Dinosaurs roamed(/r??m/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（縮寫） for Cretaceous, which is associated with the German word “Kreidezeit.”) Besides dinosaurs, many other species of mammals（/'m?ml/哺乳動(dòng)物）, amphibians（/?m'fibi?n/兩棲動(dòng)物） and plants died out at the same time. Over the years, paleontologists（/?pe?l? ?n?t ?l?d ??st/古生物學(xué)家） have proposed several theories for this extensive （/?k'stens?v/大量的）die-off. One early theory was that small mammals ate dinosaur eggs, thereby reducing the dinosaur population until it became unsustainable（/?ns?'ste?n?b(?)l/芽淡，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（/ple?g/瘟疫） decimated（/'des?me?t/大批殺害） the dinosaur population and then spread to the animals that feasted （/fi?st/享受）on their carcasses （/?kɑ:k?s/尸體）. Starvation was another possibility: Large dinosaurs required vast amounts of food and could have stripped （剝奪）bare（光禿） all the vegetation （植物，草木）in their habitat（/'h?b?t?t/棲息處）. But many of these theories are easily dismissed（/d?s'm?s/散開(kāi)）. If dinosaurs’ brains were too small to be adaptive, they would not have flourished（/'fl?r??/繁盛） for 160 million years. Also, plants do not have brains nor do they suffer from the same diseases（/d?'ziz/） as animals, so their simultaneous （/?s?m(?)l'te?n??s/同時(shí)發(fā)生的）extinction makes these theories less plausible（/'pl??z?b(?)l/似乎是真的）.
