Any of a group (sometimes considered as two separate orders) of extinct reptiles living between 205 million and 65 million years ago. Their closest living relations are crocodiles and birds. Many species of dinosaur evolved during the millions of years they were the dominant large land animals. Most were large (up to 27 m/90 ft), but some were as small as chickens. They disappeared 65 million years ago for reasons not fully understood, although many theories exist.

Classification
Dinosaurs are divisible into two unrelated stocks, the orders Saurischia ('lizard-hip') and Ornithischia ('bird-hip'). Members of the former group possess a reptile-like pelvis and are mostly bipedal and carnivorous, although some are giant amphibious quadrupedal herbivores. Members of the latter group have a bird-like pelvis, are mainly four-legged, and entirely herbivorous.

The Saurischia are divided into: theropods ('beast-feet'), including all the bipedal carnivorous forms with long hindlimbs and short forelimbs (tyrannosaurus, megalosaurus); and sauropodomorphs ('lizard-feet forms'), including sauropods, the large quadrupedal herbivorous and amphibious types with massive limbs, long tails and necks, and tiny skulls (diplodocus, brontosaurus).

The Ornithischia were almost all plant-eaters, and eventually outnumbered the Saurischia. They are divided into four suborders: ornithopods ('bird-feet'), Jurassic and Cretaceous bipedal forms (Iguanodon) and Cretaceous hadrosaurs with duckbills; stegosaurs ('plated' dinosaurs), Jurassic quadrupedal dinosaurs with a double row of triangular plates along the back and spikes on the tail (stegosaurus); ankylosaurs ('armoured' dinosaurs), Cretaceous quadrupedal forms, heavily armoured with bony plates (nodosaurus); and ceratopsians ('horned' dinosaurs), Upper Cretaceous quadrupedal horned dinosaurs with very large skulls bearing a neck frill and large horns (triceratops).

These two main dinosaur orders form part of the superorder Archosaurus ('ruling reptiles'), comprising a total of five orders. The other three are Pterosaurs ('winged lizards'), including pterodactyls, of which no examples exist today, crocodilians, and birds. All five orders are thought to have evolved from a 'stem-order', the Thecondontia.

Species
Brachiosaurus, a long-necked plant-eater of the sauropod group, was about 12.6 m/40 ft to the top of its head, and weighed 80 tonnes. Compsognathus, a meat-eater, was only the size of a chicken, and ran on its hind legs. Stegosaurus, an armoured plant-eater 6 m/20 ft long, had a brain only about 3 cm/1.25 in long. Not all dinosaurs had small brains. At the other extreme, the hunting dinosaur stenonychosaurus, 2 m/6 ft long, had a brain size comparable to that of a mammal or bird of today, stereoscopic vision, and grasping hands. Many dinosaurs appear to have been equipped for a high level of activity. Tyrannosaurus was a huge, two-footed, meat-eating theropod dinosaur of the Upper Cretaceous in North America and Asia. The largest carnivorous dinosaur was Giganotosaurus carolinii. It lived in Patagonia about 97 million years ago, was 12.5 m/41 ft long, and weighed 6–8 tonnes. Its skeleton was discovered 1995.

Theories of extinction
A popular theory of dinosaur extinction suggests that the Earth was struck by a giant meteorite or a swarm of comets 65 million years ago and this sent up such a cloud of debris and dust that climates were changed and the dinosaurs could not adapt quickly enough. The evidence for this includes a bed of rock rich in iridium – an element rare on Earth but common in extraterrestrial bodies – dating from the time.

An alternative theory suggests that changes in geography brought about by the movements of continents and variations in sea level led to climate changes and the mixing of populations between previously isolated regions. This resulted in increased competition and the spread of disease.

Archaeological findings
The term 'dinosaur' was coined 1842 by Richard Owen, although there were findings of dinosaur bones as far back as the 17th century. In 1822 G A Mantell (1790–1852) found teeth of iguanodon in a quarry in Sussex. The first dinosaur to be described in a scientific journal was in 1824, when William Buckland, professor of geology at Oxford University, published his finding of a 'megalosaurus or great fossil lizard' found at Stonesfield, a village northwest of Oxford, although a megalosaurus bone had been found in 1677.

One of the largest dinosaur species found in the UK was a Sauropod, Cetiosaurus oxoniensis, discovered in 1870 near Bletchingdon, north of Oxford. It was around 15 m/49 ft long, although specimens have been discovered in North Africa up to 18 m/60 ft long. In 1992 another large dinosaur, Iguanodon bernissartensis, was discovered near Ockley in Surrey, England, by amateur fossil hunters.

An almost complete fossil of a dinosaur skeleton was found in 1969 in the Andean foothills, South America; it had been a two-legged carnivore 2 m/6 ft tall and weighed more than 100 kg/220 lb. More than 230 million years old, it is the oldest known dinosaur. In 1982 a number of nests and eggs were found in 'colonies' in Montana, suggesting that some bred together like modern seabirds. In 1987 finds were made in China that may add much to the traditional knowledge of dinosaurs, chiefly gleaned from North American specimens. In 1989 and 1990 an articulated Tyrannosaurus rex was unearthed by a palaeontological team in Montana, with a full skull, one of only six known. Short stretches of dinosaur DNA were extracted in 1994 from unfossilized bone retrieved from coal deposits approximately 80 million years old.

Recent discoveries
The discovery of a small dinosaur was announced in China in 1996. Sinosauropteryx lived about 120 million years old and was 0.5 m/1.6 ft tall. It had short forelegs, a long tail, and short feathers, mainly on its neck and shoulders.

In 1997 US scientists claimed that 65 million-year-old remains discovered in the Atlantic Ocean were proof that a massive asteroid impact on Earth killed the dinosaurs. A sea-drilling expedition discovered three samples that have the signature of an asteroid impact. Previous evidence from sediment suggested that the dinosaurs did not become extinct at exactly the same time as an impact occurred. The new evidence appeared to substantiate the theories of geologists such as Walter Alvarez, who championed the theory that the dinosaurs disappeared from fossil history because of such an impact.

US palaeontologists discovered in 1997 a dinosaur wishbone in place in the skeleton of a velociraptor. This was the first time a wishbone had been found in place and scientists claimed that this constitutes strong evidence for birds having evolved from dinosaurs.