Dinosaur

Dinosaurs were a diverse and highly successful group of reptiles that lived on land for well over 160 million years. They first appeared during the Triassic period and dominated terrestrial ecosystems through the Jurassic and Cretaceous periods, before most of them died out in a mass extinction about 66 million years ago. Their fossils are found on every continent, and their study has transformed how scientists understand evolution, anatomy and Earth history.

Modern science regards birds as living dinosaurs. Birds evolved from a lineage of small, feathered, meat-eating dinosaurs, which means dinosaurs never entirely disappeared; they survive today in the form of the roughly ten thousand bird species around the world.

Origins and the age of dinosaurs

The earliest dinosaurs evolved in the Triassic, at a time when all the continents were joined in a single landmass. Early dinosaurs were relatively small and were not yet dominant; other reptile groups were more common. After a major extinction at the end of the Triassic, dinosaurs expanded into many of the ecological roles left open, and they became the dominant large land animals for the remainder of the Mesozoic era. The Mesozoic is often informally called the "age of reptiles" for this reason.

Major groups

Dinosaurs are traditionally divided into two main branches based on the shape of their hip bones: the Saurischia and the Ornithischia. The saurischians included the enormous long-necked plant-eaters such as sauropods, as well as the theropods, the mostly meat-eating group from which birds arose. The ornithischians were almost all plant-eaters and included forms with horns, frills, plates and thick armoured skulls, such as the horned dinosaurs and the duck-billed dinosaurs. These categories are based on anatomy and evolutionary relationships rather than on diet alone.

Size and diversity

Dinosaurs ranged from animals the size of a chicken to giants that were among the largest land animals ever to live. Some sauropods reached extraordinary lengths and weights, while small theropods were lightly built and probably fast and agile. Diets varied too: many dinosaurs ate plants, others hunted prey, and some may have been scavengers or omnivores. Evidence from fossils, including stomach contents and tooth wear, helps scientists reconstruct what they ate and how they lived.

Behaviour and biology

Fossil evidence has revealed a great deal about dinosaur behaviour. Nests, eggs and clusters of young suggest that some species cared for their offspring. Trackways show that some dinosaurs moved in groups, and bone structures indicate that many were active, possibly warm-blooded animals rather than slow, cold-blooded reptiles. The discovery of feathered dinosaurs in particular blurred the line between dinosaurs and birds and supported the idea that some dinosaurs were warm-blooded and socially complex.

The extinction

About 66 million years ago, a mass extinction wiped out the non-bird dinosaurs along with many other species, including large marine reptiles and many groups of plankton. The leading explanation is that a large asteroid struck the region that is now the Yucatán Peninsula in Mexico. The impact would have thrown vast amounts of dust and debris into the atmosphere, blocking sunlight, cooling the planet and disrupting food chains. Intense volcanism in what is now India may have added to the stress on ecosystems. Birds, being small and able to fly and feed in varied ways, survived and diversified.

How we know about dinosaurs

Almost everything known about dinosaurs comes from fossils: bones, teeth, eggs, footprints and, more rarely, preserved soft tissue and feathers. Palaeontologists compare these remains with those of living animals to infer how dinosaurs moved, grew and reproduced. New techniques, including computed tomography scanning and chemical analysis, allow researchers to study fossils in ways earlier generations could not, and new species are still being described regularly.

Common misconceptions

Several popular ideas about dinosaurs are mistaken. Dinosaurs and humans never lived at the same time; the last non-bird dinosaurs died out tens of millions of years before the first humans appeared. Pterosaurs and plesiosaurs, often shown alongside dinosaurs, were not dinosaurs at all. Not every dinosaur was enormous, and many were small. Finally, the familiar image of a scaly, dull-coloured dinosaur is outdated; many dinosaurs had feathers, and at least some were brightly coloured.

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