The fossil remains of the first flying vertebrates，the pterosaurs, have intrigued paleontologists for more than two centuries. How such large creatures，which weighed in some cases as much as a piloted hang-glider and had wingspans from 8 to 12 meters，solved the problems of powered flight, and exactly what these creatures were-reptiles or birds are among the questions scientists have puzzled over.
Perhaps the least controversial assertion about the pterosaurs is that they were reptiles. Their skulls pelvises, and hind feet are reptilian. The anatomy of their wings suggests that they did not evolve into the class of birds. In pterosaurs a greatly elongated fourth finger of each forelimb supported a wing like membrane. The other fingers were short and reptilian，with sharp claws. In birds the second finger is the principal strut of the wing，which consists primarily of feathers. If the pterosaurs walked on all fours，the three short fingers may have been employed for grasping. When a pterosaur walked or remained stationary，the fourth finger，and with it the wing，could only turn upward in an extended inverted V-shape along each side of the animal's body.
The pterosaurs resembled both birds and bats in their overall structure and proportions. This is not surprising because the design of any flying vertebrate is subject to aerodynamic constraints. Both the pterosaurs and the birds have hollow bones，a feature that represents a savings in weight. In the birds，however，these bones are reinforced more massively by internal struts.
Although scales typically cover reptiles，the pterosaurs probably had hairy coats. T.H. Huxley reasoned that flying vertebrates must have been warm blooded because flying implies a high rate of metabolism, which in turn implies a high internal temperature. Huxley speculated that a coat of hair would insulate against loss of body heat and might streamline the body to reduce drag in flight. The recent discovery of a pterosaur specimen covered in long， dense， and relatively thick hair like fossil material was the first clear evidence that his reasoning was correct.
Efforts to explain how the pterosaurs because airborne have led to suggestions that they launched themselves by jumping from cliffs，by dropping from trees or even by rising into light winds from the crests of waves. Each hypothesis has its difficulties. The first wrongly assumes that the pterosaurs' hind feet resembled a bat's and could serve as hooks by which the animal could hang in preparation for flight. The second hypothesis seems unlikely because large pterosaurs could not have landed in trees without damaging their wings. The third calls for high waves to channel updrafts. The wind that made such waves however might have been too strong for the pterosaurs to control their flight once airborne.
3.According to the passage，the skeleton of a pterosaur can be distinguished from that of a bird by the
（A） size of its wingspan
（B） presence of hollow spaces in its bones
（C） anatomic origin of its wing strut
（D） presence of hook like projections on its hind feet
（E） location of the shoulder joint joining the wing to its body