The word "dinosaur", which is derived from the Greek term for "terrible lizard", is the popular name for a group of extinct land-dwelling reptiles. They were the dominant vertebrate animals during most of the Mesozoic era, which began 225 million years ago and ended 65 million years ago. Among the dinosaurs were the largest animals that ever walked the earth, although some of the earliest dinosaurs were very small. The Mesozoic era is divided into three periods, Triassic, Jurassic, and Cretaceous, of approximately equal length. Dinosaurs first appeared in the later third of the Triassic period. Experts believe that dinosaurs developed from a group of archosauromorph reptiles such as Marasuchus, which was a lightly built flesh eater about 1.3 meters long. It was clearly a biped, running on its hind legs, and the long tail was presumably used as a balancing organ. Dinosaurs are divided into two separate orders, depending on the arrangement and shape of the hip bones, which determine thewayan animal walks and holds its body. The saurischians, or "reptile hips", as they are commonly called, arose in the early part of the Late Triassic; the ornithischians, or "bird hips", arose toward the end of the Triassic period.
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