Dinosaurs were a group of land animals that lived from about 230 million years ago until about 60 million years ago. This spans the era of the Earth's history known as the Mesozoicera, which includes, from most ancient to most recent, the Triassic, Jurassic and Cretaceous
periods. Dinosaurs grew in population and diversity during their time on Earth before becoming extinct at the end of the Cretaceous period.
No one knows exactly how many types of dinosaurs inhabited the planet. There are currently about 700 named species, but this probably represents a fraction of the dinosaurs that ever existed.
Dinosaurs ranged in size from immense to tiny, and they came in a range of shapes. Today's dinosaur classifications come from these differences in shape and size. Carnivorous dinosaurs were all theropods, bipedal animals with three-toed feet. Carnosaurs were a small, agile type of theropod. One of the most widely-known carnosaurs was Velociraptor, which is considerably smaller than depicted in the "Jurassic Park" films. Sauropods, on the other hand, were enormous, four-legged herbivores like Brachiosaurus, Apatosaurus and Diplodocus. Dinosaurs with armored bodies and spiny tails were ankylosaurs. Ceratopians -- like Triceratops -- had frills and horns on their heads.
But not every reptile that lived during the Mesozoic era was a dinosaur. In fact, a lot of extinct animals that people think of as dinosaurs aren't classified as dinosaurs. This is because they don't share one or more of dinosaurs' basic traits:
- Dinosaurs were animals with four limbs, although not all walked on all four legs.
- Although they may have ventured into the water, they were terrestrial, or land-dwelling, animals.
- Their muscles and bones had several specific features. For example, all dinosaurs had cheek muscles that extended from their jaws to the tops of their skulls.
- Their hip girdles comprised three bones -- the ilium, ischium and pubis. These bones fit together in one of two configurations: ornithischian (bird-hipped) or saurischian (lizard-hipped).
- They had an upright gait. Dinosaurs held their bodies over their legs like rhinoceroses do rather than using the sprawling gait that crocodiles do.
These traits keep some well-known prehistoric animals from being considered dinosaurs:
- Plesiosaurs were aquatic creatures with long bodies and flipper-like fins.
- Another aquatic reptile group, ichthyosaurs, had a more dolphin-like body structure.
- Pterosaurs, like Pteranodon and the Pterodactyl subgroup, were flying reptiles.
- Synapsids had an opening behind their eye socket that also occurs in mammals. One of the most well-known synapsids is Dimetrodon, a lizard-like animal with a large sail on its back.
So because of their bone structure, habitat or other traits, these animals weren't technically dinosaurs. But they did leave behind same evidence that dinosaurs did -- fossils. Next, we'll look at what fossils can and can't tell us about dinosaurs' physical appearance.