Archaeopteryx

***Archaeopteryx*** (/ˌɑːrkiːˈɒptərᵻks/), sometimes referred to by its German name ***Urvogel*** ("original bird" or "first bird"), is a genus of bird-like dinosaurs that is transitional between non-avian feathered dinosaurs and modern birds. The name derives from the ancient Greek ἀρχαῖος (*archaīos*) meaning "ancient", and πτέρυξ (*ptéryx*), meaning "feather" or "wing". Between the late nineteenth century and the early twenty-first century, *Archaeopteryx* had been generally accepted by palaeontologists and popular reference books as the oldest known bird (member of the group Avialae). Older potential avialans have since been identified, including *Anchiornis*, *Xiaotingia*, and *Aurornis*.

*Archaeopteryx* lived in the Late Jurassic 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 size to a Eurasian magpie, with the largest individuals possibly attaining the size of a raven, the largest species of *Archaeopteryx* could grow to about 0.5 m (1 ft 8 in) in length. Despite their small size, broad wings, and inferred ability to fly or glide, *Archaeopteryx* had more in common with other small Mesozoic dinosaurs than with modern birds. In particular, they shared the following features with the dromaeosaurids and troodontids: jaws with sharp teeth, three fingers with claws, a long bony tail, hyperextensible second toes ("killing claw"), feathers (which also suggest warm-bloodedness), and various features of the skeleton.

These features make *Archaeopteryx* a clear candidate for a transitional fossil between non-avian 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 single feather in 1861. That same year, the first complete specimen of *Archaeopteryx* was announced. Over the years, ten 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, 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.

## Description

*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 mass 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 to modern-day bird feathers. Despite the presence of numerous avian features, *Archaeopteryx* had many non-avian 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 features common to both birds and non-avian dinosaurs, *Archaeopteryx* has often been considered a link between them. In the 1970s, John Ostrom, following Thomas Henry 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 several avian features, such as a wishbone, flight feathers, wings, and a partially reversed first toe along with 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.

### **Plumage**

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 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). 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 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*: decomposed and fluffy, and possibly even appearing more like fur than feathers in life (although not in their microscopic structure). These occur on the remainder of the body—although some feathers did not fossilize and others were obliterated during preparation, leaving bare patches on specimens—and the lower neck.

There is no indication of feathering on the upper neck and head. While these conceivably may have been nude, this may still be an artefact 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 (death pose) 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.

#### **Colouration**

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. The feather studied was most probably a single covert, which would have partly covered the primary feathers on the wings. The study does not mean that *Archaeopteryx* was entirely black, but suggests 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 light- and dark-coloured plumage, with only the tips of its flight feathers being primarily black. However, it was later shown that this interpretation was incorrect, and that the isolated feather was indeed matte black with a darker tip.

## Palaeobiology

### **Flight**

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, but it is unclear whether *Archaeopteryx* was capable of flapping flight or simply a glider. 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, and humerus—instead of the dorsally angled arrangement found in modern birds—may 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 its ability to fly through cluttered environments such as trees and brush (similar wing shapes are seen in birds that 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, because flightless birds tend to have symmetrical feathers. 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. The degree of asymmetry seen in*Archaeopteryx* is more typical for slow flyers than for flightless birds.

In 2010, Robert L. Nudds and Gareth J. Dyke in the journal *Science* published a paper in which they analysed the rachises of the primary feathers of *Confuciusornis* and *Archaeopteryx*. The analysis suggested that the rachises on these two genera were thinner and weaker than those of modern birds relative to body mass. The authors determined that *Archaeopteryx* and *Confuciusornis*, were unable to use flapping flight. This study was criticized by Philip J. Currie and Luis Chiappe. Chiappe suggested that it is difficult to measure the rachises of fossilized feathers, and Currie speculated that *Archaeopteryx* and *Confuciusornis* must have been able to fly to some degree, as their fossils are preserved in what is believed to have been marine or lake sediments, suggesting that they must have been able to fly over deep water. Gregory Paul also disagreed with the study, arguing in a 2010 response that Nudds and Dyke had overestimated the masses of these early birds, and that more accurate mass estimates allowed powered flight even with relatively narrow rachises. Nudds and Dyke had assumed a mass of 250 g (8.8 oz) for the Munich specimen *Archaeopteryx*, a young juvenile, based on published mass estimates of larger specimens. Paul argued that a more reasonable body mass estimate for the Munich specimen is about 140 g (4.9 oz). Paul also criticized the measurements of the rachises themselves, noting that the feathers in the Munich specimen are poorly preserved. Nudds and Dyke reported a diameter of 0.75 mm (0.03 in) for the longest primary feather, which Paul could not confirm using photographs. Paul measured some of the inner primary feathers, finding rachises 1.25–1.4 mm (0.049–0.055 in) across. Despite these criticisms, Nudds and Dyke stood by their original conclusions. They claimed that Paul's statement, that an adult *Archaeopteryx* would have been a better flyer than the juvenile Munich specimen, was dubious. This, they reasoned, would require an even thicker rachis, evidence for which has not yet been presented. Another possibility is that they had not achieved true flight, but instead used their wings as aids for extra lift while running over water after the fashion of the basilisk lizard, which could explain their presence in lake and marine deposits (see Evolution of bird flight).

