dinosaur

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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, perhaps the most widely accepted being that the Earth was struck by a comet.

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).

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