Dinosaurs are animals evolved into many sizes and shapes. Dinosaurs are diverse and quite often one person will think of an animal like a long-necked sauropod, while another person will think of a large, fierce meat-eater like Tyrannosaurus rex. It should be clear then that the term "dinosaurs", or the scientific version "Dinosauria", is describing a diverse group of animals with widely different modes of living.

The term "dinosaur" has had a long history of misrepresentation. A few simple points must be kept in mind when discussing these animals:

Not everything big and dead is a dinosaur. All too often books written for a popular audience include animals such as mammoths, mastodonts, and the sail-backed reptile Dimetrodon. Dinosaurs are a specific subgroup of reptiles, whereas mammoths and mastodonts are mammals. Dimetrodon is a neither a reptile nor a mammal, but a synapsid pelycosaur.

Not all dinosaurs that we know lived at the same time. Different dinosaurs lived at different times. For example, no Stegosaurus ever saw a Tyrannosaurus, because Tyrannosaurus wasn't alive for another 60 million years.

Not all dinosaurs were huge; some were actually quite small. Based on features of the skeleton, many people studying dinosaurs consider birds to be dinosaurs. This definition would make even the smallest hummingbird a legitimate dinosaur.

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The Dinosauria contains two major groups of dinosaurs, the Ornithischia, or "bird hipped" dinosaurs, and the Saurischia, or "lizard hipped" dinosaurs. These groups are named and recognized because of the structure of their hips. The Ornithischia contains several groups of herbivorous dinosaurs, including the horned-dinosaurs like Triceratops; the armored dinosaurs like Stegosaurus and Ankylosaurus; small, bipedal, plant-eaters like Heterodontosaurus; and the duck-billed dinosaurs like Maiasaura and Edmontosaurus. The Saurischia contains three groups: the Prosauropoda, a group of largely herbivorous, dominantly quadrupedal dinosaurs such as Plateosaurus; the Sauropoda, which were long-necked, long-tailed, enormous herbivorous dinosaurs like Apatosaurus, Brachiosaurus, and Diplodocus; and the Theropoda. The theropods contain some extinct dinosaurs like Tyrannosaurus rex and Dilophosaurus, but this last group also includes the living dinosaurs--the birds.

Although dinosaur remains had been found earlier elsewhere, it was the discoveries of dinosaurs in North America in the second half of the 1800s that provided the first real glimpes of what these animals were like.

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