In 2004, scientists analysing a detailed CT scan of the braincase of the London *Archaeopteryx* 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 its inner ear. The structure more closely resembles that of modern birds than the inner ear of non-avian reptiles. These characteristics taken together suggest that *Archaeopteryx* had the keen sense of hearing, balance, spatial perception, and coordination needed to fly. *Archaeopteryx* had a cerebrum-to-brain-volume ratio 78% of the way to modern birds from the condition of non-coelurosaurian dinosaurs such as *Carcharodontosaurus* or *Allosaurus*, which had a crocodile-like anatomy of the brain and inner ear. Newer research shows that while the *Archaeopteryx* brain was more complex than that of more primitive theropods, it had a more generalized brain volume among maniraptoran dinosaurs, even smaller than that of other non-avian dinosaurs in several instances, which indicates the neurological development required for flight was already a common trait in the maniraptoran clade.

Recent studies of flight feather barb geometry reveal that modern birds possess a larger barb angle in the trailing vane of the feather, whereas *Archaeopteryx* lacks this large barb angle, indicating potentially weak flight abilities.

### **Growth**

A histological study by Erickson, Norell, Zhongue, and others in 2009 estimated that *Archaeopteryx* grew relatively slowly compared to modern birds, presumably because the outermost portions of*Archaeopteryx* bones appear poorly vascularized; in living vertebrates poorly vascularized bone is correlated with slow growth rate. They also assume that all known skeletons of *Archaeopteryx* come from juvenile specimens. Because the bones of *Archaeopteryx* could not be histologically sectioned in a formal skeletochronological (growth ring) analysis, Erickson and colleagues used bone vascularity (porosity) to estimate bone growth rate. They assumed that poorly vascularized bone grows at similar rates in all birds and in *Archaeopteryx*. The poorly vascularized bone of *Archaeopteryx* might have grown as slowly as that in a mallard (2.5 micrometres per day) or as fast as that in an ostrich (4.2 micrometres per day). Using this range of bone growth rates, they calculated how long it would take to "grow" each specimen of *Archaeopteryx* to the observed size; it may have taken at least 970 days (there were 375 days in a Late Jurassic year) to reach an adult size of 0.8–1 kg (1.8–2.2 lb). The study also found that the avialans *Jeholornis* and *Sapeornis* grew relatively slowly, as did the dromaeosaurid *Mahakala*. The avialans *Confuciusornis* and *Ichthyornis* grew relatively quickly, following a growth trend similar to that of modern birds. One of the few modern birds that exhibit slow growth is the flightless kiwi, and the authors speculated that *Archaeopteryx* and the kiwi had similar basal metabolic rate.

### **Daily activity patterns**

Comparisons between the scleral rings of *Archaeopteryx* and modern birds and reptiles indicate that it may have been diurnal, similar to most modern birds.

## Paleoecology

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 a lack of terrestrial sediments characteristic of rivers. Evidence of plants, although 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 were therefore likely to have lived on the low islands surrounding the Solnhofen lagoon rather than to have been corpses that drifted in from farther away. *Archaeopteryx* skeletons are considerably less numerous in the deposits of Solnhofen than those of pterosaurs, of which seven genera have been found. The pterosaurs included species such as *Rhamphorhynchus* belonging to the Rhamphorhynchidae, the group which dominated the niche currently occupied by seabirds, and which became extinct at the end of the Jurassic. The pterosaurs, which also included *Pterodactylus*, were common enough that it is unlikely that the specimens found are vagrants from the larger islands 50 km (31 mi) 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 closest modern analogue for the Solnhofen conditions is said to be Orca Basin in the northern Gulf of Mexico, although it is much deeper than the Solnhofen lagoons. 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 also is 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, as several species of bird live exclusively in low shrubs. Various aspects of the morphology of *Archaeopteryx* point to either an arboreal or ground existence, including the length of its legs and the elongation in its feet; some authorities consider it likely to have been a generalist capable of feeding in both shrubs and open ground, as well as along 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.

## Classification

Today, fossils of the genus *Archaeopteryx* are usually assigned to one or two species, *A. lithographica* and *A. siemensii*, but their taxonomic history is complicated. Dozens of names have been published for the handful of specimens, most of which are simply spelling errors (*lapsus*). As interpreted today, the name *A. lithographica*only referred to the single feather described by Meyer. In 1954 Gavin de Beer concluded that the London specimen was the holotype. In 1960, Swinton accordingly proposed that the name *Archaeopteryx lithographica* be placed on the official genera list making the alternative names *Griphosaurus* and *Griphornis* invalid. The ICZN, implicitly accepting de Beer's standpoint, did indeed suppress the plethora of alternative names initially proposed for the first skeleton specimens, which mainly resulted from the acrimonious dispute between Meyer and his opponent Johann Andreas Wagner (whose *Griphosaurus problematicus* – "problematic riddle-lizard" – was a vitriolic sneer at Meyer's *Archaeopteryx*). In addition, in 1977 the first specific name of the Haarlem specimen, *crassipes*, described by Meyer as a pterosaur before its true nature was realized, also was suppressed.

It has been noted that the feather, the first specimen of *Archaeopteryx* described, does not correspond 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 known so far. As the feather had been designated the type specimen, the name *Archaeopteryx* should then no longer be applied to the skeletons, thus creating significant nomenclatorial confusion. In 2007, two sets of scientists therefore petitioned the ICZN requesting that the London specimen explicitly be made the type by designating it as the new holotype specimen, or neotype. This suggestion was upheld by the ICZN after four years of debate, and the London specimen was designated the neotype on 3 October 2011.