaurs sounded, how they behaved, how they mated, what color they were, or even how to tell whether a fossil was male or female. No one knows what color or patterns the dinosaurs were.

Most dinosaurs were plant-eaters (also called herbivores). For example, Triceratops was a plant-eating dinosaur.

Some dinosaurs were meat-eaters (also called carnivores). For example, T. rex was a meat-eating dinosaur. 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 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.

When the dinosaurs lived, the Earth's continents were jammed together into a supercontinent called Pangaea and the Earth was warmer than it is now.

The dinosaurs went extinct 65 million years ago, probably because of the environmental changes brought about by an asteroid hitting the Earth. The dinosaurs went extinct about 65 million years ago, at the end of the Cretaceous period, which was a time of high volcanic and tectonic activity. There are a lot of theories why the extinction occurred. The most widely accepted theory is that an asteroid impact caused major climactic changes to which the dinosaurs couldn't adapt. Dinosaurs probably live on today as the birds. All that's left of the dinosaurs are fossils and, perhaps,the birds. Dinosaur fossils have been found all over the world, maybe even near where you live! Some dinosaurs were very bird-like and may be the ancestors of today's birds.

There are almost 500 described dinosaur genera and many more species. Every few months (sometimes weeks), new finds are unearthed. Although dinosaurs' fossils have been known since at least 1818, the term dinosaur (deinos means terrifying; sauros means lizard) was coined by the English anatomist Sir Richard Owen in 1842. The only three dinosaurs known at the time were Megalosaurus, Iguanodon, and Hylaeosaurus, very large dinosaurs. The oldest known dinosaur is Eoraptor, a meat-eater from about 228 million years ago.