Just winging it
With all of this tetrapod diversity to explore, we can only hope to visit a few points of special interest along the way – transitions that will also serve to illustrate other features of evolution that we have not yet discussed in great detail.
One transition that has long fascinated scientists is the origin of birds, with their striking adaptations of feathers and powered flight. These adaptations place modern birds at a sizeable distance from other present-day tetrapods – a fact that was troublesome to Darwin. Only a few years after the publication of his On the Origin of Species, however, a stunning stem-group bird was discovered in Germany – Archaeopteryx lithographica.
Even to a casual observer, Archaeopteryx displays a mix of “reptilian” and “avian” characteristics. Like reptiles, Archaeopteryx had a long bony tail, teeth, and forelimbs with clawed digits – but combined these features with a trait that until then had been thought to be the sole hallmark of birds – feathered wings. For Darwin, Archaeopteryx was simultaneously support for his theory as well as a reminder of the paucity of the fossil record. He would express these thoughts in a letter to a colleague in 1863:
The fossil Bird with the long tail & fingers to its wings (I hear from Falconer that Owen has not done the work well) is by far the greatest prodigy of recent times. It is a grand case for me; as no group was so isolated as Birds; & it shows how little we know what lived during former times.
Let us now look to the mutual affinities of extinct and living species. They all fall into a few grand classes; and this fact is at once explained on the principle of descent. The more ancient any form is, the more, as a general rule, it differs from living forms. But, as Buckland long ago remarked, all extinct species can be classed either in still existing groups, or between them. That the extinct forms of life help to fill up the intervals between existing genera, families, and orders, cannot be disputed. For if we confine our attention either to the living or to the extinct alone, the series is far less perfect than if we combine both into one general system. With respect to the vertebrata, whole pages could be filled with illustrations from Owen, showing how extinct animals fall in between existing groups… Another distinguished palæontologist, M. Gaudry, shows that very many of the fossil mammals discovered by him in Attica connect in the plainest manner existing genera. Even the wide interval between birds and reptiles has been shown by Professor Huxley to be partially bridged over in the most unexpected manner, by, on the one hand, the ostrich and extinct Archeopteryx (sic), and on the other hand, the Compsognathus, one of the Dinosaurians—that group which includes the most gigantic of all terrestrial reptiles.
So, even in Darwin’s time, the evidence supported the hypothesis that Archaeopteryx was a transitional form in the sense that we have been discussing – as a stem group on the lineage leading to modern birds – with that stem rooted within theropod dinosaurs.
Theropods of a feather, group together
Despite the early discovery of Archaeopteryx, other fossil species that “fill up the interval” between crown-group birds and extinct theropods were unknown until over 100 years later. In the mid 1990s, however, the first of what would be a number of significant discoveries was made in deposits of the Yixian Formation in China – namely, a feathered, non-avian theropod dinosaur named Sinosauropteryx prima. This fossil was noteworthy not merely because it was feathered and related to Compsognathus, but also because of the nature of its feathers – this theropod possessed only relatively simple “protofeathers” – unbranched filaments that could serve as insulation. Sinosauropteryx seems to have branched off the avian lineage at a time when feathers were not (yet) even branched, let alone adapted for flight.