第1个回答 2012-02-03
Dinosaurs 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.[29][30] Radiometric dating of the rock formation that contained fossils from the early dinosaur genus Eoraptor establishes its presence in the fossil record at this time. Paleontologists think that Eoraptor resembles the common ancestor of all dinosaurs;[31] if this is true, its traits suggest that the first dinosaurs were small, bipedal predators.[32] The discovery of primitive, dinosaur-like ornithodirans such as Marasuchus and Lagerpeton in Argentinian Middle Triassic strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators.
When dinosaurs appeared, terrestrial habitats were occupied by various types of archosaurs and therapsids, such as aetosaurs, cynodonts, dicynodonts, ornithosuchids, rauisuchians, and rhynchosaurs. Most of these other animals became extinct in the Triassic, in one of two events. First, at about the boundary between the Carnian and Norian faunal stages (about 215 million years ago), dicynodonts and a variety of basal archosauromorphs, including the prolacertiforms and rhynchosaurs, became extinct. This was followed by the Triassic–Jurassic extinction event (about 200 million years ago), that saw the end of most of the other groups of early archosaurs, like aetosaurs, ornithosuchids, phytosaurs, and rauisuchians. These losses left behind a land fauna of crocodylomorphs, dinosaurs, mammals, pterosaurians, and turtles.[11] The first few lines of early dinosaurs diversified through the Carnian and Norian stages of the Triassic, most likely by occupying the niches of the groups that became extinct.
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, and there was a worldwide dinosaur fauna mostly composed of coelophysoid carnivores and early sauropodomorph herbivores.[33] Gymnosperm plants (particularly conifers), a potential food source, radiated in the Late Triassic. Early sauropodomorphs did not have sophisticated mechanisms for processing food in the mouth, and so must have employed other means of breaking down food farther along the digestive tract.[34] The general homogeneity of dinosaurian faunas continued into the Middle and Late Jurassic, where most localities had predators consisting of ceratosaurians, spinosauroids, and carnosaurians, and herbivores consisting of stegosaurian ornithischians and large sauropods. Examples of this include the Morrison Formation of North America and Tendaguru Beds of Tanzania. Dinosaurs in China show some differences, with specialized sinraptorid theropods and unusual, long-necked sauropods like Mamenchisaurus.[33] Ankylosaurians and ornithopods were also becoming more common, but prosauropods had become extinct. Conifers and pteridophytes were the most common plants. Sauropods, like the earlier prosauropods, were not oral processors, but ornithischians were evolving various means of dealing with food in the mouth, including potential cheek-like organs to keep food in the mouth, and jaw motions to grind food.[34] Another notable evolutionary event of the Jurassic was the appearance of true birds, descended from maniraptoran coelurosaurians.[14]
By the Early Cretaceous and the ongoing breakup of Pangaea, dinosaurs were becoming strongly differentiated by landmass. The earliest part of this time saw the spread of ankylosaurians, iguanodontians, and brachiosaurids through Europe, North America, and northern Africa. These were later supplemented or replaced in Africa by large spinosaurid and carcharodontosaurid theropods, and rebbachisaurid and titanosaurian sauropods, also found in South America. In Asia, maniraptoran coelurosaurians like dromaeosaurids, troodontids, and oviraptorosaurians became the common theropods, and ankylosaurids and early ceratopsians like Psittacosaurus became important herbivores. Meanwhile, Australia was home to a fauna of basal ankylosaurians, hypsilophodonts, and iguanodontians.[33] The stegosaurians appear to have gone extinct at some point in the late Early Cretaceous or early Late Cretaceous. A major change in the Early Cretaceous, which would be amplified in the Late Cretaceous, was the evolution of flowering plants. At the same time, several groups of dinosaurian herbivores evolved more sophisticated ways to orally process food. Ceratopsians developed a method of slicing with teeth stacked on each other in batteries, and iguanodontians refined a method of grinding with tooth batteries, taken to its extreme in hadrosaurids.[34] Some sauropods also evolved tooth batteries, best exemplified by the rebbachisaurid Nigersaurus.[35]
There were three general dinosaur faunas in the Late Cretaceous. In the northern continents of North America and Asia, the major theropods were tyrannosaurids and various types of smaller maniraptoran theropods, with a predominantly ornithischian herbivore assemblage of hadrosaurids, ceratopsians, ankylosaurids, and pachycephalosaurians. In the southern continents that had made up the now-splitting Gondwana, abelisaurids were the common theropods, and titanosaurian sauropods the common herbivores. Finally, in Europe, dromaeosaurids, rhabdodontid iguanodontians, nodosaurid ankylosaurians, and titanosaurian sauropods were prevalent.[33] Flowering plants were greatly radiating,[34] with the first grasses appearing by the end of the Cretaceous.[36] Grinding hadrosaurids and shearing ceratopsians became extremely diverse across North America and Asia. Theropods were also radiating as herbivores or omnivores, with therizinosaurians and ornithomimosaurians becoming common.[34]
The Cretaceous–Paleogene extinction event, which occurred approximately 65 million years ago at the end of the Cretaceous period, caused the extinction of all dinosaur groups except for the neornithine birds. Some other diapsid groups, such as crocodilians, sebecosuchians, turtles, lizards, snakes, sphenodontians, and choristoderans, also survived the event.[37]
The surviving lineages of neornithine birds, including the ancestors of modern ratites, ducks and chickens, and a variety of waterbirds, diversified rapidly at the beginning of the Paleogene period, entering ecological niches left vacant by the extinction of Mesozoic dinosaur groups such as the arboreal enantiornithines, aquatic hesperornithine, and even the larger terrestrial theropods (in the form of Gastornis, mihirungs, and "terror birds"). However, mammals were also rapidly diversifying during this time, and out-competed the neornithines for dominance of most terrestrial niches.[38]本回答被提问者采纳
第3个回答 2011-02-01
in 1842, Britain ancient lived the scientist Charlie Germany · Irving to found “the dinosaur” this noun. English dinosaur from Greek article deinos (meaning is terror) Saurosc (meaning is lizard or reptile). Regarding Irving then, this “the terror lizard” or “the terror reptile” is the reptile which referred to a big way exterminates. In fact, discovered at that time the dinosaur are not many. After 1989 antarctica discovery dinosaur, the world seven continents had dinosaur's vestige. At present in the world the dinosaur which describes is had at least 650 to 700 many kinds of (in biology species). along with dinosaur's massive discoveries and research thorough, dinosaur's meaning had the archery target change. This essence's change is: The dinosaur was not only exterminated, the huge ancient times reptile, moreover was the ancient times reptile which could stand erect walks. Some ancient times' reptile, for instance with dinosaur same time pterodactyl, fish dragon, plesiosaurus and so on, is also huge after the animal which exterminates. But they cannot use two or four legs walk on the land, therefore they are not the dinosaur. Reptile some are the advance by creeping, walks when the abdomen clings the tread, the four limbs tries the center of resistance in body's both sides, like nowadays's turtle, alligator and so on; Some walk when the body leaves the ground slightly, but can only take the very short road, like one kind of named dragon Parker alligator's ancient times alligator, as well as modern certain alligators; Moreover is the dinosaur can equally likely in the organic evolution history the most successful place. Therefore we may say like this: The dinosaur was the life is about 23,500,000,000 to 65,000,000 year ago, can walk, already a kind of terrestrial reptile which by the hind legs support body erectness exterminated. (Shanghai Science and technology Education Publishing house "Chinese Dinosaur" author: Zhen Shuona)