See L. M. Witmer, The Search for the Origin of Birds (1995); A. Feduccia, The Origin and Evolution of Birds (1996); S. Chatterjee, The Rise of Birds (1997); P. Shipman, Taking Flight: Archaeopteryx and the Evolution of Bird Flight (1998).
Oldest known fossil animal that is generally accepted as a bird (classified as genus Archaeopteryx). It flourished during the Late Jurassic period (159–144 million years ago). Fossil specimens indicate that archaeopteryx ranged in size from as small as a blue jay to as large as a chicken. Archaeopteryx shared anatomical characteristics with both birds (well-developed wings and birdlike skull) and theropod dinosaurs (well-developed teeth and a long tail).
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Archaeopteryx, sometimes referred to by its German name Urvogel ("original bird" or "first bird"), is the earliest and most primitive bird known. The name is from the Ancient Greek ἀρχαῖος archaios meaning 'ancient' and πτέρυξ pteryx meaning 'feather' or 'wing'; .
Archaeopteryx lived in the late Jurassic Period around 155–150 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 size and shape to a European Magpie, Archaeopteryx could grow to about 0.5 metres (1.6 ft) in length. Despite its small size, broad wings, and ability to fly, 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 was a primitive bird that lived during the Tithonian stage of the Jurassic Period, around 155–150 million years ago. The only specimens of Archaeopteryx that have been discovered come from Bavaria in southern Germany.
Archaeopteryx was roughly the size of a medium-sized modern-day bird, with broad wings that were rounded at the ends and a long tail compared to its body length. In all, Archaeopteryx could reach up to 500 millimeters (1.6 ft) in body length. Archaeopteryx feathers, although less documented than its other features, were very similar in structure and design to modern-day bird feathers. However, despite the presence of numerous avian features, 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—possibly the first bird in its change from a land dweller to a bird. 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 preserves 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.
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 in the origin of birds, transitional fossils debate and the 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.
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 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), but in part they are firm and thus capable of supporting flight.
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.
However, 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, 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.
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 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%.
In 2004, scientists analyzing a detailed CT scan of Archaeopteryx' 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. 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 it 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 the species was not particularly specialized for running on the ground or for perching. Considering the current knowledge of flight-related morphology, a scenario outlined by Elżanowski in 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.
The richness and diversity of the Solnhofen limestones in which all specimens of Archaeopteryx have been found have shed light on an ancient Jurassic Bavaria strikingly different from the present day. The latitude was similar to Florida, though the climate was likely to have been drier, as evidenced by fossils of plants with adaptations for arid conditions and lack of terrestrial sediments characteristic of rivers. Evidence of plants, though scarce, include cycads and conifers while animals found include a large number of insects, small lizards, pterosaurs and Compsognathus.
The excellent preservation of Archaeopteryx fossils and other terrestrial fossils found at Solnhofen indicates that they did not travel far before becoming preserved. The Archaeopteryx specimens found are likely therefore to have lived on the low islands surrounding the Solnhofen lagoon rather than been corpses that drifted in from further away. Archaeopteryx skeletons are considerably less numerous in the deposits of Solnhofen than those of pterosaurs such as Rhamphorhynchus, the group which dominated the niche currently occupied by seabirds, yet are common enough that it is unlikely that the specimens found are vagrants from the larger islands 50 km (31 miles) to the north.
The islands that surrounded the Solnhofen lagoon were low lying, semi-arid and sub-tropical with a long dry season and little rain. The flora of these islands was adapted to these dry conditions and consisted mostly of low (3 m [10 ft]) shrubs. Contrary to reconstructions of Archaeopteryx climbing large trees, these seem to have been mostly absent from the islands; few trunks have been found in the sediments and fossilized tree pollen is also absent.
The lifestyle of Archaeopteryx is difficult to reconstruct and there are several theories regarding it. Some researchers suggest that it was primarily adapted to life on the ground, while other researchers suggest that it was principally arboreal. The absence of trees does not preclude Archaeopteryx from an arboreal lifestyle; several species of extant bird live exclusively in low shrubs. Various aspects of the morphology of Archaeopteryx point to either an arboreal or ground existence, the length of its legs, the elongation in its feet; and some authorities consider it likely to have been a generalist capable of feeding in both shrubs, open ground and even alongside the shores of the lagoon. It most likely hunted small prey, seizing it with its jaws if it was small enough or with its claws if it was larger.
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, quarried for centuries, near Solnhofen, Germany.
The initial discovery, a single feather, was unearthed in 1860 and described a year later by Christian Erich Hermann von Meyer. It is currently located at the Humboldt Museum für Naturkunde in Berlin. This is generally assigned to Archaeopteryx and was the initial holotype, but whether it actually is a feather of this species or another, as yet undiscovered, proto-bird is unknown. There are some indications it is indeed not from the same animal as most of the skeletons (the "typical" A. lithographica).
