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What is a Dinosaur?

Dinosaurs were reptiles that lived through three periods of Earth's geologic history: Triassic (208-245 million years ago), Jurassic (145-208 million years ago), and Cretaceous (65-145 million years ago). Different species of dinosaurs flourished and then became extinct throughout these three periods.

All dinosaurs were reptiles, but not all reptiles were dinosaurs. Even though the name "dinosaur" means "terrible lizard," dinosaurs were not lizards at all.

Some Key Dinosaur Facts

  • All dinosaurs had four limbs. Some walked on all four legs; others, on their two back legs.

[Drawing of a Tyrannosaurus rex -- a large animal with two strong back legs and two small forelegs that are more like arms.  T. rex has a large head with many sharp teeth.]

[Illustration of a typical lizard.]

Dinosaur legs were positioned below their bodies, making it possible for them to walk upright.

Lizard legs are positioned outward from their bodies, giving them a sprawling posture.


  • Dinosaur legs were positioned under their bodies. They were able to walk and run very efficiently.

  • Dinosaurs lived on land. They did not fly in the air or live in water.

  • Some dinosaurs were carnivores (meat-eaters), but most were herbivores (plant-eaters).

  • Not all dinosaurs were large. The smallest, Compsognathus, was about 3 feet long (1 m). One of the largest, Barosaurus, reached 80 feet (24 m). Tyrannosaurus rex measured 46 feet (14 m).


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