The word "dinosaur" comes from the ancient Greek words "deinos" ("terrible") and "sauros" ("lizard"). Strictly speaking, dinosaurs refer to land dwelling creatures that dominated life on earth during the Mesozoic Era (225 million to 65 million years ago). They lived alongside with marine reptiles (such as ichthyosaurs, mosasaurs, plesiosaurs) and flying reptiles. As a group, dinosaurs existed for over 150 million years, occupying every type of environment and climate on Earth. They ranged in size from those as small as a chicken to others over 100 feet long and weighing nearly 100 tons. Dinosaurs did not achieve their great size or domination overnight - they evolved alongside reptiles and mammals, but managed to outcompete them. Were it not for the great Cretaceous extinction 65 million years ago, the mammals would not have evolved as they did, nor would human beings as we know them exist.