When Dinosaurs Lived
Paleontologists now know, for example, that dinosaurs lived throughout most of the Mesozoic Era (248 million to 65 million years ago), which scientists divide into three periods. Dinosaurs first appeared about 230 million years ago during the Triassic Period (248 million to 213 million years ago). The largest dinosaurs that ever lived, such as the plant-eating Brachiosaurus, lived during the Jurassic Period (213 million to 145 million years ago). T. rex did not appear until late in the Cretaceous Period (145 million to 65 million years ago) and was the largest member of a family of meat-eating dinosaurs called tyrannosaurids. The T. rex and its relatives died out with the last dinosaurs ever to walk the Earth.
Paleontologists and geologists have also learned that the world in which T. rex lived was very different from our world. Throughout most of the Cretaceous Period, a long seaway extending from the Arctic region to the Gulf of Mexico divided the continent of North America. The width of the sea varied with changes in the global sea level. During the reign of T. rex, the sea stretched as far west as the western part of present-day South Dakota and as far east as present-day Wisconsin. The region in which T. rex lived, along the western coast of the sea, was a lush, forested region with ferns, pine trees, redwoods, and flowering plants. And dinosaurs were not the only inhabitants of the region. Streams and riverbank communities included a wide range of fish, amphibians, lizards, crocodiles, and snakes. Small plant-eating mammals shared the forests with the dinosaurs.
During decades of excavations, paleontologists have also learned much about the characteristics of T. rex. By 2000, researchers had recovered the remains of some 40 tyrannosaurs, ranging from juveniles to adults. About 20 of the specimens were fairly complete, and the largest of them exceeded 12 meters (40 feet) in length. As a result of these finds, T. rex has become one of the best documented and best understood dinosaur species.
Much of our knowledge about tyrannosaurs comes from studies of their skulls, which are up to 1.6 meters (5 feet) long. The upper and lower jaws of T. rex skulls are still filled with the large pointed teeth that made the Tyrannosaurus such a potent killer. A T. rex's teeth, serrated on the edges somewhat like the blade of a bread knife, could exceed 30 centimeters (12 inches) in length. More than half of the length of the teeth consisted of the roots, which were anchored in a massive jaw, giving the teeth great strength. These characteristics made it obvious to paleontologists from the start that T. rex was a very formidable dinosaur.
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