Anchiornis

''Anchiornis'' is a genus of small, four-winged paravian dinosaurs, with only one known species, the type species ''Anchiornis huxleyi'', named for its similarity to modern birds. The Latin name ''Anchiornis'' derives from a Greek word meaning "near bird", and ''huxleyi'' refers to Thomas Henry Huxley, a contemporary of Charles Darwin.

''Anchiornis'' fossils have been found only in the Tiaojishan Formation of Liaoning, China, in rocks dated to the Late Jurassic, about 160 million years ago. It is known from hundreds of specimens, and given the exquisite preservation of some of these fossils, it became the first Mesozoic dinosaur species for which almost the entire life appearance could be determined, and an important source of information on the early evolution of birds.

Description

''Anchiornis huxleyi'' was a small, bipedal theropod dinosaur with a triangular skull bearing several details in common with dromaeosaurids, troodontids, and primitive avialans. Like other early paravians, ''Anchiornis'' was small, about the size of a crow. It had long, wing-bearing arms, long legs, and a long tail. Like all paravians, it was covered in feathers, though it also had scales on certain parts of the body. The wings, legs, and tail supported long but relatively narrow vaned feathers. Two types of simpler, downy (plumaceous) feathers covered the rest of the body, as in ''Sinornithosaurus'': down feathers made up of filaments attached at their bases, and more complex down feathers with barbs attached along a central quill. Long, simple feathers covered almost the entire head and neck, torso, upper legs, and the first half of the tail. The rest of the tail bore pennaceous tail feathers (rectrices). Long feathers on the head (crown) may have formed a crest. While the first specimen of ''Anchiornis'' preserved only faint traces of feathers around the preserved portion of the body, many more well-preserved fossils have since been found. Studies of ''Anchiornis'' specimens using laser fluorescence have revealed not only more details of the feathers, but also of the skin and muscle tissue. Taken together, this evidence has given scientists a nearly complete picture of ''Anchiornis'' anatomy. Additional studies indicate that ''Anchiornis'' had body plumage that consisted of short quills with long and independent, flexible barbs. These barbs stuck out from the quills at low angles on two opposing blades. This also gave each feather an overall forked shape and resulted in the theropod possessing a softer textured and "shaggier" appearing plumage than is seen in modern birds. 'Shaggy' contour feathers probably influenced thermoregulatory and water repellence abilities, and, in combination with open-vaned wing feathers, would have decreased aerodynamic efficiency.

Its size was originally estimated at and , but some specimens show that is could in fact have grown somewhat larger, up to and . ''Anchiornis'' had a wing span of up to .

Wings

Like other early paravians, ''Anchiornis'' had large wings made up of pennaceous feathers attached to the arm and hand. The wing of ''Anchiornis'' was composed of 11 primary feathers and 10 secondary feathers. The primary feathers in ''Anchiornis'' were about as long as the secondaries, and formed a rounded wing. The wing feathers had curved but symmetrical central quills, with small and thin relative size, and rounded tips, all indicating poor aerodynamic ability. In the related dinosaurs ''Microraptor'' and ''Archaeopteryx'', the longest wing feathers were closest to the tip of the wing, making the wings appear relatively long and pointed. However, in ''Anchiornis'', the longest wing feathers were those nearest the wrist, making the wing broadest in the middle and tapering near the tip for a more rounded, less flight-adapted profile.

Like other maniraptorans, ''Anchiornis'' had a propatagium, a flap of skin connecting the wrist to the shoulder and rounding out the front edge of the wing. In ''Anchiornis'', this part of the wing was covered in covert feathers which smoothed the wing and covered the gaps between the larger primary and secondary feathers. However, unlike modern birds, the covert feathers of ''Anchiornis'' were not arranged in tracts or rows. The arrangement of the covert feathers was also more primitive in ''Anchiornis'' than in birds and more advanced paravians. In modern birds, the coverts usually cover only the upper portion of the wing, with most of the wing surface made up of uncovered flight feathers. In some ''Anchiornis'' fossils, on the other hand, several layers of covert feathers seem to extend down to cover most of the wing's surface, so that the wing is essentially made of multiple layers of feathers, rather than a layer of broad feathers with only their bases hidden by layers of coverts. This multi-layered wing arrangement might have helped strengthen the wing, considering that the primary and secondary feathers themselves were narrow and weak.

The wing included three clawed fingers; however, unlike in some more primitive theropods, the longest two fingers were not separate, but were bound together by the skin and other tissue forming the wing, so ''Anchiornis'' was functionally two-fingered. These bound fingers were incorporated into a post-patagium, or flap of skin and other tissues that helped support the bases of the main wing feathers. Like the toes, the skin around the bottom of the fingers was covered in tiny, rounded scales. Unlike the toes, the flesh around the underside of the finger bones was twice as thick as the bones themselves and lacked distinct pads; instead, the fingers were straight and smooth without any major creases at the joints. Scales and skin around the fingers is very rarely preserved in fossils of early pennaraptorans, the only notable exceptions being ''Anchiornis'' and ''Caudipteryx'', which had similar thick, scaly fingers associated with its wings.

