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.
Discovery and history
The first known fossil of Anchiornis 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, 160.89 to 160.25 million years old. Anchiornis was 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 with the specimen number BMNHC PH828. Another specimen from the same fossil quarry as the type specimen was found by a local fossil dealer and sold to the Yizhou Fossil & Geology Park and catalogued there as YTGP-T5199. This fossil, a nearly complete skeleton, was prepared and studied by scientists at the Geology Park and identified as an Anchiornis. It was then used for a scanning electron microscope study of Anchiornis feather microstructure. The study also examined the well-preserved melanosomes of the feathers to determine their color. The scientists involved in the study found that the coloration found for this specimen was different than the color reported for BMNHC PH828, and they noted that the BMNHC specimen may not in fact be Anchiornis, as it was described before similar species from the same formation had been discovered.
The Shandong Tianyu Museum of Nature in Pingyi County, China, for example, was reported to hold 255 specimens of Anchiornis in its collections in 2010. Among their collection is a very well preserved fossil with visible color patterns, catalogued as STM 0-214. While this specimen has yet to be fully described, it was photographed for a 2011 article in National Geographic and was used in a study of Anchiornis covert feathers and wing anatomy the following year.
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 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. Long feathers on the head 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.The holotype, belonging to a subadult or young adult individual, measured long and weighed. The largest specimens measured in total body length, nearly in wingspan and weighed about.
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.