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Revision:A Level Geology - Dinosaur ExtinctionTSR Wiki > Study Help > Subjects and Revision > Revision Notes > Geology > Dinosaur Extinction DinosaursDinosaurs diverged from their archosaur ancestors approximately 230 million years ago during the Middle to Late Triassic period, roughly 20 million years after the Permian-Triassic extinction event wiped out an estimated 95% of all life on Earth. They are divided into two orders, Saurischia and Ornithischia, depending upon pelvic structure. Saurischia includes those taxa sharing a more recent common ancestor with birds than with Ornithischia, while Ornithischia includes all taxa sharing a more recent common ancestor with Triceratops than with Saurischia. Saurischians ('lizard-hipped', from the Greek sauros (σαυρος) meaning 'lizard' and ischion (ισχιον) meaning 'hip joint') retained the hip structure of their ancestors, with a pubis bone directed cranially, or forward. Reptile Hipped Dinosaurs The reptile-hipped dinosaurs have the more primitive hip structure, as seen in the archosaurs. In this type of hip, one of the three main bones (the pubis) points forward. Included in the reptile-hipped dinosaurs are the well-known carnivores such as Velociraptor, and the large herbivores such as Diplodocus. The saurischian dinosaurs include the largest land animals ever to have lived, such as Seismosaurus, which is estimate to have weighed up to 100 tonnes. Bird Hipped Dinosaurs The bird hipped (or ornithischian) dinosaurs, as their name reflects, have a very similar hip structure to living birds. All of these dinosaurs were herbivores, and the main groups include forms such as Hypsilophodon and the 'duck-billed' hadrosaurs such as Maiasaura (an ornithopod), the horned dinosaurs such as Triceratops (a ceratopsian) and other large herbivores such as Stegosaurus and the club-tailed Ankylosaurus. All major groups of dinosaurs can be grouped together to form an evolutionary 'tree' that illustrates their evolutionary relationships. The diagram below shows the arrangement of bones in the ornithischian hip, in which the pubis points backward parallel with the ischium. Evolution Dinosaur evolution after the Triassic follows changes in vegetation and the location of continents. In the Late Triassic and Early Jurassic, the continents were connected as the single landmass Pangaea, there was a worldwide dinosaur fauna mostly composed of coelophysoid carnivores and prosauropod herbivores. Gymnosperm plants (particularly conifers), a potential food source, radiated in the Late Triassic. Prosauropods did not have sophisticated mechanisms for processing food in the mouth, so must have employed other means of breaking down food farther along the digestive tract.
Mass extinctionsNon-avian dinosaurs suddenly became extinct approximately 65 million years ago. Many other groups of animals also became extinct at this time, including ammonites (nautilus-like mollusks), mosasaurs, plesiosaurs, pterosaurs, herbivorous turtles and crocodiles, most birds, and many groups of mammals. This mass extinction is known as the Cretaceous–Tertiary extinction event. The nature of the event that caused this mass extinction has been extensively studied since the 1970s; at present, several related theories are supported by paleontologists. Though the general consensus is that an impact event was the primary cause of dinosaur extinction, some scientists cite other possible causes, or support the idea that a confluence of several factors was responsible for the sudden disappearance of dinosaurs from the fossil record. The 'big-five' are:
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