Dinosaurs: Ancient Fossils, New Discoveries

The Problem of Size
Big animals alive today have a lot in common. They are heavy. They have few predators. And they tend to move slowly. But have big animals always behaved this way? Scientists continue to disagree about the speed—fast or slow?—of one very big extinct animal, Tyrannosaurus rex.

The most famous of the upright, largely meat-eating dinosaurs called theropods, T. rex would have weighed between 5,000 and 7,000 kilograms (11,000-15,500 pounds) with skin and flesh on its huge bones. That's about as much as the largest African elephant.

Some claim T. rex could run at speeds up to 72 kilometers (45 miles) per hour. But biomechanics researchers argue that the animal moved much more slowly, perhaps only about 16 kilometers (10 miles) per hour—about as fast as an average human runner. These experts say size slowed T. rex down. But why is it so hard to be big and fast? Bones, muscles and posture all play a part.

FAST FACTS: *T. rex*
	Pronunciation: "Tie-RAN-uh-sore-us rex"
	Size:
		LENGTH: 12 meters (40 feet)
		HEIGHT: 3.6 meters (12 feet)
		WEIGHT: 5,000-7,000 kg (about 11,000-15,500 pounds)
	Food: Other animals
	Lifespan: About 28 years
	When it lived: 99-65.5 million years ago
	Fun fact: At the hip, this adult T. rex is three times the height of an average seven-year old—but more than 260 times the weight of that child!

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