Dinosaurs

Dinosaurs are a highly distinctive group of vertebrates that played a dynamic role in terrestrial ecosystems for 165 million years of Earth's history. Fossils of their teeth and bones are found in abundance all over the world, sometimes along with evidence of soft tissues, nests, eggs, feces, and footprints. Collectively these remains comprise an excellent record extending from their emergence in the Triassic period up to the Cretaceous extinction, testifying to the great diversity of dinosaur species. First described in the early 19th century, dinosaurs were once thought to be sluggish, lizard-like reptiles. Our conception of these animals has changed with each new discovery; modern science shows that many dinosaurs were energetic, dynamic creatures more akin to birds and mammals in their activities. Their majesty and variety inspire perennial fascination, and scientific research on the group has vastly improved our knowledge of evolution and survival strategies in ecosystems under pressure.

Dinosaurs arose from archosaurian reptiles during the mid-Triassic period, approximately 230 million years ago. They became a hugely successful group, surviving several mass extinction events and evolving into a vast array of forms throughout their 165-million-year reign. During this time the dinosaurs achieved remarkable feats of specialization. The smallest were about the size and weight of a modern-day crow, while the largest attained lengths over 40 meters and weights up to 60 tons or more, the most gigantic land animals ever known. Some were quadrupeds, others bipeds, and some could move in either mode. Some were frightful, sharp-toothed carnivores; others were herbivores of such voracity that they altered the landscape with their feeding. Their range of forms and adaptability ensured that while not being the most abundant of terrestrial species, they ruled the top of the food chain in whatever niche they occupied and dominated their habitats over an exceptionally long period of life's history on Earth.

The term dinosaur is derived from the taxonomie group Dinosauria, a term coined by Sir Richard Owen in 1842. The fossils of many large and fantastic animals were being discovered in England at the time, including the carnivorous Megalosaurus, the herbivorous Iguanodon, and the armored Hylaeosaurus. Owen noted that the anatomy of these creatures differed significantly from that of modern and fossil reptiles but felt obligated to respect earlier research classifying the huge beasts as reptiles. He thus invented the name Dinosauria, meaning “fearfully great lizards.”

One modern way of defining groups of organisms is through phylogenetic taxonomy, whereby organisms are related strictly by common ancestry. In this manner, Dinosauria can be defined as all members of the group descended from the most recent common ancestor of Megalosaurus, Iguanodon, and Hylaeosaurus, as these are all quite early forms of the two great dinosaur lineages Ornithischia and Saurischia. Dinosaurs can also be broadly defined as large terrestrial vertebrates that lived only during the Mesozoic era. Contrary to images in movies and popular culture, the prehistoric aquatic reptiles are not dinosaurs, nor are the flying pterosaur reptiles. Dinosaurs were predominantly terrestrial in nature, although fossil traces testify that some dinosaurs swam on occasion. Likewise, although the skeletons and feathers of certain species suggest some sort of flying ability, and theropod dinosaurs did indeed give rise to birds, this entry follows common usage in restricting the term dinosaur to nonavian dinosaurs unless specifically noted.

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