There is no
evidence, however, that these hollow bones were filled with air from
the lungs, as in the case of the bones of birds. The foot is
bird-like; the hand is still more so; in fact, no dinosaur hand has
ever before been found which so closely mimics that of a bird in the
great elongation of the first or index-finger, in the abbreviation of
the thumb and middle finger, and in the reduction of the ring-finger.
These fingers, with sharp claws, were not strong enough for climbing,
and the only special fitness we have been able to imagine is that they
were used for the grasping of a light and agile prey (see figs. 17,
18.)
Another reason for the venture of designating this animal as the
"bird-catcher" is that the Jurassic birds (not thus far discovered in
America, but known from the _Archaeopteryx_ of Germany) were not so
active or such strong fliers as existing birds; in fact, they were not
unlike the little dinosaur itself. They were toothed, long-tailed,
short-armed, the body was feathered instead of scaled; they rose
slowly from the ground. This renders it probable that they were the
prey of the smaller pneumatic-built dinosaurs such as the present
animal.
This hypothetical bird-catcher seems to have been designed to spring
upon a delicately built prey, the structure being the very antipode of
that of the large carnivorous dinosaurs. A difficulty in the
bird-catching theory, namely, that the teeth are not as sharp as one
would expect to find them in a flesh-eater, is somewhat offset by the
similarity of the teeth to those of the bird-eating monitor lizards
(_Varanus_), which are not especially sharp.
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