| Yi | |
|---|---|
 | |
| Replica fossil ofY. qi | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Scansoriopterygidae |
| Genus: | †Yi Xuet al., 2015 |
| Type species | |
| Yi qi Xuet al., 2015 | |
| Species | |
| |
Yi is a genus ofscansoriopterygiddinosaur from theLate Jurassic ofChina. Its only species,Yi qi (Mandarin pronunciation:[î tɕʰǐ]; fromChinese:翼;pinyin:yì;lit. 'wing' and奇;qí; 'strange'), is known from a singlefossil specimen of an adult individual found inMiddle orLate JurassicTiaojishan Formation of Hebei, China, approximately 159 million years ago. It was a small, possibly tree-dwelling (arboreal) animal. Like other scansoriopterygids,Yi possessed an unusual, elongated third finger that appears to have helped to support a membranousgliding plane made of skin. The planes ofYi 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 ofbats.
The first and only known specimen ofYi qi was a fossil of an adult individual found by a farmer, Wang Jianrong, in a quarry near Mutoudeng Village (Qinglong County, Hebei). Wang sold the fossil to the Shandong Tianyu Museum of Nature in 2007, at which point Ding Xiaoqing, a technician at the museum, began further preparation of the fossil. Because many of the unique features and soft tissues of the specimen were uncovered by museum staff during preparation rather than amateur fossil sellers before the purchase, the scientists who studied it were confident that the specimen was authentic and unaltered, which was confirmed by aCAT scan. The initial study ofYi was published in the journalNature.[1]
The team of scientists who authored this initial study were led byXu Xing and also included Zheng Xiaoting, Corwin Sullivan, Wang Xiaoli, Xing Lida, Wang Yan, Zhang Xiaomei, Jingmai O'Connor, Zheng Fucheng Zhang and Pan Yanhong. They named and described thetype speciesYi qi. The generic nameYi means "wing" inMandarin. Thespecific nameqi means "strange".[1]Yi is notable for having the shortest generic name of any dinosaur, containing only two letters. Its four-letter binomial name,Yi qi, is also the shortest possible under articles 11.8.1 and 11.9.1 of theInternational Code of Zoological Nomenclature, and shares this distinction with thegreat evening batIa io.[2]
Yi qi is known only from a single partial skeleton (holotype specimen STM 31-2) currently in the collections of theShandong Tianyu Museum of Nature [zh]. 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 otherparavian 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 theulna 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 ofYi qi preserved a heavy covering of feathers. Unusually, based on its classification as an advancedtheropod in the cladePennaraptora (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. Themetatarsus 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 ofYi 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 ofconvergent evolution.[1][3] 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 likeflying squirrels. Thegreater glider, and the prehistoric gliding rodentEomys quercyi, also have a similarly long cartilaginous styliform element.[1]
On twelve positions the fossil was checked by anelectron microscope for the presence ofmelanosomes, pigment-bearingorganelles. All nine feather locations showed eumelanosomes. In the head feathers also phaeomelanosomes were present. On the membranes, only one observation had a positive result, of phaeomelanosomes. The eumelanosomes of the calf feathers were exceptionally large.[1]
Yi was placed in theScansoriopterygidae, a group ofmaniraptorantheropods. Acladistic analysis failed to resolve its exact relationships with the other known scansoriopterygids,Epidendrosaurus andEpidexipteryx. In the analysis the Scansoriopterygidae was recovered as the most basalclade of theParaves.[1]
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 ofYi 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 thatYi 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 thehumerus being small—and the cumbersome nature of the styliform, make it more likely thatYi qi was an exclusive glider. The only indication of powered flight comes from the researchers who conducted the initial study of the only knownYi specimen, who concluded that its mode of flight should be considered uncertain.[1]
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 byflying frogs. In this last case, no membrane would have formed an inner wing but possibly the arm feathers would have generated some lift.[1]
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 (24 in). The narrow wing of the maniraptor model would have resulted in a 320 cm2 (50 sq in) wing surface with awing loading of 1.19 g/cm2 (2.4 lb/sq ft). For the broad bat model wing these values would have been 638 cm2 (98.9 sq in) and 0.6 g/cm2 (1.2 lb/sq ft) respectively. In both cases the wing loading is well below the critical 2.5 g/cm2 (25 kg/m2; 5.1 lb/sq ft) upper limit for bird flight. The maniraptor model loading is typical forducks, though these have a relatively larger wingspan and a loweraspect 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 thatYi 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, itscenter 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 relativeEpidexipteryx. Its stability might also have been improved by a few flapping movements of its wings. A modern analogue is thekākāpō, that likewise glides from trees but also flaps to control its descent.[1]
In 2020, T. Alexander Dececchi and colleagues found that, thoughYi and other scansoriopterygids wereglidingarboreal animals, they had notorious deficiencies in flapping behaviors such asWAIR (Wing-assisted incline running) or taking-off from ground compared to other small paravians, and had similar gliding abilities to those of nearly-sized mammalian gliders, such asbats. Their results suggest that scansoriopterygids might have been specialized maniraptorans in mainly closed forests. While the high glide speed and average glide ratios would have been more efficient for travelling across small gaps in thecanopy, longer flights would have been less efficient with higherpredation risks. The poor gliding abilities of scansoriopterygids likeYi andAmbopteryx along with their difficulty to take-off, would have made them highly susceptible to beout-competed by more capable aerial vertebrates. Moreover, their likely specialist life-style may have further contributed to this disadvantage.[4]
The only knownYi qi fossil was found in rocks assigned to theTiaojishan Formation, dating to theCallovian-Oxfordian age of theMiddle-Late Jurassic,[1] dated to approximately 159 million years ago.[5] This is the same formation (and around the same age) as the other known scansoriopterygidsEpidexipteryx andScansoriopteryx. The ecosystem preserved in the Tiaojishan Formation is a forest dominated bybennettitales,ginkgo trees,conifers, andleptosporangiate ferns. These forests surrounded large lakes in the shadow of active volcanoes, ash from which was responsible for the remarkable preservation of many of the fossils. Based on the Tiajishan's plant life, its climate would have beensubtropical totemperate, warm andhumid.[6] Other vertebrate fossils found in the same rock quarry asYi qi, which would have been close contemporaries, includedsalamanders likeChunerpeton tianyiensis, the flyingpterosaursChangchengopterus pani,Dendrorhynchoides mutoudengensis, andQinglongopterus guoi, dinosaurs likeTianyulong confuciusi, basal birds likeAnchiornis huxleyi,Caihong juji, andEosinopteryx brevipenna, and finally as the early gliding[7]mammaliaform speciesArboroharamiya jenkinsi.[1]
{{cite journal}}: CS1 maint: DOI inactive as of July 2025 (link)
.jpg)