Soon after, the first skeleton, known as the London Specimen (BMNH 37001) was unearthed in 1861 near Langenaltheim, Germany and given to a local physician Karl Häberlein in return for medical services. He then sold it to the Natural History Museum in London, where it remains. Missing most of its head and neck, it was described in 1863 by Richard Owen as Archaeopteryx macrura, who assumed it did not belong to the same species as the feather. In the subsequent 4th edition of his On the Origin of Species (chap. 9 p. 367), Charles Darwin described how some authors had maintained "that the whole class of birds came suddenly into existence during the eocene period; but now we know, on the authority of Professor Owen, that a bird certainly lived during the deposition of the upper greensand; and still more recently, that strange bird, the Archeopteryx, with a long lizard-like tail, bearing a pair of feathers on each joint, and with its wings furnished with two free claws, has been discovered in the oolitic slates of Solenhofen. Hardly any recent discovery shows more forcibly than this how little we as yet know of the former inhabitants of the world."
The Greek term "pteryx" (πτερυξ) primarily means "wing", but can also designate merely "feather". Von Meyer suggested this in his description. At first he referred to a single feather which appeared like a modern bird's remex (wing feather), but he had heard of and been shown a rough sketch of the London specimen, to which he referred as a "Skelet eines mit Federn bedeckten Thiers" ("skeleton of an animal covered in feathers"). In German, this ambiguity is resolved by the term Schwinge which does not necessarily mean a wing used for flying. Urschwinge was the favored translation of Archaeopteryx among German scholars in the late 19th century. In English, "ancient pinion" offers a rough approximation.
Since then nine specimens have been recovered:
The Berlin Specimen (HMN 1880) was discovered in 1876 or 1877 on the Blumenberg near Eichstätt, Germany, by Jakob Niemeyer. He exchanged this precious fossil for a cow, with Johann Dörr. Placed on sale in 1881, with potential buyers including O.C. Marsh of Yale University's Peabody Museum, it was bought by the Humboldt Museum für Naturkunde, where it is now displayed. The transaction was financed by Ernst Werner von Siemens, founder of the famous company that bears his name. Described in 1884 by Wilhelm Dames, it is the most complete specimen, and the first with a complete head. Once classified as a new species, A. siemensii, a recent evaluation supports the A. siemensii species definition.
Composed of a torso, the Maxberg Specimen (S5) was discovered in 1956 or 1958 near Langenaltheim and described in 1959 by Heller. It is currently missing, though it was once exhibited at the Maxberg Museum in Solnhofen. It belonged to Eduard Opitsch, who loaned it to the museum. After his death in 1991, the specimen was discovered to be missing and may have been stolen or sold. The specimen is missing its head and tail, although the rest of the skeleton is mostly intact.
The Haarlem Specimen (TM 6428, also known as the Teyler Specimen) was discovered in 1855 near Riedenburg, Germany and described as a Pterodactylus crassipes in 1875 by von Meyer. It was reclassified in 1970 by John Ostrom and is currently located at the Teylers Museum in Haarlem, the Netherlands. It was the very first specimen, despite the classification error. It is also one of the least complete specimens, consisting mostly of limb bones and isolated cervical vertebrae and ribs. The Eichstätt Specimen (JM 2257) was discovered in 1951 or 1955 near Workerszell, Germany and described by Peter Wellnhofer in 1974. Currently located at the Jura Museum in Eichstätt, Germany, it is the smallest specimen and has the second best head. It is possibly a separate genus (Jurapteryx recurva) or species (A. recurva).
The Solnhofen Specimen (BSP 1999) was discovered in the 1960s near Eichstätt, Germany and described in 1988 by Wellnhofer. Currently located at the Bürgermeister-Müller-Museum in Solnhofen, it was originally classified as Compsognathus by an amateur collector. It is the largest specimen known and may belong to a separate genus and species, Wellnhoferia grandis. It is missing only portions of the neck, tail, backbone, and head.
The Munich Specimen (S6, formerly known as the Solnhofen-Aktien-Verein Specimen) was discovered in 1991 near Langenaltheim and described in 1993 by Wellnhofer. It is currently located at the Paläontologisches Museum München in Munich. What was initially believed to be a bony sternum turned out to be part of the coracoid, but a cartilaginous sternum may have been present. Only the front of its face is missing. It may be a new species, A. bavarica.
An eighth, fragmentary specimen, the Bürgermeister-Müller Specimen was discovered in 1997 and it is currently kept at the Bürgermeister-Müller Museum. Other than the above remains discovered, a further fragmentary fossil was found in 2004.
