page_title: Yi (dinosaur)
section_title: Paleobiology
hierarchical_section_title: Yi (dinosaur) / Paleobiology
caption_attribution_description: English: Restoration of the membrane-winged scansoriopterygid Yi qi
context_page_description: Yi is a genus of scansoriopterygid dinosaurs from the Late Jurassic of China. Its only species, Yi qi, is known from a single fossil specimen of an adult individual found in Middle or Late Jurassic Tiaojishan Formation of Hebei, China, approximately 159 million years ago. It was a small, possibly tree-dwelling animal. Like other scansoriopterygids, Yi possessed an unusual, elongated third finger, that appears to have 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.
context_section_description: Yi qi, and presumably other scansoriopterygids, possessed a type of wing unknown among any other prehistoric bird relatives. Unlike other paravian dinosaurs, they seem to have replaced bird-like feathers with membranous wings, in what may have been one of many independent evolutionary experiments with flight close to the origin of birds.
  The membranous wings of Yi qi are unique among dinosaurs and difficult to interpret. That the arm could in principle function as a wing, is shown by being longer than the already elongated hindlimb and the sufficient thickness of its long bones. Also it is hard to explain the styliform element outside a flight context. The presence of a long styliform bone adding support to the membrane, found only in other animals that glide, suggests that Yi qi was specialized for gliding flight. While it is possible that some form of flapping flight was also used by this animal, the lack of evidence for large pectoral muscles—the deltopectoral crest of the humerus being small—and the cumbersome nature of the styliform, make it more likely that Yi qi was an exclusive glider. At best, the researchers who conducted the initial study of the only known Yi specimen concluded that its mode of flight should be considered uncertain.
  The authors proposed three main models for the wing configuration. In the "bat model", the styliform element would have pointed straight to the rear, a membrane connecting styliform and torso. This would have resulted in a broad wing. A variant of the bat model might be the "pterosaur model" in which the styliform bone would have been directed obliquely to the outside, with a narrower wing as a result. The second main possibility is the "maniraptor model", in which the styliform element was pointing towards the body, reinforcing the trailing edge of a narrow membrane, possibly widened by feathers, on the top or the underside, sticking out. A last configuration would be the "frog model", the styliform bone enlarging a membranous hand plane, like that used by flying frogs. In this last case, no membrane would have formed an inner wing but possibly the arm feathers would have generated some lift.
  A preliminary analysis was made of the flight characteristics of the bat model and the maniraptor model. For both models it was assumed that the wingspan was about sixty centimetres. The narrow wing of the maniraptor model would have resulted in a 320 cm² wing surface with a wing loading of 1.19 gr/cm². For the broad bat model wing these values would have been 638 cm² and 0.6 gr/cm² respectively. In both cases the wing loading is well below the critical 2.5 gr/cm² upper limit for bird flight. The maniraptor model loading is typical for ducks, though these have a relatively larger wingspan and a lower aspect ratio. The bat model has a loading typical for shore birds, though again their wingspan is (much) larger while their aspect ratio to the contrary is higher.
  A problem for the hypothesis that Yi was specialised for gliding flight, resides in the fact that because of it having a forelimb wing, instead of a gliding skin along its torso as with most gliders, its center of mass seems to be behind its control and main lift surfaces, causing flight instability. This problem might have been lessened by a short fleshy tail and long tail feathers, as known from its relative Epidexipteryx. Its stability might also have been improved by a few flapping movements of its wings. A modern analogue is the kakapo, that likewise glides from trees but also flaps to control its descent.
caption: Life restoration by Emily Willoughby
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