托福阅读official64 In paragraph 2, what evidence suggests that early pterosaurs walked on their hind legs only? 答案解析【小站备考】

The extinct flying reptiles called pterosaurs were the second group of animals (after insects) to evolve flight. Most pterosaurs were about the size of modern seagulls. A few were as small as sparrows, but some of the later species were the largest flying animals that have ever lived. In 1817 Theodore Von Soemmerring published the first description of a pterosaur fossil, and thinking that it was that of an unusual bat species, he drew his reconstruction with a very batlike posture and wing. His early reconstruction of a pterosaur has haunted the public and scientific perception of pterosaurs ever since. Soemmerring’s reconstruction is understandable given that he was the first to try to describe a pterosaur, that few naturalists of the time accepted the idea of major groups of extinct animals, and that both pterosaurs’ and bats’ wings consist of a membrane supported by enormously elongated finger bones. Soemmerring showed his pterosaur with the laterally directed legs and reoriented feet of bats and with the wing membrane stretching from the arm and finger along the sides of the body and legs all the way to the ankle. The reconstruction also included a membrane stretching between the legs, similar to that in bats. Even though other scientists developed less batlike descriptions of pterosaurs in the late 1800s, the popular literature, and even some scientific literature, continued to describe pterosaurs as batlike into the 1980s.

Bats perch by hanging upside down from tree limbs and roofs of caves. Though many are surprisingly agile climbers, bats are generally awkward when crawling on level surfaces. Did pterosaurs also hang upside down and avoid landing on the ground? Until recently, some paleontologists thought they did, but most scientists now agree that pterosaurs got around on the ground reasonably well. What is still uncertain is whether pterosaurs walked on all fours or just on their hindlegs. Pterosaurs’ ancestors were bipedal (two-footed) and used their tails to balance their forward-tilted trunks and heads. Early pterosaurs also had long tails and probably could have run on their hindlegs, certainly handy for an animal with wings for forelimbs. These early pterosaurs, however, could have used their forelimbs for walking because their arm and hand bones were only slightly enlarged—most of the wing was supported by the gigantic fourth finger.

Later pterosaurs are more enigmatic: their arms and especially their hands seem too long to be used comfortably for walking, but their tails were too short to counterbalance their bodies if they walked just on their hindlegs. Birds also have short, stubby tail skeletons, but they manage to walk quite well on their hindlegs. Birds manage this by angling the thighs forward to get their feet under the body’s center of gravity. They hold their thighs at this unstable angle with extensive hip and thigh muscles. Some researchers have suggested that pterosaurs’ hipbones were too small to anchor extensive thigh-positioning muscles, but others have responded that pterosaurs’ leg and foot bones are so strikingly birdlike that pterosaurs must surely have walked like birds. Recently, however, some pterosaur experts have concluded that a number of fossil trackways—trails of preserved footprints—were made by pterydactyloid pterosaurs, and these animals clearly walked on all four limbs. Perhaps some early pterosaurs walked on their hindlegs, but according to current evidence, most species probably walked on all fours. In any case, large pterosaurs, with eight- or ten-meter wingspans and weighing as much as an adult human, do not seem likely candidates for a batlike existence confined to clambering about in trees and hanging upside down from branches.

Pterosaurs also possessed some obvious adaptations for powered flight. They had large sternums (breastbones) for attaching powerful flight muscles, well-developed shoulder bones to carry the body’s weight in flight, and air-filled bones to lighten the skeleton. Some even had a furcula (a fused breastbone also found in birds), perhaps to flex like a spring and help raise the wings during the upstroke. How competent were they at flying? The original batlike reconstructions, along with their classification as reptiles, suggested to many earlier biologists that pterosaurs were only gliders. Biologists now, however, generally agree that pterosaurs were capable of powered, flapping flight. Indeed, the shoulder joint is clearly specialized for the down-and-forward, up-and-back movement of normal flapping.

OFFICIAL64 In paragraph 2, what evidence suggests that early pterosaurs walked on their hind legs only?

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