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Dinosaurs roamed(/rəʊm/漫游) the earth for 160 million years until their sudden demise （死）some 65.5 million years ago, in an event now known as the Cretaceous-Tertiary, or K-T, extinction event. (“K” is the abbreviation（缩写） for Cretaceous, which is associated with the German word “Kreidezeit.”) Besides dinosaurs, many other species of mammals（/'mæml/哺乳动物）, amphibians（/æm'fibiən/两栖动物） and plants died out at the same time. Over the years, paleontologists（/ˌpeɪlɪ ɔnˈt ɔləd ʒɪst/古生物学家） have proposed several theories for this extensive （/ɪk'stensɪv/大量的）die-off. One early theory was that small mammals ate dinosaur eggs, thereby reducing the dinosaur population until it became unsustainable（/ʌnsə'steɪnəb(ə)l/，不成立的）. Another theory was that dinosaurs’ bodies became too big to be operated by their small brains. Some scientists believed a great plague（/pleɪg/瘟疫） decimated（/'desɪmeɪt/大批杀害） the dinosaur population and then spread to the animals that feasted （/fiːst/享受）on their carcasses （/ˈkɑ:kəs/尸体）. Starvation was another possibility: Large dinosaurs required vast amounts of food and could have stripped （剥夺）bare（光秃） all the vegetation （植物，草木）in their habitat（/'hæbɪtæt/栖息处）. But many of these theories are easily dismissed（/dɪs'mɪs/散开）. If dinosaurs’ brains were too small to be adaptive, they would not have flourished（/'flʌrɪʃ/繁盛） for 160 million years. Also, plants do not have brains nor do they suffer from the same diseases（/dɪ'ziz/） as animals, so their simultaneous （/ˌsɪm(ə)l'teɪnɪəs/同时发生的）extinction makes these theories less plausible（/'plɔːzɪb(ə)l/似乎是真的）.
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