The fossil remains of the first flying vertebrates, the pterosaurs, have intrigued paleontologists for

more than two centuries. How such large creatures, which weighted 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 fingure is the principle strut of the wing, which consists primarily of feathers. If the

pterosaur walked or remained stationary, the fourth finger, and with it the wing, could only turn

upward in an extended inverted V-shape along 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 saving in weight. In the

birds, however, these bones are reinforced more massively by internal struts.

According to the passage the skeleton of pterosaurs can be distinguished from that of a bird by the: