Dinosaurs: One of a group of extinct reptiles that lived from about 230 million to about 65 million years ago.

The word dinosaur was coined in 1842 by British anatomist Sir Richard Owen, derived from the Greek words deinos, meaning “marvelous” or “terrible,” and sauros, meaning “lizard.” For more than 140 million years, dinosaurs reigned as the dominant animals on land.

Owen distinguished dinosaurs from other prehistoric reptiles by their upright rather than sprawling legs and by the presence of three or more vertebrae supporting the pelvis, or hipbone. Dinosaurs are classified into two orders according to differences in pelvic structure: Saurischia, or lizard-hipped dinosaurs, and Ornithischia, or bird-hipped dinosaurs. Dinosaur bones occur in sediments that were deposited during the Mesozoic Era, the so-called era of middle animals, also known as the age of reptiles. This era is divided into three periods: the Triassic (240 million to 205 million years ago), the Jurassic (205 million to 138 million years ago), and the Cretaceous (138 million to 65 million years ago).

Historical references to dinosaur bones may extend as far back as the 5th century bc. Some scholars think that Greek historian Herodotus was referring to fossilized dinosaur skeletons and eggs when he described griffins—legendary beasts that were part eagle and part lion—guarding nests in central Asia. “Dragon bones” mentioned in a 3rd century ad text from China are thought to refer to bones of dinosaurs.

In the late 19th and early 20th centuries, as the science of paleontology grew and the search for dinosaur remains was extended around the world, new kinds of dinosaurs were discovered. At present, about 350 different varieties of dinosaur have been identified from bones found on all of the continents as well as on the islands of Greenland, Madagascar, and New Zealand.