Dinosaur, an extinct reptile that lived on land. Dinosaurs lived from about 230,000,000 to 65,000,000 years ago, during the Mesozoic Era. Fossil skeletons and other fossil remains of dinosaurs have been found on every continent. Dinosaur fossils are especially plentiful in the United States, particularly in Colorado, Montana, Wyoming, New Mexico, and Utah. Other extinct animals that lived at the same time as the dinosaurs include the pterosaurs, which were flying reptiles, and the plesiosaurs and ichthyosaurs, which lived in water.

DinosaursDinosaurs varied greatly in size and shape.

Scientists disagree as to whether dinosaurs were, like most other reptiles, cold-blooded or, like birds and mammals, warm-blooded. (Cold-blooded animals have a body temperature that varies with the temperature of their surroundings; warm-blooded animals can maintain a constant body temperature.)

The word dinosaur means terrible lizard, but not all dinosaurs were large and ferocious. Some were about one foot (30 cm) long. The largest dinosaurs were more than 80 feet (24 m) long and weighed more than 80 tons (72,600 kg). About 800 species have been identified.

DinosaursDinosaurs of the Jurassic Period lived roughly 150 million to 200 million years ago.

Some dinosaurs lived in herds, foraged for plants, walked on all fours, and were slow and lumbering. Other dinosaurs lived alone, hunted other animals, walked on their hind legs, and were fast and agile. Many fossilized dinosaur eggs have been found. Scientists believe that most types of dinosaurs laid eggs and that some dinosaurs, unlike most reptiles today, defended their recently hatched young and brought food to them.

Some dinosaurs had small, simple brains; others, especially those that hunted, had large, complex brains. Large dinosaurs probably had specialized nerve centers in the pelvic and shoulder regions of the spinal cord to help control the movements of the limbs and tail. Large dinosaurs probably had very powerful hearts and efficient lungs to supply the body with blood and oxygen.

Dinosaurs developed from a group of primitive reptiles, the thecodonts. Herrerasaurus is one of the earliest known dinosaurs, having lived 230,000,000 years ago. Fossil bones, found in Argentina, reveal that it was about six feet (1.8 m) long and weighed about 300 pounds (136 kg). Coelophysis lived about 215,000,000 years ago in what is now New Mexico. It was about eight feet (2.4 m) long and weighed about 50 pounds (23 kg).