Tuesday, November 3, 2009

Tyrannosaurus Rex

Tyrannosaurus Rex and its subgroup, the Tyrannosaurids, were some of the largest land predators the world has ever known. They were several times larger than the largest land predators of today, such as the grizzly bear, which weighs less than one metric ton. Tyrannosaurus Rex weighed more than five metric tons. The mouth was wide enough to swallow an adult human, and the bite was three times as powerful as that of a lion. Possibly the best known of all dinosaurs, it was considered the "king of the dinosaurs" until the discovery of larger allosaurids, such as gigantosaurus in the 1990s.

About 20 skeletons of Tyrannosaurus Rex have been excavated, so the appearance of this dinosaur is known with some confidence. It was present at the mass extinction event, 65 million years ago, which ended the ended the age of dinosaurs.

It was first discovered in the Hell Creek Formation in Montana, probably the best record anywhere on earth of life at the end of the Cretaceous era, just before and during the mass extinction event that wiped out 35 percent of all species including the dinosaurs. This formation has yielded thousands of fossils, including the T-Rex skeletons in the American Museum of Natural History and the Smithsonian Institution. During the Cretaceous era the area was warm and humid with open forests and rivers. It was not swampy.

Scientists debate the lifestyle of Tyrannosaurus: was it a fearsome predator, as has always been portrayed? Or was it a slow-moving animal, living as a scavenger on the corpses of animals that had died or had been killed by more active hunters?

Evidence for the first theory includes the position of the eyes. The fields of vision overlap, so the animal had good depth perception, essential for a hunter of fast-moving prey. The teeth and skull certainly seem very strong, able to withstand the stress caused by struggling prey. Although they are much larger, the teeth are almost exactly the same shape as those of monitor lizards, the most vicious predatory reptiles of modern times. The ear structure is like that of crocodiles, which have good hearing.
Marks that match Tyrannosaurus teeth have been found on the bones of Triceratops, and Tyrannosaurus teeth have been found wedged in the bones of the duck-billed dinosaur Hypacrosaurus.

On the other hand, animals as big as Tyrannosaurus might not have been capable of much sustained speed or activity. Once up to speed the animal could stumble easily, resulting in a crash that might have been fatal.

It is very likely that both theories are true to a degree: Tyrannosaurus was probably an occasional hunter when threatened with starvation, but did not pass up the chance to devour any corpse it came across. Trackways suggest that it might have followed great herds of plant-eating dinosaurs, preying on easy victims such as the young and the injured.

The small arms of Tyrannosaurus Rex have always been a mystery. What good is a pair of arms that is too short to reach the mouth, or anything else?

There are three main theories: First, that they were used to pull prey toward the chest so that the jaws could reach it. Second, that they were used to grasp the female while mating. Third, that they could help the animal get up from a lying down position.
The arms were surprisingly powerful: the same length as a human arm, but three times as thick. Still more peculiar, the arms end in only two clawed fingers, useless for grasping.

No one can explain any of this for certain, and it's a good example of how some aspects of dinosaur behavior cannot be solved by the fossil record.

There are a lot of terrific books about dinosaurs, perfect for holiday gifts. A few of my favorites include the Illustrated Encyclopedia of Dinosaurs, by Dougal Dixon; National Geographic Dinosaurs, by Paul Barrett; and Dinosaurus: The Complete Guide to Dinosaurs, by Steve Parker. All are available online at our sponsor, Barnes & Noble.

For my previous posts on dinosaurs, click here.

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