Dinosaur

Dinosaurs are a group of now-extinct, terrestrial reptiles in the order Dinosauria that lived from about 225 million years ago to 66 million years ago, during the Mesozoic era. Species of dinosaurs ranged from chicken-sized creatures such as the 2 lb (1 kg) predator Compsognathus to colossal, herbivorous animals known as "sauropods," which were larger than any terrestrial animals that lived before or since. Some dinosaurs were enormous, awesomely fierce predators, while others were mild-mannered herbivores, or plant eaters, that reached an immense size. The word "dinosaur" is derived from two Greek words, meaning "terrible lizard." The term refers to some of the huge and awesome predatory dinosaurs—the first of these extinct reptiles to be discovered that were initially thought to be lizard-like in appearance and biology. But Richard Owen (1804-1892), the British expert in comparative anatomy, also coined the word in awe of the complexity of this wide variety of creatures that lived so long ago and yet were so well-adapted to their world.

Dinosaurs were remarkable and impressive animals but are rather difficult to define as a zoological group. They were terrestrial animals that had upright legs, rather than legs that sprawled outward from the body. Their skulls had two temporal openings on each side (in addition to the opening for the eyes), as well as other common and distinctive features. The dinosaurs were distinguished from other animals, however, by distinctive aspects of their behavior, physiology, and ecological relationships. Unfortunately, relatively little is known about these traits because we can only learn about dinosaurs using their fossil traces, which are rare and incomplete. It is clear from the available evidence that some species of dinosaurs were large predators, others were immense herbivores, and still others were smaller predators, herbivores, or scavengers. Sufficient information is available to allow paleontologists to assign scientific names to many of these dinosaurs and to speculate about their evolutionary and ecological relationships.

Although they are now extinct, the dinosaurs were among the most successful large animals ever to live on Earth. The dinosaurs arose during the interval of geologic time known as the Mesozoic (middle life) era, often called the "golden age of reptiles" or "the age of dinosaurs." Radiometric dating of volcanic rocks associated with dinosaur fossils suggests they first evolved 225 million years ago, during the late Triassic Period and became extinct 66 million years ago, at the end of the Cretaceous period. Dinosaurs lived for about 160 million years and were the dominant terrestrial animals on Earth throughout the Jurassic and Cretaceous periods—a span of over 100 million years.

Interestingly, mammal-like animals co-existed almost continuously with the dinosaurs and obviously prospered after the last of the dinosaurs became extinct. Although they co-existed in time with dinosaurs, mammals were clearly subordinate to these reptiles. It was not until the disappearance of the last dinosaurs that an adaptive radiation of larger species of mammals occurred, and they then became the dominant large animals on Earth.

It is not known exactly what caused the last dinosaurs to become extinct. It must be stressed, however, that dinosaurs were remarkably successful animals. These creatures were dominant on Earth for an enormously longer length of time than the few tens of thousands of years that humans have been a commanding species.