Legs

In addition to the front wings, ''Anchiornis'' had long, vaned feathers on the hind legs. This has led many scientists to call ''Anchiornis'' a four-winged dinosaur, along with similar animals like ''Microraptor'' and ''Sapeornis''. However, the feathers on the hind legs in ''Anchiornis'' did not have the shape or arrangement expected from flight feathers, and it is likely that their primary role was in display rather than flight.

''Anchiornis'' had very long legs, which is usually an indication that an animal is a strong runner. However, the extensive leg feathers indicate that this may be a vestigial trait, as running animals tend to have reduced, not increased, hair or feathers on their legs. Like most paravians, ''Anchiornis'' had four toes on the foot, with the third and fourth toes the longest. The first toe, or hallux, was not reversed as in perching species. The hindwings of ''Anchiornis'' were also shorter than those of ''Microraptor'', and were made up of 12 to 13 flight feathers anchored to the tibia (lower leg) and 10 to 11 to the tarsus (upper foot). Also unlike ''Microraptor'', the hindwing feathers were longest closer to the body, with the foot feathers being short and directed downward, almost perpendicular to the foot bones.

Unlike many other paravians, the feet of ''Anchiornis'' (except for the claws) were completely covered in feathers, though these were much shorter than the ones making up the hindwing. Some specimens have preserved scales on the toes, tarsus, and even lower leg (tibia), suggesting that scales existed beneath the feathers. The underside of the toes were formed into fleshy pads with distinct creases at the joints. The foot pads were covered in small, pebble-like scales. Scales were also present on the top of the feet but these are very hard to see in all known fossils.

Color

In 2010, a team of scientists examined numerous points among the feathers of an extremely well-preserved ''Anchiornis'' specimen in the Beijing Museum of Natural History''), ''Anchiornis'' became the first Mesozoic dinosaur for which almost the entire life coloration was known (note that the tail of this specimen was not preserved). The study found that most of the body feathers of this ''Anchiornis'' specimen were gray and black. The crown feathers were mainly rufous with a gray base and front, and the face had rufous speckles among predominantly black head feathers. The forewing and hindwing feathers were white with black tips. The coverts (shorter feathers covering the bases of the long wing feathers) were gray, contrasting the mainly white main wings. The larger coverts of the wing were also white with gray or black tips, forming rows of darker dots along mid-wing. These took the form of dark stripes or even rows of dots on the outer wing (primary feather coverts) but a more uneven array of speckles on the inner wing (secondary coverts). The shanks of the legs were gray other than the long hindwing feathers, and the feet and toes were black.

In 2015, a second ''Anchiornis'' fossil at the Yizhou Fossil & Geology Park was subjected to a similar study that included a survey of melanosome shapes across all the feathers. In contrast to the 2010 study, only gray-black type melanosomes were found. Even when the crown feathers were examined, none of the rounder, rufous-type melanosomes were seen. The scientists who conducted this second study suggested several possible explanations for this discrepancy. First, the different preservation of melanosomes or different investigative techniques might have influenced the results of the original study. Second, because the Beijing Museum specimen was smaller, it is possible that the rufous color was replaced as these animals aged. Third, it is possible that there were regional differences or even different species of ''Anchiornis'' which had different color patterns in their plumage.

Discovery and history

The first known fossil of ''Anchiornis'' (its type specimen) was dug up in the Yaolugou area of Jianchang County, Liaoning, China. These rocks have been difficult to date, but most studies have concluded that they belong to the Tiaojishan Formation of rocks dated to the late Jurassic period ( Oxfordian age), 160.89 to 160.25 million years old. studied and described by paleontologist Xu Xing and colleagues in a paper accepted to the ''Chinese Science Bulletin'' in 2009. The specimen is currently in the collection of the Institute of Vertebrate Paleontology and Paleoanthropology with the catalogue number IVPP V14378. It is an articulated skeleton missing the skull, part of the tail, and the right forelimb. The name ''Anchiornis huxleyi'' was chosen by Xu and colleagues in honor of Thomas Henry Huxley, an early proponent of biological evolution, and one of the first to propose a close evolutionary relationship between birds and dinosaurs. The generic name ''Anchiornis'' comes from combining the Ancient Greek words for "nearby" and "bird", because it was interpreted as important in filling a gap in the transition between the body plans of birds and dinosaurs.

A second specimen came to light around the same time, and was given to a team of scientists from Shenyang Normal University by a local farmer when they were excavating at the Yaolugou dig site. According to the farmer, this second specimen had been found nearby in the area of Daxishan, also from Tiaojishan Formation rocks of about the same age as the first ''Anchiornis''. Two scientists visited the site in order to compare the new fossil with the rock types found there, and were able to confirm that the new specimen probably did come from the area the farmer described. They were able to dig up several fish fossils and a third ''Anchiornis'' fossil. The farmer's fossil underwent study which was published on September 24, 2009, in the journal ''Nature''. It was assigned the catalogue number in the Liaoning Paleontological Museum. It is larger and much more complete than the first specimen, and preserved long wing feathers on the hands, arms, legs and feet, showing that it was a four-winged dinosaur similar to ''Microraptor''.

While only a few specimens have been described in detail, many more have been identified and are held in both private collections and museums. One of these, a nearly complete skeleton missing the tail, also preserving extensive feather remains, was reported in 2010. This fossil also showed evidence that ''Anchiornis'' had a feathered crest on its head, and was used to determine the animal's life coloration. It is housed in the Beijing Museum of Natural History

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