Tyrannosaurus rex

Tyrannosaurus rex or T. rex, is easily the most famous of the Tryannosaurids (tyrant reptile). Despite its popularity, T. rex appears to have had a limited range in North America and Asia, and existed for a relatively short period of time. T. rex appeared during the late Cretaceous Period, about 85 million to 65 million years ago. This was toward the end of the Mesozoic Era or the Age of Reptiles, and just prior to the dinosaurs' extinction. Many people grow up believing that T. rex was the "King of the dinosaurs," as implied by the addition of rex after its name. However, it was neither the largest dinosaur of its time nor the most intelligent, and at least one paleontologist argues that although it was a teropod or meat-eating dinosaur, T. rex was not even a predator.

But there is no doubt that it was big. Based on fossils founds, T. rex weighed 5-6 tons, stood about 15 ft (4 m) tall, was 20-46 ft (6.5-15 m) long, and had 6 in (15 cm) long, sharp, serrated teeth. It is currently believed that T. rex was the largest terrestrial carnivore of all time.

Its skull was one unified whole, solidly constructed, with no moving parts except at the joint of the jaw. The compartments in the skull and in the lower jaw that housed the muscles were enlarged more than in any other predator. Its snout was sharply pinched to clear its field of vision. Its eyes faced forward to provide some overlap between visual fields from the right and left eyes, permitting stereoscopic vision.

This lizard-hipped dinosaur walked upright on two powerful hind legs, which ended in birdlike feet with three forward-pointing toes with large claws. These were its weapons. Evolution shortened its torso for balance and speed. Some say the beast was surprisingly slender-limbed, graceful, and fast, able to attack other slower plant-eating dinosaurs such as the Triceratops. Although it possessed two small and muscular forelimbs, many paleontologists believe they were of little practical use.

At least one paleontologist, Jack Horner, of the Museum of the Rockies (Bozeman, MT), recently raised the question as to whether or not T. rex was an effective hunter, given its small eyes, small arms, and relatively slow gait. Horner theorizes that T. rex scavenged its food from other animals' kills instead of killing its own. He supports this theory citing the beast's large olfactory lobes, which would be able to smell dead animals from a great distance, and its powerful legs with the thigh about the size of the calf as in humans, which were built for walking long stretches. As with any theory, there are many arguments refuting this hypothesis. Dr. Kenneth Carpenter from the Denver Museum of Natural History found a healed T. rex tooth mark on the tail of a hadrosaur (a duck-billed dinosaur), proving to many that T. rex was an active predator. Other experts say that the small eyes do not necessarily imply poor vision, the forearms were not needed for predation, and its gait, in spite of its bulk, was far from slow.

A coprolite (fossilized feces) believed to be from a T. rex was found in Saskatchewan, Canada by a team led by Karen Chin. This 65 million-year-old specimen contained chunks of bones believed to be from the head frill of a Triceratops, an herbivorous (plant-eating) dinosaur eaten by the T. rex. The coprolite is a whitish-green rock that is 17 in (44 cm) long, 6 in (15 cm) high and 5 in (13 cm) wide. This fossil provides evidence that T. rex crushed bones before swallowing them, since the bones in this coprolite were broken up.