Any extinct carnivorous or herbivorous reptile belonging to the predominantly terrestrial orders Saurischia or Ornithischia that existed during the Mesozoic era. Evidence of the earliest dinosaurs can be dated back to around 230 million years ago, and every dinosaur species became extinct within a relatively short time period around 65 million years ago. The term ‘dinosauria’ was coined in 1842 by the English palaeontologist Richard Owen.
Many species of dinosaur evolved during the millions of years they were the dominant land animals. Most were large (up to 27 m/90 ft), but some were as small as modern chickens. Although 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, stenonychosaurus, a hunting dinosaur 2 m/6 ft long, had a brain size comparable to that of a modern mammal or bird, as well as stereoscopic vision and grasping hands.
Classification Dinosaurs are divisible into two 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 and are mainly four-legged and almost entirely herbivorous.
The Saurischia are subdivided into: theropods (‘beast-feet’) and sauropodomorphs (‘lizard-feet forms’). Theropods include all the bipedal carnivorous forms with long hindlimbs and short forelimbs (e.g. tyrannosaurus and Megalosaurus). Sauropodomorphs include sauropods, the large quadrupedal herbivorous and amphibious types with massive limbs, long tails and necks, and tiny skulls (e.g. diplodocus and apatosaurus).
The Ornithischia were almost all plant-eaters, and eventually outnumbered the Saurischia. They are divided into four suborders: ornithopods (‘bird-feet’), stegosaurs (‘plated’ dinosaurs), ankylosaurs (‘armoured’ dinosaurs), and ceratopsians (‘horned’ dinosaurs). Ornithopods include Jurassic and Cretaceous bipedal forms (e.g. iguanodon) and Cretaceous hadrosaurs with duckbills. Stegosaurs are Jurassic quadrupedal dinosaurs with a double row of triangular plates along the back and spikes on the tail (e.g. Stegosaurus). Ankylosaurs are Cretaceous quadrupedal forms, heavily armoured with bony plates (e.g. nodosaurus). Ceratopsians are Upper Cretaceous quadrupedal horned dinosaurs with very large skulls bearing a neck frill and large horns (e.g. 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.
Extinction The most popular theory of dinosaur extinction suggests that the Earth was struck by a giant asteroid or comet 65 million years ago. This is thought to have sent up enough debris and dust into the atmosphere that the Earth's climate was changed into one unsuitable for dinosaurs and they could not adapt quickly enough. Around 70% of all species on Earth died at this time in what is known as the cretaceous tertiary mass extinction event (or KT extinction), which marked the end of the Cretaceous period. The evidence for this includes a high concentration of iridium, an element rare on Earth but common in extraterrestrial bodies, in sedimentary rocks around the world (known as the KT boundary layer) dating from this time.
Bird: Fossils of Early Birds
Dinosaur: Cloning from Preserved DNA
Dinosaur: The Biggest and the Oldest
Extinction: A Race against Time
What Killed the Dinosaurs?
Early Dinosaur Discoveries in North America
101 Crazy Theories About Dinosaur Extinction
World's First Dinosaur Skeleton: Hadrosaurus Foulkii
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