Dinosaurs

Abstract

Dinosaurs are a group of extinct reptiles that had an upright limb posture. Their fossil record extends from the Late Triassic (225–230Ma) until their extinction at the end of the Cretaceous (66Ma). Dinosaur body fossils (skeletons, bones and teeth) have long been collected on all of the continents, and at least 500 different genera of dinosaurs have received scientific names. There is also an important record of dinosaurs preserved in their trace fossils: skin impressions, footprints, gizzard stones (gastroliths), tooth marks (dentalites) and feces (coprolites). Vertebrate paleontologists have long identified dinosaurs as a monophyletic, single clade united by evolutionary novelties that indicate that dinosaurs held the hind limb upright and directly under the body. Cladistic analysis has upheld a twofold division of all but a few dinosaurs into the two clades, Saurischia and Ornithischia. Saurischia consist of two clades, Theropoda (the meat-eating dinosaurs) and the Sauropodomorpha (includes the largest dinosaurs). Ornithischia comprises three clades: Ornithopoda (mostly iguanodonts and hadrosaurs), Thyreophora (stegosaurs and ankylosaurs) and Marginocephalia (ceratopsians and pachycephalosaurs). During the Late Triassic, the theropods and sauropodomorphs diverged, and basal thyreophorans and ornithopods did not appear until the Early or Middle Jurassic. The ankylosaur-stegosaur split in the thyreophorans took place by Middle Jurassic time, and marginocephalians did not diverge from ornithopods until the Middle Jurassic. During the Late Triassic, most dinosaurs were small and rare, and other reptiles dominated the landscape. Dinosaurs dominated life on land during the Jurassic and Cretaceous. During the Jurassic, sauropods, stegosaurs and small- to medium-size ornithopods were the dominant plant eaters. Megalosaurs and carnosaurs were the large meat eaters. A transition in dinosaurs took place between the end of the Jurassic and the Late Cretaceous, so that Early Cretaceous ornithopods were generally larger, and sauropods were generally smaller. Nodosaurid ankylosaurs first became common, whereas stegosaurs virtually disappeared. Dinosaurs of the Late Cretaceous were very different. They were mostly hadrosaurid ornithopods, ceratopsians, ankylosaurs and tyrannosaurid and abelisaurid theropods. Sauropods were common only in South America. Dinosaur extinction was part of a more pervasive extinction that occurred at the end of the Cretaceous.