Archaeopteryx

[Barry the Baryonyx]

Archaeopteryx

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Archaeopteryx (from Ancient Greek archaios meaning 'ancient' and meaning 'feather' or 'wing'; pronounced /(är'kē-ŏp'tər-ĭks), from the late Jurassic Period (Kimmeridgian stage, 155-150 million years ago) of what is now Germany, is the earliest and most primitive known avian. Archaeopteryx was similar in size and shape to a magpie, with broad, rounded wings and a long tail, and reached up to 0.5 meters (1.6 feet) in length. Its feathers resembled those of modern birds but Archaeopteryx was rather different from any bird known today, in that it had jaws lined with sharp teeth, three 'fingers' ending in curved claws and a long bony tail. In 1862, the description of the first intact specimen of Archaeopteryx, just two years after Charles Darwin published The Origin of Species, set off a firestorm of debate about evolution and the role of transitional fossils that endures to this day.

Archaeopteryx and the origins of birds

In the 1970s, John Ostrom argued that the birds evolved from theropod dinosaurs. Archaeopteryx provides a critical piece of this argument, as it preserves a number of avian features (a wishbone, flight feathers, wings, a partially reversed first toe) and a number of dinosaur and theropod features (for instance, a long ascending process of the astragalus, 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. 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 Chinese feathered dinosaurs.





Archaeopteryx is probably close to the ancestry of modern birds - it shows most of the features one would expect in an ancestral bird - but it may not be the direct ancestor of living birds, and it is arguable how much divergence was already present in the early birds at its time.





Plumage


Archaeopteryx specimens were most notable for their well-developed remiges (flight feathers). These are markedly asymmetrical and show the structure of flight feathers of modern birds, with vanes given stability by a barb-barbule-barbicel arrangement. The tail feathers are less asymmetrical, again in line with the situation in modern birds, and also have firm vanes. The thumb did not bear a separately movable tuft of stiff feathers (alula) yet.


Body plumage is less well documented, and only properly researched in the well-preserved Berlin specimen. Thus, as more than one species seems to be involved, the following does not necessarily hold true for all of them. 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 (i.e., lack barbicels as in ratites: Christiansen & Bonde, 2004), but at least in part they are firm and thus capable of supporting flight (Longrich, 2006).


There was a patch of pennaceous feathers running along the back which was quite similar to the contour feathers of the body plumage of modern birds in being symmetrical and firm (though not as 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 (though 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 (Christiansen & Bonde, 2004).





On the other hand, there is no indication of feathering on the upper neck and head; while these may conceivably have been nude as in many closely related feathered dinosaurs for which good specimens are available, 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 back in the sea - the head and neck and the tail are generally bent downwards 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 was already softened and loose (further evidence is provided by the fact that in some specimens, the flight feathers were starting to detach at the point of embedding in the sediment), and in specimens moving along the ground in shallow water, this would cause the head and upper neck, but not the more firmly attached tail feathers to slough off (Elżanowski, 2002).


It must be mentioned that the feather, the initial specimen described, does not agree too well with the flight-related feathers of Archaeopteryx. It certainly is a remix of a contemporary species, but its size and proportions indicate that it probably belongs to an as of yet undiscovered species of primitive bird or possibly bird-like dinosaur. As the feather was the original type specimen, this has created quite some nomenclatorial confusion.





Flight ability


The flight feathers of Archaeopteryx were highly asymmetrical, as in the wings of modern birds, and the tail feathers are rather broad. This implies that the wings and tail were used for lift generation, but 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 cartilagenous sternum. The sideways orientation of the glenoid (shoulder) joint between scapula, coracoid and humerus - instead of the dorsally angled arrangement found in modern birds - suggests that Archaeopteryx was unable to lift its wings above its back, a requirement for the upstroke found in modern flapping flight. Thus, it seems likely that Archaeopteryx was indeed unable to use flapping flight as modern birds do, but it may well have utilized a downstroke-only flap-assisted gliding technique (Senter, 2006).


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 could also have improved Archaeopteryx' ability to fly through cluttered environments such as trees and brush (similar wing shapes are seen in birds which 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, would also have added to the aerial mobility of Archaeopteryx. The first detailed study of the hind wings by Longrich (2006) suggested that the structures formed up to 12% of the total airfoil. Considering that it is not certain to what extent such feathers capable of supporting flight were present on the legs, this would have reduced stall speed by up to 6% and turning radius by up to 12%, in addition to the stall and turning radius reduction provided by the primary wing and tail feathers.





In 2004, scientists analyzing a detailed CT scan of Archaeopteryx's braincase, concluded that its brain was significantly larger than that of most dinosaurs, indicating that it possessed the brain size necessary for flying. The overall brain anatomy was reconstructed using the scan. The reconstruction showed that the regions associated with vision took up nearly one-third of the brain. Other well-developed areas involved hearing and muscle coordination (Winter, 2004). The skull scan also revealed the structure of the inner ear. The structure more closely resembles that of modern birds than the inner ear of reptiles. These characteristics taken together suggest that Archaeopteryx had the keen sense of hearing, balance, spatial perception and coordination needed to fly.





Archaeopteryx continues to play an important part in scientific debates about the origin and evolution of birds. Some scientists see Archaeopteryx as a semi-arboreal climbing animal, following the idea that birds evolved from tree-dwelling gliders (the "trees down" hypothesis for the evolution of flight proposed by O.C. Marsh). Other scientists see Archaeopteryx as running quickly along the ground, supporting the idea that birds evolved flight by running (the "ground up" hypothesis proposed by Samuel Wendell Williston). Still others suggest that Archaeopteryx might have been at home both in the trees and on the ground, like modern crows, and this latter view is what today is considered best-supported by morphological characters. Altogether, it appears that it was a species which was neither particularly specialized for running on the ground, nor for perching. Considering the current knowledge of flight-related morphology, a scenario as outlined by Elżanowski (2002), namely that Archaeopteryx used its wings mainly to escape predators by glides punctuated with shallow downstrokes to reach successively higher perches, and alternatively to cover longer distances by (mainly) gliding down from cliffs or treetops, appears quite reasonable.


Given that it is now well established that several lineages of theropods evolved feathers and flight independently, the question of how precisely the ancestors of Archaeopteryx became able to fly has lost dramatically in importance for the time being. Since it is quite likely that this species belongs to a lineage of birds unrelated to the Neornithes (the Jurassic ancestor of which remains unknown), how exactly flying ability was gained in Archaeopteryx may be a moot point, having little bearing on how this happened in the ancestors of modern birds.





Taxonomy


The relationships of the specimens are problematic. Most specimens have been given their own species at one point or another. The Berlin specimen has been designated as Archaeornis siemensii, the Eichstätt specimen as Jurapteryx recurva, the Munich specimen as Archaeopteryx bavarica and the Solnhofen specimen was designated as Wellnhoferia grandis.


Recently, it has been argued that all the specimens belong to the same species. However, significant differences exist among the specimens. In particular, the Munich, Eichstätt, Solnhofen and Thermopolis specimens differ from the London, Berlin, and Haarlem specimens in being smaller or much larger, having different finger proportions, having more slender snouts, lined with forward-pointing teeth and possible presence of a sternum. These differences are as large as or larger than the differences seen today between adults of different bird species. However, it is also possible that these differences could be explained by different ages of the living birds.





Fossils


Over the years, ten body fossil specimens of Archaeopteryx and a feather that may belong to it have been found. All of the fossils come from the limestone deposits near Solnhofen, Germany.
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