Yi (dinosaur)

Yi is a genus of scansoriopterygid dinosaurs from the Late Jurassic of China. Its only species, Yi qi (Mandarin pronunciation: [î tɕʰǐ]; from Chinese: 翼; pinyin: yì; literally: "wing" and 奇; qí; "strange"), is known from a single fossil specimen of an adult individual found in Middle or Late Jurassic of Hebei, China, approximately 160 million years ago. It was a small, possibly tree-dwelling (arboreal) animal. Like other scansoriopterygids, Yi possessed an unusual, elongated third finger, that helped to support a membranous gliding plane made of skin. The planes of Yi qi were also supported by a long, bony strut attached to the wrist. This modified wrist bone and membrane-based plane is unique among all known dinosaurs, and might have resulted in wings similar in appearance to those of bats.

Description
Yi qi is known only from a single partial skeleton (holotype specimen STM 31-2) currently in the collections of the Shandong Tianyu Museum of Nature. The fossil was compressed and is visible on a stone plate and a counterplate. It is largely articulated, including the skull, lower jaws, neck and limb bones but lacking most of the backbone, pelvis and tail. Yi was a relatively small animal, estimated to weigh about 380 grams (0.84 lb).[1]

Like other scansoriopterygids, the head was short and blunt-snouted, with a downturned lower jaw. Its few teeth were present only in the tips of the jaws, with the four upper front teeth per side being the largest and slightly forward-pointing, and the front lower teeth being angled even more strongly forward.[1] The long, slender forelimbs were similar, overall, to those of most other paravian dinosaurs. Like other scansoriopterygid dinosaurs, the first finger was shortest and the third was the longest. Unlike all other known dinosaurs, a long, pointed wrist bone known as a "styliform element", exceeding both the third finger and the ulna in length, extended backward from the forelimb bones. This styliform, an adaptation to help support the membrane, may have been a newly evolved wrist bone, or a calcified rod of cartilage. It was slightly curved and tapered at its outer end.[1]

The only known specimen of Yi qi preserved a heavy covering of feathers. Unusually, based on its classification as an advanced theropod in the clade Pennaraptora (a group containing theropods with advanced, bird-like feathers), the feathers were all very simple in structure and "paintbrush-like", with long quill-like bases topped by sprays of thinner filaments. All these structures were rather stiff.[1] The feathers covered most of the body, starting near the tip of the snout. The head and neck feathers were long and formed a thick coat, and the body feathers were even longer and denser, making it difficult for scientists to study their detailed structure. The longest feathers, with a length of about six centimetres, were present behind the upper arm and the shinbone. The metatarsus of the foot had a feather covering also.[1]

Small patches of wrinkled skin were also preserved, between the fingers and the styliform bone, indicating that unlike all other known dinosaurs, the planes of Yi qi were formed by a skin membrane rather than flight feathers.[1] The membrane stretched between the shorter fingers, the elongated third finger, the styliform bone, and possibly connected to the torso, though the inner part of the wing membrane was not preserved in the only known fossil.[1] This would have given the animal an appearance similar to modern bats, in an example of convergent evolution.[1][2] However, in bats, the membrane stretches between the fingers only, no styliform wrist bone being present. Ossified styliform bones are found, however, in the wings of some modern gliding animals like flying squirrels. The Greater glider, as well as the prehistoric gliding rodent Eomys quercyi, has a long cartilaginous styliform element.[1]

On twelve positions the fossil was checked by an electron microscope for the presence of melanosomes, pigment-bearing organelles. All nine feather locations showed eumelanosomes, which cause a black colour. In the head feathers also phaeomelanosomes were present, rendering a more yellow-brownish hue. On the membranes, only one observation had a positive result, of phaeomelanosomes. The eumelanosomes of the calf feathers were exceptionally large. =Phylogeny== Yi was placed in the Scansoriopterygidae, a group of maniraptoran theropods. A cladistic analysis failed to resolve the exact relationships with other known scansoriopterygids as Epidendrosaurus and Epidexipteryx. In the analysis the Scansoriopterygidae were recovered as the most basal clade of the Paraves.