The dinosaurs are a large group of reptiles that lived from 230 to 65 million years ago. Some, such as the well-known Tyrannosaurus rex, were enormous meat-eating animals. Others, however, were small and timid creatures that nibbled on plants.
Scientists divide the dinosaurs into two orders. One order is Saurischia, which includes the theropods (THAIR-oh-pods) that walked on their two hind legs and were mostly meat-eating dinosaurs and the sauropods (SAWR-oh-pods) that walked on all fours and ate plants. The theropods had more primitive features, including jagged teeth, and some, such as the Ceratosaurus, had hornlike knobs jutting out of their skulls. Tyrannosaurus rex was a theropod. Although it was quite large at 40 to 50 feet (12.2 to 15.2 meters) long, its ancestors only grew to about 10 feet (3 meters) long.
The sauropods looked much different than the theropods. They had very long necks and tiny heads. Some of them were able to lift their front legs off the ground and grab leaves or other things with their hands. Other species, including Brachiosaurus, had longer front legs than hind legs, similar to the arrangement in current-day giraffes. Their tall front legs, combined with their overly long necks, helped them easily reach food even at the tops of most trees.
The other order within the dinosaurs is Ornithischia, which included those dinosaurs that ate plants and had hip bones that looked like those found in present-day birds. Many of them had crests, beaks, horns, or helmets, and some had armor-like plates, called scutes, covering their bodies and occasionally spikes. Stegosaurs are an example of an Orinithischian. These dinosaurs had armor-like spines down the middle of the back and spiked tails. The Ornithischia also includes the duckbill dinosaurs with their wide snouts.
Within these two orders of dinosaurs, the animals are further split into several hundred smaller groups, called genera (jen-AIR-uh). One or more species is grouped into each genus (JEAN-us), which is the singular of genera.
Although the name dinosaur actually means "terrible lizards," dinosaurs are not lizards and are different from all other groups of reptiles. One of the major differences between dinosaurs and other reptiles is in the way they moved. Lizards and crocodiles walk with their legs held out to the side, in the same type of position a person's arms take when doing pushups. A few dinosaurs sprawled their front legs like a lizard, but the vast majority of them walked like a dog or cat—with the legs directly below the hips and shoulders.
Many scientists also now suspect that at least some of the dinosaurs were warm-blooded, instead of cold-blooded like other reptiles. A warm-blooded animal, more properly called an endothermic (EN-doe-THER-mik) animal, uses its own energy to keep its body at a constant temperature. Cold-blooded, or ectothermic (EK-toe-THER-mik), animals get their body heat from an outside source, like the warmth of the sun.
Dinosaurs came in many shapes and sizes. The Seismosaurus, or "earth-shaking dinosaur," may have been the longest at 120 to 150 feet (36.6 to 45.7 meters) long. The heaviest may have been the Argentinosaurus, which grew to 100 to 130 feet (30.5 to 39.6 meters) long and weighed 110 tons (99,800 kilograms). Other enormous dinosaurs include the Supersaurus at 100 feet (30.5 meters) long and about 50 tons (45,000 kilograms) and the Brachiosaurus at 85 feet (25.9 meters) long and about 75 tons (68,000 kilograms). The Tyrannosaurus rex, a name that is often shortened to T. rex, was considerably smaller at 40 to 50 feet (12.2 to 15.2 meters) long and 6 tons (5,400 kilograms) in weight. Since T. rex stood on its hind legs rather than on all fours, it towered over most other dinosaurs. Other similarly sized meat-eating dinosaurs were the Gigantosaurus, Spinosaurus, and Carcharodontosaurus. All dinosaurs were not giants, however. Some, such as the Saltopus and Lesothosaurus, were only 24 to 36 inches (61 to 91 centimeters) long, and the tiny Microraptor's full-grown size may have been only about 16 inches (41 centimeters) long.