The London specimen of Archaeopteryx, discovered only two years after the publication of On the Origin of Species

Transitional fossils (or missing links) are the fossilised remains of lifeforms that show features typical of two distinct groups. It may be a rare fossil, such as Archaeopteryx, which shows early features of a group which later becomes widespread.

The rarity of transitional forms is interesting. It suggests the new forms are not yet dominant in the ecology of their time, and their populations are relatively small. It follows that their fossils would be scarce. In the Solnhofen limestone where Archaeopteryx was found, there are more examples of pterosaurs of several genera than there are of the proto-bird. In the Jurassic, the pterosaurs were dominant in the air.