Dinosaurs are a diverse group of animals belonging to the life-form group called Dinosauria. They first appeared during the Triassic period, some 230 million years ago, and were the dominant terrestrial vertebrates for 135 million years, extending from the beginning of the Jurassic (about 200 million years ago) until the end of the Cretaceous (65 million years ago), when the end-Cretaceous extinction event led to the extinction of most dinosaur groups at the close of the Mesozoic Era.

The term "Dinosauria" was invented by Sir Richard Owen in 1842 to describe these "fearfully great reptiles," specifically Megalosaurus, Iguanodon, and Hylaeosaurus. The Dinosauria superorder was divided into the two orders Ornithischia and Saurischia by Harry Seeley in 1887. This division is based on the evolution of the pelvis into a more bird-like structure (although birds did not descend from these dinosaurs), details in the vertebrae and armour and the possession of a 'predentary' bone. The predentary is an extra bone in the front of the lower jaw, which extends the dentary (the main lower jaw bone). Together they form a beak-like apparatus used to clip off plant material.

The first dinosaurs appearing in the Late Triassic period apparently were not major components of the fauna. However, by the Early Jurassic (about 30 million years later), after many other Triassic vertebrates had gone extinct, dinosaurs were diversifying rapidly. By then they had become dominant occupants of many major terrestrial adaptive environments, judging from their frequently large size and considerable morphological and taxonomic diversity. The Late Jurassic (about 145 million years ago) through the Late Cretaceous (about 65 million years ago) was the heyday of the dinosaurs. It was also the Late Jurassic that saw the bird lineage diverge from its flightless Theropod ancestors, and these first birds enjoyed an explosion in diversity in the Cretaceous period and beyond to the present-day.