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Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”

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Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post Updated on: 05 May 2018, 00:31
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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.


1. The primary purpose of the passage is to

(A) present counter evidence to two hypotheses concerning the origins of bird flight
(B) propose and alternative to two hypotheses concerning the origins of bird flight correct certain misconceptions about hypotheses concerning the origins of bird flight
(C) dismiss counter evidence to two 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



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

(A) discussion of some of the features of Archaeopteryx
(B) description of the environment known to have been inhabited by bird ancestors
(C) possible reason why bird ancestors might have been engaging in activities that eventually evolved into flight
(D) 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



3. 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



4. 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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Originally posted by JarvisR on 15 Jun 2015, 20:12.
Last edited by hazelnut on 05 May 2018, 00:31, edited 3 times in total.
Edited the question.
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 15 Jun 2015, 22:00
second question is hard even I do not understand oa.
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 16 Jun 2015, 11:23
I chose D for ques 2.....need explanation for its OA
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 30 Aug 2015, 05:01
campus1995 wrote:
I chose D for ques 2.....need explanation for its OA


we are asked to choose an option that refers CH but not AH.

Correct Answer says - Reason for achieving the flight by the animals.

CH Says - The early animals choose to fly either to escape predators or to hunt.

But no such reason was mentioned in AH on why the animals choose to climb trees.

Hope its clear now.

Even i got it wrong when i practiced under timer.. but later understood.. :)
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 05 Aug 2016, 06:45
Interesting passage . Took 3 mins to read and 8 mins and 40 seconds to answer :read

-The author talks about 2 hypotheses for the origin of flight in birds
-The author then evaluates both the hypotheses

1. E - evaluate competing hypotheses concerning the origins of bird flight

2. “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"
This reasoning as to why bird ancestors might have engaged in “flight” type activities is not provided in the discussion of arboreal hypothesis. Hence option (C ) is correct.

3. “and there were no plants taller than a few meters in the environments where Archaeopteryx fossils have been found"
the fact that small animals can climb does not in and of itself undermine the arboreal hypothesis. For C to be correct, it needs to present the entire undermining fact: that many small animals can climb but cannot fly.

also, D is clearly a piece of evidence used to undermine the hypothesis - the earliest known bird did not have trees high enough to glide from, so it probably developed flight in a different way. Once you have D, there really is no reason to go and talk yourself into choosing a trap answer choice such as C.
Option (D) is a clear word justification of the above, Hence correct.

4. “Even if the animals could climb trees, this ability is not synonymous with gliding ability"
The above suggests that scientists were unsure on whether the Archaeopteryx was capable of climbing trees. Option (E) is hence the right answer choice.
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 22 Aug 2018, 00:13
In Q4 why is option A incorrect??
It is stated that there is no convincing analysis to suggest that archaeopteryx could climb trees and fly with its forelimbs

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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 31 Aug 2018, 21:34
MithilaGauri wrote:
In Q4 why is option A incorrect??
It is stated that there is no convincing analysis to suggest that archaeopteryx could climb trees and fly with its forelimbs

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And that's what option (E) states: "Scientists are uncertain whether it was capable of climbing trees." Hence, it's the correct answer.

On the other hand, Option (A) is incorrect as there is no mention in the paragraph about presence of feathers on Archaeopteryx's limbs.
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial” &nbs [#permalink] 31 Aug 2018, 21:34
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