2016年10月28日托福阅读真题答案(宁波新东方版)

　　2016年10月28日托福阅读真题第二篇 Extinctions at the End of the Cretaceous(白垩纪)(生命/物质类)

　　【20160109&20150614】旧题

　　原文回顾：白垩纪的物种大灭绝。这个话题大家中文背景知道都有一定的了解。这篇文章主要关注点在于海洋生物和陆地生物，是同步减少的。首先他说，有35%的不如动物和75%的植被都在大灭绝中消失，同时在海洋中，大量的生物比如micro plankton(浮游生物)也灭绝了。这也导致了陆地上以叶子为食物链基础的食物链也断了，树木减少，恐龙赖以生存的栖息地和食物也不断下降，最终导致恐龙灭绝。但是鸟类和部分爬行动物幸存下来，因为他们是腐蚀性动物，动物尸体为他们的食物。

　　第二个灭绝的原因是恐龙的繁殖速度太慢，但是像鳕鱼繁殖力较大的就能存活。后面介绍了kt界层线的标志是浮游生物的数量变化。活下来的浮游生物获得一种特殊的能力。界线两边的浮游生物化石有变化。在一层沉积层中，科学家发现浮游生物化石不多，但是有一种不同的生物化石数目很多，认为两者有一定的联系。1860年间，还没有很多证据证明kt界层线的具体形成原因。同年有其他学者找到证据认为kt是因为小行星a撞击地球造成的。因为在kt中发现了很多铱元素，这种元素在a中大量存在。但是也有人认为，这种元素可能来自地球自身的火山运动产生。

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　　相关背景学习

　　The Cretaceous–Paleogene (K–Pg) extinction event, also known as the Cretaceous–Tertiary (K–T) extinction, was a mass extinction of some three-quarters of the plant and animal species on Earth that occurred over a geologically short period of time approximately 66 million years ago. With the exception of some ectothermic species like the leatherback sea turtle and crocodiles, no tetrapod weighing more than 55 pounds (25 kilos) survived. It marked the end of the Cretaceous period and with it, the entire Mesozoic Era, opening the Cenozoic Era that continues today.

　　In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows high levels of the metal iridium, which is rare in the Earth's crust but abundant in asteroids.

　　As originally proposed in 1980 by a team of scientists led by Luis Alvarez, it is now generally thought that the K–Pg extinction was triggered by a massive comet or asteroid impact 66 million years ago and its catastrophic effects on the global environment, including a lingering impact winter that made it impossible for plants and plankton to carry out photosynthesis. The impact hypothesis, also known as the Alvarez hypothesis, was bolstered by the discovery of the 180-kilometre-wide (112 mi) Chicxulub crater in the Gulf of Mexico in the early 1990s, which provided conclusive evidence that the K–Pg boundary clay represented debris from an asteroid impact. The fact that the extinctions occurred at the same time as the impact provides strong situational evidence that the K–Pg extinction was caused by the asteroid. It was possibly accelerated by the creation of the Deccan Traps. However, some scientists maintain the extinction was caused or exacerbated by other factors, such as volcanic eruptions, climate change, or sea level change, separately or together.

　　A wide range of species perished in the K–Pg extinction. The most well-known victims are the non-avian dinosaurs. However, the extinction also destroyed a plethora of other terrestrial organisms, including certain mammals, pterosaurs, birds, lizards, insects, and plants. In the oceans, the K–Pg extinction killed off plesiosaurs and the giant marine lizards (Mosasauridae) and devastated fish, sharks, mollusks (especially ammonites, which became extinct) and many species of plankton. It is estimated that 75% or more of all species on Earth vanished. Yet the devastation caused by the extinction also provided evolutionary opportunities. In the wake of the extinction, many groups underwent remarkable adaptive radiations—a sudden and prolific divergence into new forms and species within the disrupted and emptied ecological niches resulting from the event. Mammals in particular diversified in the Paleogene, producing new forms such as horses, whales, bats, and primates. Birds, fish and perhaps lizards also radiated.

　　Complex Cretaceous–Paleogene clay layer (gray) in the Geulhemmergroeve tunnels near Geulhem, The Netherlands. Finger is on the actual Cretaceous-Paleogene boundary.