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Ages of the Earth => Dinosaurs => Topic started by: Majir on January 19, 2011, 01:25:51 pm

Title: Archaeopteryx
Post by: Majir on January 19, 2011, 01:25:51 pm
Archaeopteryx (pronounced /ˌɑrkiːˈɒptərɨks/ AR-kee-OP-tər-iks), sometimes referred to by its German name Urvogel ("original bird" or "first bird"), is the earliest and most primitive bird known. The name derives from the Ancient Greek ἀρχαῖος (archaīos) meaning "ancient", and πτέρυξ (ptéryx), meaning "feather" or "wing".

Archaeopteryx lived in the Late Jurassic Period around 150–148 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, and possibly the size of a raven,[1] Archaeopteryx could grow to about 0.5 metres (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.[2][3] The features above make Archaeopteryx a clear candidate for a transitional fossil between dinosaurs and birds.[4][5] 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 direct evidence of such structures. Moreover, because these feathers are an advanced form (flight feathers), these fossils are evidence that the evolution of feathers began before the Late Jurassic.[6]

Title: Re: Archaeopteryx
Post by: Majir on January 19, 2011, 01:26:39 pm

Scientific classification [ e ]
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: †Archaeopterygiformes
Family: †Archaeopterygidae
Genus: †Archaeopteryx
Meyer, 1861
Species: †A. lithographica
Meyer, 1861 [nomen conservandum]
Archaeopteryx crassipes Meyer, 1887 (Ostrom, 1972) [originally Pterodactylus nomen rejectum]Griphosaurus problematicus Wagner, 1862 [nomen rejectum]Griphornis longicaudatus Woodward, 1862 [nomen rejectum]Archaeopteryx macrura Owen, 1862 [nomen rejectum]Archaeornis siemensii (Dames, 1897) Petronievics, 1917 [originally Archaeopteryx]Archaeopteryx owenii Petronievics, 1917 [nomen rejectum]Jurapteryx recurva? (Howgate, 1985) Howgate, 1984 [originally Archaeopteryx]Wellnhoferia grandis? Elżanowski, 2001

Title: Re: Archaeopteryx
Post by: Majir on January 19, 2011, 01:27:51 pm
Archaeopteryx was a primitive bird that lived during the early Tithonian stage of the Jurassic Period, around 150.8–148.5 million years ago.[7] The only 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.[8][9]

Archaeopteryx was roughly the size of a raven,[1] 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).[1] Archaeopteryx feathers, although less documented than its other features, were very similar in structure and design to modern-day bird feathers.[8] However, despite the presence of numerous avian features,[10] Archaeopteryx had many theropod dinosaur characteristics. Unlike modern birds, Archaeopteryx had small teeth[8] as well as a long bony tail, features which Archaeopteryx shared with other dinosaurs of the time.[11]

Because it displays a number of features common to both birds and dinosaurs, Archaeopteryx has often been considered a link between them—possibly the first bird in its change from a land dweller to a bird.[8] In the 1970s, John Ostrom, following T. H. Huxley's lead in 1868, argued that birds evolved from 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, and 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.[12][13][14][15][16][17][18][19][20]

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. However, it may not be the direct ancestor of living birds, and it is uncertain how much evolutionary divergence was already present among other birds at the time.

Title: Re: Archaeopteryx
Post by: Majir on January 19, 2011, 01:28:28 pm
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.[21] 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 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),[22] but in part they are firm and thus capable of supporting flight.[23]

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.[22]

However, there is no indication of feathering on the upper neck and head. While these may conceivably have been **** 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, 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.[24]

Title: Re: Archaeopteryx
Post by: Majir on January 19, 2011, 01:29:33 pm

Eight specimens compared to a human in scale.