Temporal range: Middle Jurassic
|Anchiornis: for colours see text|
Xu Xing 2009
A. huxleyi Xu et al. 2009 (type)
Anchiornis ('near bird') is a genus of small, feathered, troodontid dinosaurs. Anchiornis huxleyi is named in honour of Thomas Henry Huxley, the first to propose a close evolutionary relationship between birds and dinosaurs. A complete individual would be 34 cm (13 in) and weigh only 110 g (3.9 oz), making it the smallest known feathered dinosaur.
Anchiornis is a small, early troodontid dinosaur with a triangular skull like other troodontids. Also, Anchiornis had long legs, usually an indication of a strong runner. However, the extensive leg feathers indicate that long legs may be an vestigial trait, because running animals tend to have reduced, not increased, hair or feathers on their legs. The forelimbs of Anchiornis were also very long, unusual among troodontids (which tend to be short-armed) but similar to dromaeosaurids and early birds, emphasizing its basal ('primitive') position among dinobirds.
While the first specimen of Anchiornis preserved only faint traces of feathers around the preserved portion of the body, the well-preserved second specimen showed nearly complete feather preservation, allowing researchers to identify the structure of the feathers and how they were distributed.
As in other early birds, such as Microraptor, Anchiornis had large wings, made up of flight feathers attached to the arm and hand (as in modern birds) as well as flight feathers on the hind legs, forming an arrangement of fore and hind wings. The forewing of Anchiornis was composed of 11 primary feathers and 10 secondary feathers. Unlike Microraptor, the primary feathers in Anchiornis were about as long as the secondaries and formed a more rounded wing, with curved but symmetrical central vanes, a small and thin relative size, and rounded tips, all indicating poorer aerodynamic ability compared to its later relative. In Microraptor and Archaeopteryx, the longest forewing feathers were closest to the tip of the wing, making the wings appear long, narrow, and pointed. However, in Anchiornis, the longest wing feathers anchored near the wrist, making the wing broadest in the middle and tapering near the tip for a more rounded, less flight-adapted profile.
The hind wings of Anchiornis were also shorter than those of Microraptor, and were made up of 12–13 flight feathers anchored to the tibia (lower leg) and 10–11 to the metatarsus (upper foot). Also unlike Microraptor, the hind wing feathers were longest closer to the body, with the foot feathers being short and directed downward, almost perpendicular to the foot bones. Unlike any other known Mesozoic dinosaur, the feet of Anchiornis (except for the claws) were completely covered in feathers (much shorter than the ones making up the hind wing).
Two types of simpler, downy (plumaceous) feathers covered the rest of the body, as in Sinornithosaurus. Long downy 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).
In 2010, a team examined numerous points among the feathers of an extremely well-preserved Anchiornis specimen to survey the distribution of melanosomes, the pigment cells that give feathers their colour. By studying the melanosomes and comparing them with those of modern birds, the scientists were able to map the colours and patterning present on Anchiornis when it was alive. Though this technique had been used before, Anchiornis became the first Mesozoic dinosaur for which almost the entire life colouration was known.
Most of the body feathers of Anchiornis were grey and black. The crown of head feathers was mainly reddish with a grey base and front, and the face had rufous speckles among predominantly black head feathers. The fore and hind wing feathers were white with black tips. The coverts (shorter feathers covering the bases of the long wing feathers) were grey, contrasting the mainly white main wings. The larger coverts of the wing were also white with grey 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 grey other than the long hind wing feathers, and the feet and toes were black.
Like many modern birds, Anchiornis exhibited a complex pattern of coloration with different colours in speckled patterns across the body and wings, or "within- and among-feather plumage coloration". In modern birds, such colour patterning is used in communication and display, either to members of the same species (e.g. for mating or territorial threat display) or to threaten and warn off competing or predatory species.
Anchiornis is notable for its long forelimbs, which were 80% of the total length of the hind limbs. This is similar to Archaeopteryx; long forelimbs are necessary for flight. Anchiornis also had a more avian wrist than other non-avialan theropods. The authors first thought that Anchiornis could fly or glide. However, further finds showed that the wings of Anchiornis, while well-developed, were short when compared to later species like Microraptor, with relatively short primary feathers that had rounded, symmetrical tips, unlike the pointed, aerodynamically proportioned feathers of Microraptor. So the animal could glide, but probably not fly.
Anchiornis has long hind legs, suggesting a fast-running lifestyle. However, the legs and even feet and toes of Anchiornis were covered in feathers, making it unlikely that Anchiornis was a capable ground runner. A tree-based glider is the probable life-style.
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