Similar in size and shape to a European Magpie, Archaeopteryx could grow to about 0.5 meters (1.6 ft) in length. Despite its small size, broad wings, and inferred ability to fly or glide, Archaeopteryx has more in common with small theropod dinosaurs than it does with modern birds. In particular, it shares the following features with the deinonychosaurs (dromaeosaurs and troodontids): jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), feathers (which also suggest homeothermy), and various skeletal features.
The features above make Archaeopteryx the first clear candidate for a transitional fossil between dinosaurs and birds. Thus, Archaeopteryx plays an important role not only in the study of the origin of birds but in the study of dinosaurs.
The first complete specimen of Archaeopteryx was announced in 1861, only two years after Charles Darwin published On the Origin of Species, and it became a key piece of evidence in the debate over evolution. Over the years, nine more fossils of Archaeopteryx have surfaced. Despite variation among these fossils, most experts regard all the remains that have been discovered as belonging to a single species, though this is still debated.
Many of these eleven fossils include impressions of feathers—among the oldest (if not the oldest) direct evidence of feathers. Moreover, because these feathers are an advanced form (flight feathers), these fossils are evidence that feathers had been evolving for quite some time.
Archaeopteryx lived during the early Tithonian stage of the Jurassic period, approximately 150.8–148.5 million years ago. Most of the specimens of Archaeopteryx that have been discovered come from the Solnhofen limestone in Bavaria, southern Germany, which is a lagerstätte, a rare and remarkable geological formation known for its superbly detailed fossils.
Archaeopteryx was roughly the size of a raven, with broad wings that were rounded at the ends and a long tail compared to its body length. It could reach up to 500 millimetres (20 in) in body length, with an estimated weight of 0.8 to 1 kilogram (1.8 to 2.2 lb). Archaeopteryx feathers, although less documented than its other features, were very similar in structure and design to modern-day bird feathers. Despite the presence of numerous avian features, however, Archaeopteryx had many theropod dinosaur characteristics. Unlike modern birds, Archaeopteryx had small teeth, as well as a long bony tail, features which Archaeopteryx shared with other dinosaurs of the time.
Because it displays a number of features common to both birds and dinosaurs, Archaeopteryx has often been considered a link between them. In the 1970s, John Ostrom, following T. H. Huxley's lead in 1868, argued that birds evolved within theropod dinosaurs and Archaeopteryx was a critical piece of evidence for this argument; it had a number of avian features, such as a wishbone, flight feathers, wings, and a partially reversed first toe along with a number of dinosaur and theropod features. For instance, it has a long ascending process of the ankle bone, interdental plates, an obturator process of the ischium, and long chevrons in the tail. In particular, Ostrom found that Archaeopteryx was remarkably similar to the theropod family Dromaeosauridae.
The first remains of Archaeopteryx were discovered in 1861, just two years after Charles Darwin published On the Origin of Species. Archaeopteryx seemed to confirm Darwin's theories and has since become a key piece of evidence for the origin of birds, the transitional fossils debate, and confirmation of evolution. Indeed, further research on dinosaurs from the Gobi Desert and China has since provided more evidence of a link between Archaeopteryx and the dinosaurs, such as the Chinese feathered dinosaurs. Archaeopteryx is close to the ancestry of modern birds, and it shows most of the features one would expect in an ancestral bird. It may not be the direct ancestor of living birds, however, and it is uncertain how much evolutionary divergence already was present among other birds at the time.
Specimens of Archaeopteryx were most notable for their well-developed flight feathers. They were markedly asymmetrical and showed the structure of flight feathers in modern birds, with vanes given stability by a barb-barbule-barbicel arrangement. The tail feathers were less asymmetrical, again in line with the situation in modern birds and also had firm vanes. The thumb, however, did not yet bear a separately movable tuft of stiff feathers.
The body plumage of Archaeopteryx is less well documented and has only been properly researched in the well-preserved The Berlin specimen. Thus, as more than one species seems to be involved, the research into the Berlin specimen's feathers does not necessarily hold true for the rest of the species of Archaeopteryx. In the Berlin specimen, there are "trousers" of well-developed feathers on the legs; some of these feathers seem to have a basic contour feather structure, but are somewhat decomposed (they lack barbicels as in ratites). However, in part they are firm and thus capable of supporting flight.
A patch of pennaceous feathers is found running along its back, which was quite similar to the contour feathers of the body plumage of modern birds in being symmetrical and firm, although not so stiff as the flight-related feathers. Apart from that, the feather traces in the Berlin specimen are limited to a sort of "proto-down" not dissimilar to that found in the dinosaur Sinosauropteryx, being decomposed and fluffy, and possibly even appeared more like fur than like feathers in life (although not in their microscopic structure). These occur on the remainder of the body, as far as such structures are both preserved and not obliterated by preparation, and the lower neck.
There is no indication of feathering on the upper neck and head, however. While these conceivably may have been nude, this may still be an artifact of preservation. It appears that most Archaeopteryx specimens became embedded in anoxic sediment after drifting some time on their backs in the sea — the head and neck and the tail are generally bent downward, which suggests that the specimens had just started to rot when they were embedded, with tendons and muscle relaxing so that the characteristic shape of the fossil specimens was achieved. This would mean that the skin already was softened and loose, which is bolstered by the fact that in some specimens the flight feathers were starting to detach at the point of embedding in the sediment. So it is hypothesized that the pertinent specimens moved along the sea bed in shallow water for some time before burial, the head and upper neck feathers sloughing off, while the more firmly attached tail feathers remained.
In 2011, graduate student Ryan Carney and colleagues performed the first colour study on an Archaeopteryx specimen. Using scanning electron microscopy technology and energy-dispersive X-ray analysis, the team was able to detect the structure of melanosomes in the single-feather specimen described in 1861. The resultant structure was then compared to that of 87 modern bird species and was determined with a high percentage of likelihood to be black in colour. The feather studied was most probably a single covert, which would have partly covered the primary feathers on the wings. While the study does not mean that Archaeopteryx was entirely black, it does suggest that it had some black colouration which included the coverts. Carney pointed out that this is consistent with what we know of modern flight characteristics, in that black melanosomes have structural properties that strengthen feathers for flight. In a 2013 study published in the Journal of Analytical Atomic Spectrometry, new analyses of Archaeopteryx's feathers revealed that the animal may have had complex coloring in the form of light and dark colored plumage, with the tips of its flight feathers being primarily black as opposed to the entire feather being dark in color. This may have been integral for display and flight, but this remains unknown at present.