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
(35) metabolism, which in turn implies a high internal tem-
perature. 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
(40) relatively thick hairlike fossil material was the first clear
evidence that his reasoning was correct.
Efforts to explain how the pterosaurs became air-
borne have led to suggestions that they launched them-
selves by jumping from cliffs, by dropping from trees.
(45) 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 rese-
mbled a bat's and could serve as hooks by which the
animal could hang in preparation for flight. The second
(50) 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
(55) control their flight once airborne.
1. It can be inferred from the passage that scientists now generally agree that the