Two opposing scenarios,
the "arboreal" hypothesis and
the "cursorial" hypothesis, have
Line traditionally been put forward con-
(5)cerning the origins of bird flight.
The "arboreal" hypothesis holds
that bird ancestors began to fly
by climbing frees and gliding
down from branches with the
(10) 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,
(15) flapping flight evolved and birds
finally became fully air-borne.
This hypothesis makes intuitive
Sense, but certain aspects are
Troubling. Archaeopteryx (the
(20) earliest known bird) and its
maniraptoran dinosaur cousins
have no obviously arboreal
adaptations, such as feet fully
adapted for perching. Perhaps
(25) 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
(30) 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
(35) synonymous with gliding ability.
(Many small animals, and even
some goats and kangaroos,
are capable of climbing trees
but are not gliders.) Besides,
(40) Archaeopteryx shows no obvious
features of gliders, such as
a broad membrane connecting
forelimbs and hind limbs.
The "cursorial"(running)
(45) 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,
(50) to avoid predators. Even rudimentary
feathers on forelimbs could have expanded
the arm's surface area to enhance lift
slightly. Larger feathers could
(55) have increased lift incrementally,
until sustained flight was gradually achieved.
Of course, a leap into the air does not provide the
acceleration produced by drop-
(60) ping 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
(65) hypothesis is strengthened by
the fact that the immediate the ropod
dinosaur ancestors of birds were terrestrial,
and they had the traits needed for high
(70) 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,
(75) which cannot be said for other
reptiles of their time.
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
Please provide explanation with your answer.
the "arboreal" hypothesis and
the "cursorial" hypothesis, have
Line traditionally been put forward con-
(5)cerning the origins of bird flight.
The "arboreal" hypothesis holds
that bird ancestors began to fly
by climbing frees and gliding
down from branches with the
(10) 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,
(15) flapping flight evolved and birds
finally became fully air-borne.
This hypothesis makes intuitive
Sense, but certain aspects are
Troubling. Archaeopteryx (the
(20) earliest known bird) and its
maniraptoran dinosaur cousins
have no obviously arboreal
adaptations, such as feet fully
adapted for perching. Perhaps
(25) 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
(30) 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
(35) synonymous with gliding ability.
(Many small animals, and even
some goats and kangaroos,
are capable of climbing trees
but are not gliders.) Besides,
(40) Archaeopteryx shows no obvious
features of gliders, such as
a broad membrane connecting
forelimbs and hind limbs.
The "cursorial"(running)
(45) 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,
(50) to avoid predators. Even rudimentary
feathers on forelimbs could have expanded
the arm's surface area to enhance lift
slightly. Larger feathers could
(55) have increased lift incrementally,
until sustained flight was gradually achieved.
Of course, a leap into the air does not provide the
acceleration produced by drop-
(60) ping 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
(65) hypothesis is strengthened by
the fact that the immediate the ropod
dinosaur ancestors of birds were terrestrial,
and they had the traits needed for high
(70) 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,
(75) which cannot be said for other
reptiles of their time.
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
Please provide explanation with your answer.
