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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
winglike membrane. The other fingers were short and reptilian, with sharpclaws.
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 hairlike fossil material was the first clear evidence that his
reasoning was correct.
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