Long in a private collection, the Thermopolis Specimen (WDC CSG 100) was discovered in Germany and described in 2005 by Mayr, Pohl, and Peters. Donated to the Wyoming Dinosaur Center in Thermopolis, Wyoming, it has the best-preserved head and feet; most of the neck and the lower jaw have not been preserved. The "Thermopolis" specimen was described in the December 2, 2005 Science journal article as "A well-preserved Archaeopteryx specimen with theropod features"; it shows that the Archaeopteryx lacked a reversed toe—a universal feature of birds—limiting its ability to perch on branches and implying a terrestrial or trunk-climbing lifestyle. This has been interpreted as evidence of theropod ancestry. The specimen also has a hyperextendible second toe. "Until now, the feature was thought to belong only to the species' close relatives, the deinonychosaurs." This tenth and latest specimen was assigned to Archaeopteryx siemensii in 2007. The specimen itself, currently on loan to the Forschungsinstitut Senckenberg in Frankfurt, is considered the most complete and well preserved Archaeopteryx remains yet.
Some reports claim that they provide evidence to prove that Archaeopteryx is a fake. However, such reports are not confirmed. The issue is further discussed below.
Today, the fossils are usually assigned to a single species A. lithographica, but the taxonomic history is complicated. Dozens of names have been published for the handful of specimens, most of which are simply spelling errors (lapsus). Originally, the name A. lithographica only referred to the single feather described by von Meyer. In 1960, Swinton proposed that the name Archaeopteryx lithographica be officially transferred from the feather to the London specimen. The ICZN did suppress the plethora of alternative names initially proposed for the first skeleton specimens, which mainly resulted from the acrimonious dispute between von Meyer and his opponent Johann Andreas Wagner (whose Griphosaurus problematicus—"problematic riddle-lizard"—was a vitriolic sneer at von Meyer's Archaeopteryx). In addition, descriptions of Archaeopteryx fossils as pterosaurs before their true nature was realized were also suppressed.
The relationships of the specimens are problematic. Most subsequent 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.
Finally, it is worth noting that the feather, the first specimen of Archaeopteryx described, does not agree well with the flight-related feathers of Archaeopteryx. It certainly is a flight feather of a contemporary species, but its size and proportions indicate that it may belong to another, smaller species of feathered theropod, of which only this feather is so far known. As the feather was the original type specimen, this has created significant nomenclatorial confusion.
The last 4 taxa may be valid genera and species.
"Archaeopteryx" vicensensis (Anon. fide Lambrecht, 1933) is a nomen nudum for what appears to be an undescribed pterosaur.
Charig et al. pointed to the presence of hairline cracks in the slabs running through both rock and fossil impressions, and mineral growth over the slabs that had occurred before discovery and preparation, as evidence that the feathers were original. Spetner et al. then attempted to show that the cracks would have naturally propagated through their postulated cement layer, but neglected to account for the fact that the cracks were old and had been filled with calcite, and thus were not able to propagate. They also attempted to show the presence of cement on the London specimen through X-ray spectroscopy, and did find something that was not rock. However, it was not cement, either, and is most probably from a fragment of silicone rubber left behind when molds were made of the specimen. Their suggestions have not been taken seriously by paleontologists, as their evidence was largely based on misunderstandings of geology, and they never discussed the other feather-bearing specimens, which have increased in number since then.
Nonetheless, Archaeopteryx is so often used as a model of the true ancestral bird that it has seemed almost heretical to suggest otherwise. Several authors have done so. Lowe (1935) and Thulborn (1984) questioned whether Archaeopteryx truly was the first bird. They suggested that Archaeopteryx was a dinosaur that was no more closely related to birds than were other dinosaur groups. Kurzanov (1987) suggested that Avimimus was more likely to be the ancestor of all birds than Archaeopteryx. Barsbold (1983) and Zweers and Van den Berge (1997) noted that many maniraptoran lineages are extremely birdlike, and suggested that different groups of birds may have descended from different dinosaur ancestors.
Archaeopteryx is the best known early bird and has thus received widespread attention. Its easily recognizable appearance and the intense public interest in dinosaurs have caused Archaeopteryx to become a feature of worldwide popular culture. For example, the second book in the Time Machine series, Search for Dinosaurs, takes the reader on a journey to the Mesozoic to find and photograph an Archaeopteryx. In one of the "strangest" appearances of Archaeopteryx in popular culture, Alfred Jarry's play Ubu cocu, ou l'Archéopteryx ('Ubu cuckolded, or the Archaeopteryx') includes an Archaeopteryx as an important character. The iconic appearance of the Berlin Specimen has been adapted into the logo of Arc'teryx Equipment Inc. The company's name is a contraction of Archaeopteryx. A main belt asteroid discovered in 1991, 9860 Archaeopteryx, was named in honour of the genus. The name is mentioned regularly in the children's novel, Skellig. In Neil Cicierega's Dinosaurchestra album, there is a song named after and written about Archaeopteryx.