Dinosaurs came in many different sizes and shapes. They belong to a group of reptiles called archosaurs. The group also includes crocodiles and pterosaurs. They stood with their legs straight under their bodies. Lizards and crocodiles sprawl instead. Flying pterosaurs and sea reptiles such as plesiosaurs were not dinosaurs. They lived in the same age, though. Dimetrodon, the animal with a sail on its back, was not one either.

Dinosaurs lasted well over 100 million years on land before the end of the Cretaceous. They first evolved in the Triassic. They did not become very varied until the Jurassic, after a mass extinction at the end of the Triassic. Their age was the Mesozoic Era. It had three periods: the Triassic, the Jurassic and the Cretaceous.

We learn about these animals from two kinds of fossils. Body fossils are things such as bones and teeth. Trace fossils are things such as footprints. Most of these are footprints. Some keep impressions of the skin, which can show scales. Dung left behind also became fossil, and it gives clues about what they ate.

The first Iguanodon fossils were found in southern England in 1822. They were teeth. They were first thought to belong to a giant iguana. Iguanodon's spiky thumb bone was put on its nose like a horn. Better fossils showed it belonged on its hand.

The word Dinosauria was made up in the early 1840s by Richard Owen. He made it from three British fossils. They were Megalosaurus, Iguanodon and Hylaeosaurus. He described the animals as fearfully great reptiles.

Becoming a fossil is rare. The ones we know from fossils are only a tiny share of all that ever lived. Most fossils found are of sea animals such as shellfish. So a fossil of a big land animal is unusual. Most dinosaur fossils are found in rock laid down by ancient rivers, which buried the bones. Some of the biggest, such as Argentinosaurus, are known only from very incomplete fossils. Their sizes are estimates.

As of November 2026, scientists link a giant asteroid that hit Earth about 66 million years ago to the end of the dinosaurs. As of November 2026, it hit just off the Yucatan Peninsula in Mexico. It left a buried crater more than 90 miles (150 kilometres) across, called Chicxulub.

Birds were the only dinosaurs that survived when the rest died out. Some mammals, small reptiles, fish and amphibians also survived the extinction. They included the lines that led to animals alive today.

Words to know

• archosaurs: reptile group
• sprawl: spread wide
• fossils: ancient remains
• estimates: good guesses