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

(5) 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 ques-

tions scientists have puzzled over.

Perhaps the least controversial assertion about the

(10) 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 winglike membrane.

(15) 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

(20) 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

(25)their overall structure and proportions. This is not sur-

prising because the design of any flying vertebrate is

subject to aerodynamic constraints. Both the pterosaurs

and the birds have hollow bones, a feature that repre-

sents a savings in weight. In the birds, however, these

(30)bones are reinforced more massively by internal struts.

Although scales typically cover reptiles, the

pterosaurs probably had hairy coats. T.H. Huxley rea-

soned that flying vertebrates must have been warm-

blooded because flying implies a high rate of