A modern bird's skeleton is very different from that of any reptile, and the origins of birds could not be deduced with certainty until, by great good luck, Archaeopteryx was discovered. This creature was preserved in Late Jurassic limestones, and provides an ideal intermediate, in both time and structure, between the reptiles and the birds.
The surprising, fortunate feature is that Archaeopteryx was fossilized in such fine-grained sediments that the clear impression of its feathers can be seen, spread around the skeleton. Both the individual feathers, and the shape of the wings that they formed, are exactly like those found in living, actively flying birds. So there is little doubt that Archaeopteryx was not merely a passive, gliding animal. This conclusion is supported by the fact that it possessed a wishbone, which in living birds forms one of the attachment sites for the flight muscles.
Archaeopteryx shows some primitive features that later birds have lost (see p. 173). For example, it had small, sharp teeth in its jaws, replaced in later birds by a horny, toothless beak. It had 3 clawed fingers on each forelimb, each finger separate and distinct from the others (unlike their partly fused arrangement in modern birds). Finally, Archaeopteryx had a long, bony tail, and the impressions in the rocks show that this was fringed on either side by a series of long feathers. These would have provided this early flyer with a source of uplift from behind to balance the uplift produced by the wings in front.
Paleontologists suggest that Archaeopteryx probably lived in trees, eating insects that it found there, and using both flapping and gliding flight to get from tree to tree. Inevitably, from time to time it would have landed on the ground, and then its sharp, clawed fingers would have helped it to climb back up into the trees.
Apart from the proportions of its wings, the skeleton of Archaeopteryx is strikingly similar to that of a small, lightly built, running dinosaur, such as the coelurosaur Compsognathus (right). This animal lived in Europe in Late Jurassic times, too (see pp. 106, 108).
Most paleontologists believe that Archaeopteryx evolved from just such a small, bipedal dinosaur, whose young may have fed on a more insectivorous diet than the adults. The young may have climbed into trees and bushes, using their clawed hands, in search of their prey. Perhaps even the feathers of birds evolved from a covering that kept warm the tiny bodies of these juvenile dinosaurs.
An alternative theory is that Archaeopteryx may have evolved from a reptile similar to early crocodiles. However, it is difficult to find strong specializations that Archaeopteryx shares with these crocodiles, and there are no fossils linking it to these Triassic forms.
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