In 1859, a British naturalist named Charles Darwin published a book that shook Victorian England to its core. Titled On the Origin of Species, Darwin’s book argued that all living organisms (and, by extension, humans too) had evolved from earlier more primitive ancestors over a long period of time through a process of evolution. And just two years after Origin of Species was published, “Darwin’s Theory” was spectacularly confirmed by a stunning fossil found in Germany.
A cast of Archaeopteryx and two dromeiosaur skeletons, on display at the Field Museum in Chicago
The example of the evolutionary transition from dinosaurs to birds, as represented by the fossilArchaeopteryx, is without a doubt the most famous in the world, and nearly every biology textbook that discusses evolution cites Archaeopteryx as an example. Six specimens ofArchaeopteryx lithographica have been found. In appearance, the specimens resemble the skeletons of small theropod dinosaurs–it is only the unmistakable imprints of feathers surrounding the fossil bones which indicate that we are dealing with a bird (in fact, the resemblance is so close that one of the skeletons was misidentified for several decades as a small theropod, and another was misidentified as a pterodactyl–a mistake not corrected until someone noticed the faint impressions of feathers).
One almost could not ask for a better example of a transitional fossil than Archaeopteryx. It exhibits an unmistakeable mixture of dinosaurian and avian characteristics. A bird, of course, is defined by the presence of feathers, and the flight feathers of Archaeopteryx are well-preserved, and are virtually indistinguishable from those of modern birds. They possess the central shaft and side barbules found in any songbird of today. The feathers are also asymmetrical and are wider on the trailing edge than the front edge–an adaptation shown by flying birds but not by flightless birds such as penguins or ostriches. This indicates that Archaeopteryx was probably capable of flight (although the fossil lacks the large keeled breastbone which all modern birds use to attach their flight muscles, and the attachment points were themselves much smaller than in modern birds–thus it is possible that Archaeopteryx was only a glider and was not capable of powered flight). The large contour feathers are the only kind found on Archaeopteryx skeletons–no smaller downy feathers have been found, although these are possessed by all modern birds.
Apart from the feathers, however, Archaeopteryx exhibits a number of characteristics which are not birdlike at all, but are shared by the theropod dinosaurs, and some of these are found in no other group of animals. Among the dinosaurian characteristics exhibited by Archaeopteryx are: simple concave articulation points on the cervical vertebrae, rather than the elongated saddle-shaped articulation found in birds; vertebrae in the trunk region which are free and mobile, rather than fused together as in birds; the presence of gastralia, or abdominal ribs, which are found in reptiles and theropods but not in birds; a rib cage which lacks uncinate processes and does not articulate with the sternum, rather than the strutlike uncinates and sternum articulations found in all birds; a sacrum consisting of only 6 vertebrae, rather than the 11-23 found in birds; mobile joints in the bones of the elbow, wrist and fingers, rather than the fused joints found in birds; a shoulder socket that faces downward like a theropod’s, rather than outward like a bird’s; solid bones which lack pneumatic sacs, rather than the hollow air-permeated bones found in birds; and a long bony tail with free vertebrae, rather than the short fused pygostile found in birds;
The Archaeopteryx skull is also typically dinosaurian in structure, exhibiting: a number of openings or “fenestrae” in the skull, arranged as in theropod dinosaurs and not birds; a heavy but short quadratic bone which is inclined forward as in dinosaurs; a bend in the jawbones behind the tooth row; a long retro-articular process, which is found in dinosaurs but not in birds; a thin straight jugal bone as in theropods; a preorbital bar separating the anteorbital fenestra and the eye socket (a reptilian characteristic); an occipital condyle and foramen magnum that are located above the dorsal end of the quadrate bone as in theropods, rather than below the quadrate as in all other birds; and a brain structure which exhibits elongated and slender cerebral hemispheres which do not overlap the midbrain (in birds, the cerebral hemispheres are heavy and extend over top of the midbrain).
There are also some features present in Archaeopteryx which are present in primitive form in the theropods but in more advanced form in the birds. In the theropods, for instance, the hallux, or big toe, is located on the back of the foot and forms a short claw that doesn’t reach the ground. In birds, this toe is greatly elongated and is used for perching. In Archaeopteryx, the hallux is reversed, but is elongated to an extent midway between the theropods and the birds. In theropods, the fingers of the front arms are long; in birds, the fingers are reduced to tiny nubbins. Archaeopteryx is midway between these conditions, In birds, the wings are supported by the furcula, or wishbone, which is composed of the two fused clavicles, and Archaeopteryxalso possesses a fused furcula (though not as strong as that in modern birds). A few of the theropods had clavicles, including such birdlike species as Velociraptor. And a theropod species known as Oviraptor possessed a fused furcula, as in birds.
Our picture of the evolution between dinosaurs and birds became even clearer in the late 1990’s, with the discovery of several fossils in China. In 1996, Chinese paleontologists discovered a typical small theropod skeleton with a surprise—it was covered in fine downy threadlike fibers, which accorded perfectly with incipient feathers. The dinosaur, named Sinosauropteryx (“Chinese lizard-bird”), probably used the featherlike covering as insulation. Today, there is a whole series of excellently-preserved fossils showing a smooth evolutionary transition from small dromaeiosaur theropod dinosaurs to modern feathered birds.