Dinosaurs are a group of terrestrial reptiles that first appeared in the Late (or perhaps Middle) Triassic Period, about 230 million years ago. Although dinosaur bones have been discovered throughout human history, these animals were only described scientifically in the early nineteenth century. Since that time, scientists have identified nearly a thousand different dinosaur species, from all continents, and ranging in size from a few kilograms to tens of tons.
Dinosaurs are also a popular and evocative group of animals, familiar to children and adults around the world. For a century they embodied the concepts of failure, sloth, and obsolescence, but the recent renaissance in dinosaur paleontology has led us to understand them as active, successful organisms that dominated terrestrial ecosystems for over 160 million years.
Source From Dinosaur Source http://paleobiology.si.edu/dinosaurs/info/everything/what.html
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