从今天开始更新阅读钩钩

1109双天蝎

顶起来顶起来~上个月的很好~希望这个月更好~虽然21就考了55

XYXB

恩恩，我杀T后再回来看这里的XDJM
ohohh，你抢沙发老是抢不过我，哈哈~

LVICHIGO

gwd27-Q3--Q6 考了。
Two opposing scenarios, the “arboreal” hypothesis and the “cursorial” hypothesis, have traditionally been put forward concerning the origins of bird flight. The “arboreal” hypothesis holds that bird ancestors began to fly by climbing trees and gliding down from branches with the help of incipient feathers: the height of trees provides a good starting place for launching flight, especially through gliding. As feathers became larger over time, flapping flight evolved and birds finally became fully air-borne. This hypothesis makes intuitive sense, but certain aspects are troubling. Archaeopteryx (the earliest known bird) and its maniraptoran dinosaur cousins have no obviously arboreal adaptations, such as feet fully adapted for perching. Perhaps some of them could climb trees, but no convincing analysis has demonstrated how Archaeopteryx would have both climbed and flown with its forelimbs, and there were no plants taller than a few meters in the environments where Archaeopteryx fossils have been found. Even if the animals could climb trees, this ability is not synonymous with gliding ability. (Many small animals, and even some goats and kangaroos, are capable of climbing trees but are not gliders.) Besides, Archaeopteryx shows no obvious features of gliders, such as a broad membrane connecting forelimbs and hind limbs.

 The “cursorial”(running) hypothesis holds that small dinosaurs ran along the ground and stretched out their arms for balance as they leaped into the air after insect prey or, perhaps, to avoid predators. Even rudimentary feathers on forelimbs could have expanded the arm’s surface area to enhance lift slightly. Larger feathers could have increased lift incrementally, until sustained flight was gradually achieved. Of course, a leap into the air does not provide the acceleration produced by dropping out of a tree; an animal would have to run quite fast to take off. Still, some small terrestrial animals can achieve high speeds. The cursorial hypothesis is strengthened by the fact that the immediate theropod dinosaur ancestors of birds were terrestrial, and they had the traits needed for high lift off speeds: they were small, agile, lightly built, long-legged, and good runners. And because they were bipedal, their arms were free to evolve flapping flight, which cannot be said for other reptiles of their time.  
非常谢谢darkangelpp同学~~

附原题两个及中文版

Q 5:

Which of the following is included in the discussion of the cursorial hypothesis but not in the discussion of the arboreal hypothesis?



A.     A discussion of some of the features of Archaeopteryx

B.     A description of the environment known to have been inhabited by bird ancestors

C.     A possible reason why bird ancestors might have been engaging in activities that eventually evolved into flight

D.     A description of the obvious features of animals with gliding ability

E.      An estimate of the amount of time it took for bird ancestors to evolve the kind of flapping flight that allowed them to become completely airborne



-------------------------------------------------

Q 6:

The passage suggests which of the following regarding the climbing ability of Archaeopteryx?



A.     Its ability to climb trees was likely hindered by the presence of incipient feathers on its forelimbs.

B.     It was probably better at climbing trees than were its maniraptoran dinosaur cousins.

C.     It had certain physical adaptations that suggest it was skilled at climbing trees.

D.     Scientists have recently discovered fossil evidence suggesting it could not climb trees.

E.     Scientists are uncertain whether it was capable of climbing trees 两种解释鸟飞的假设：树栖和疾走。
树栖论说鸟祖先爬上树滑行下来，随着羽毛越来越大，最终飞了起来。
树栖论的问题：始祖鸟和M恐龙没有明显的栖树适应性，如合适的脚。
没分析显示始祖鸟曾用前肢爬树和飞，始祖鸟化石发现的地方也没有发现高 树。即使它会爬树也不表明会滑翔。没明显的滑翔特征。
疾走论认为鸟为了躲避猎食者，奔跑并张开双臂平行,前肢的原始特征能帮助身体稍微升高,然后鸟慢慢就飞起来了。
兽脚亚目恐龙祖先拥有各种疾走的特点。
此外，它们用两足走，双臂有空用来拍打。

gaiqihan

顶了

kerby

是不是就在这里更新了呀？？

会不会开新帖呢？？我先收藏这个帖子了

饺子joy

先收藏了~！
LZ辛苦了~
20号考……再拼拼RP……TAT

欧郁姿

祝願你1月RP大爆炸,拿INTERVIEW拿到手软~~~~O(∩_∩)O哈哈~

og12

这月有救了，大牛赞~

jiayini85

感谢记题、贴题、找题、整理的英雄们 谢谢谢谢

kerby

鸟类进化，有哥们找到了背景资料，放上链接：

http://forum.chasedream.com/GMAT_Math/thread-420193-1-1.html