Archaeopteryx (/ˌɑrkiːˈɒptərɨks/ ar-kee-op-tər-iks), sometimes referred to by its German name Urvogel (“original bird” or “first bird”), is a genus of early bird that is transitional betweenfeathered dinosaurs and modern birds. The name derives from the ancient Greek ἀρχαῖος (archaīos) meaning “ancient”, and πτέρυξ (ptéryx), meaning “feather” or “wing”. Since the late nineteenth century, it had been generally accepted by palaeontologists, and celebrated in lay reference works, as being the oldest known bird (member of the group Avialae). However, older potential avialans have since been identified, including Anchiornis, Xiaotingia, and Aurornis.
Archaeopteryx lived in the Late Jurassic period around 150 million years ago, in what is now southern Germany during a time when Europe was an archipelago of islands in a shallow warm tropical sea, much closer to the equator than it is now. Similar in shape to a European Magpie, with the largest individuals possibly attaining the size of a raven, Archaeopteryx could grow to about 0.5 m (1 ft 8 in) in length. Despite its small size, broad wings, and inferred ability to fly or glide, Archaeopteryx has more in common with other small Mesozoic dinosaurs than it does withmodern 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.
These features make Archaeopteryx a 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. It was named from a feather in 1861. That same year, the first complete specimen of Archaeopteryx was announced. Over the years, ten more fossils ofArchaeopteryx have surfaced. Despite variation among these fossils, most experts regard all the remains that have been discovered as belonging to a single species, although this is still debated.
Most of these eleven fossils include impressions of feathers. Because these feathers are of an advanced form (flight feathers), these fossils are evidence that the evolution of feathers began before the Late Jurassic. The type specimen of Archaeopteryx was discovered 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.
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.
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. The scanning, done by using a synchotron radiation light source to emit an X-ray beam to identify various pigments, was done only on a few parts of one feather, leaving the full coloration of Archaeopteryx in speculation.
As in the wings of modern birds, the flight feathers of Archaeopteryx were somewhat asymmetrical and the tail feathers were rather broad. This implies that the wings and tail were used for lift generation, however, it is unclear whether Archaeopteryx was simply a glider, or capable of flapping flight. The lack of a bony breastbone suggests that Archaeopteryx was not a very strong flier, but flight muscles might have attached to the thick, boomerang-shaped wishbone, the platelike coracoids, or perhaps, to a cartilaginous sternum. The sideways orientation of the glenoid (shoulder) joint between scapula, coracoid, andhumerus — instead of the dorsally angled arrangement found in modern birds—might indicate that Archaeopteryx was unable to lift its wings above its back—a requirement for the upstroke found in modern flapping flight. According to a study by Philip Senter in 2006, Archaeopteryx was indeed unable to use flapping flight as modern birds do, but it may well have used a downstroke-only flap-assisted gliding technique.
Archaeopteryx wings were relatively large, which would have resulted in a low stall speed and reduced turning radius. The short and rounded shape of the wings would have increased drag, but also could have improved the Archaeopteryx’ability to fly through cluttered environments such as trees and brush (similar wing shapes are seen in birds who fly through trees and brush, such as crows and pheasants). The presence of “hind wings”, asymmetrical flight feathers stemming from the legs similar to those seen in dromaeosaurids such as Microraptor, also would have added to the aerial mobility of Archaeopteryx. The first detailed study of the hind wings by Longrich in 2006, suggested that the structures formed up to 12% of the total airfoil. This would have reduced stall speed by up to 6% and turning radius by up to 12%.
The feathers of Archaeopteryx were asymmetrical. This has been interpreted as evidence that it was a flyer, as flightless birds tend to have symmetrical feathers, however some scientists, including Thomson and Speakman, have questioned this. They studied more than 70 families of living birds, and found that some flightless types do have a range of asymmetry in their feathers, and that the feathers of Archaeopteryx fall into this range. However, the degree of asymmetry with Archaeopteryx is more typical for slow flyers than for flightless birds.