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.
Q 3: The primary purpose of the passage is to
A.present counterevidence to two hypotheses concerning the origins ofbird flight
B.propose and alternative to two hypotheses concerning the origins of bird flight
C.correct certain misconceptions about hypotheses concerning the origins of bird flight
D.refute a challenge to a hypothesis concerning the origins of bird flight
E.evaluate competing hypotheses concerning the origins of bird flight
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Q 4: The passage presents which of the following facts as evidence that tends to undermine the arboreal hypothesis?
A.Feathers tend to become larger over time
B.Flapping flight is thought to have evolved gradually over time
C.Many small animals are capable of climbing trees.
D.Plants in Archaeopteryx’s known habitats were relatively small
E.Leaping into the air does not provide as much acceleration as gliding out of a tree
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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
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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
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发表于 2011-1-30 07:57:46
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