Yixian Formation
| Yixian Formation | |
|---|---|
| Stratigraphic range:Barremian ~ | |
| Type | Geological formation |
| Unit of | Jehol Group |
| Sub-units | Jianshangou Bed Lujiatun Bed |
| Underlies | Jiufotang Formation |
| Overlies | Dabeigou Formation?,Tuchengzi Formation |
| Lithology | |
| Primary | Basalt |
| Other | Siliciclastic |
| Location | |
| Coordinates | 41°31′59″N121°14′18″E/ 41.5330°N 121.2383°E |
| Approximate paleocoordinates | 44°18′N122°48′E/ 44.3°N 122.8°E |
| Region | Liaoning |
| Country |  China |
| Type section | |
| Named for | Yixian,Liaoning |
| Named by | Amadeus William Grabau |
| Year defined | 1923[1] |
  Yixian Formation (China)  Yixian Formation (Liaoning) | |
TheYixian Formation(simplified Chinese:Nghĩa huyện tổ;traditional Chinese:Nghĩa huyện tổ;pinyin:Yìxiàn zǔ;formerlytranscribedasYihsien Formation[1]) is ageological formationinJinzhou,Liaoning,People's Republic of China,that spans theBarremianstage of the Early Cretaceous. It is known for its exquisitely preservedfossils,and is mainly composed ofbasaltsinterspersed withsiliciclasticsediments.[2]
History
[edit]Japanese occupation
[edit]The potential importance of the Yixian Formation was initially recognized during the time theEmpire of Japanoccupied China'sRehe ( "Jehol" ) Provinceafter theDefense of the Great Wallin 1933. Many Japanese scientists had noticed fossil remains of extinct fish and reptiles, possibly thechampsosaurs.These initial fossil discoveries made by Japanese scientists vanished onceWorld War IIended in 1945.
Chinese rediscovery
[edit]By 1949, when administration of the area passed to theChinese Communist Partyand its leaderMao Zedong,the fossils of Yixian were studied only by Chinese scientists. It was not until the 1990s when remarkable fossils of birds and other non-avian dinosaurs were excavated. Since 1996, a number ofdinosaurfossils that have revolutionized knowledge of these animals have been found at Yixian; among them are the first known non-aviantheropodswithfeathers.
Dating
[edit]For some time, the formation was believed to be from theLate Jurassic–Early Cretaceousboundary, some 145mya(million years ago).Radiometric datinghas since resolved it to be younger; it is now considered to have been deposited in theBarremian,some 125.8–124.1 mya.[3]
The Yixian Formation forms the lowest part of theJehol group,defined by Gu (1962 and 1983) as a group of geological formations including the Jehol Coal-bearing Beds, the Jehol Oil Shale Beds, and the Jehol Volcanic Rocks.[4]The Yixian Formation is preceded by the olderDaohugou Beds,of uncertain Jurassic or Early Cretaceous age, which are sometimes considered part of the Jehol group. The Yixian Formation (including the synonymous Jingangshan, Tuhulu, Jianchang, Lower Volcanic and Volcanic Rock formations) is followed stratigraphically by the slightly youngerJiufotang Formationand theFuxin Formation.[5]Chiappeet al.argued in 1999 that the oldest beds of the Yixian (those bearing a fauna dominated byconfuciusornithid birds) were best separated as a distinct formation, called theChaomidianzi Formation,with a type locality at the village of Sihetun, approximately 25 km south of Beipiao City.[6]However, this classification has fallen out of favor, and the Chaomidianzi Formation is disused as a synonym for the Jianshangou Bed of the Yixian Formation.[7]
TheDabeigou Formationin Fengning,Hebei Provincemay immediately precede the Yixian, or it may be equivalent to the oldest Yixian beds.[2]The Yixian also overlies theTuchengzi Formationin places.[8]
The Yixian Formation is informally divided into several subunits and the stratigraphy remains controversial[3]so a conservative list is give here, from most recent to oldest: (ages when available from Zhonget al., 2021[3])
- Upper Yixian (124.1 Ma)
- Upper Lava Unit
- Jianshangou Unit (125.46 Ma)
- Lower Lava Unit
- Lujiatun Unit (125.76-125.68 Ma)
Ecology
[edit]The Yixian Formation represents the second of three major faunal phases that characterize theJehol Biota,mainly based on changes ininvertebratediversity. In the Yixian,ostracods(seed shrimp) had diversified considerably, despite a very low diversity in the earlierDabeigou Formation.Other major invertebrate groups in the Yixian includeclam shrimpandinsects.Insects, as a group, experienced their largest diversification of the entireMesozoicera in the Yixian. On the other hand, some invertebrate groups, such asbivalvesandgastropods(snails and slugs), were numerous but low in diversity, being mainly represented by one or two dominant species (Arguniellain the case of the bivalves).[2]
Studies ofvertebrateshave shown support for the division of the Jehol into phases, and the diversity offishin the Yixian was distinct from older and younger formations, withLycopteraas the dominant species. The Yixian preserves the first Jeholdinosaursandpterosaurs(which have not been found in the older Dabeigou Formation), and the first major radiation ofbirds(only one bird species is known from the Dabeigou). The Yixian also preserves the largest (and only)mammalradiation so far known from the Jehol group. Most vertebrates showed a tendency to climb trees or becomearboreal,including many tree-dwelling birds, and climbing mammals and lizards.[2]
Plantlife reached its Jehol biota peak in the Yixian. Five species offlowering plantwere present (three ofArchaefructus,one ofArchaeamphoraand one ofHyrcantha(formerlySinocarpus), as were a variety ofhorsetailsthat closely resembled modern species. It is possible that increasing animal and plant diversity were linked. The Yixian is characterized by extensive forests, dominated by trees such asginkgoes,conifers,cycads,andseed ferntrees. Ground cover plants includedlycopods,horsetails,ferns,and primitive flowering plants, which were rare compared to the others.[2]
This plant life grew around a series of freshwaterlakes,provided with abundant minerals thanks to periodicvolcanic eruptions.Volcanic activity, along with periodicwildfires,andnoxious gasses released from the lake bottomscaused the ecosystem to be continually destroyed and regrown. This, along with the wide diversity of habitats in the surrounding region, may have contributed to the fast diversification of life forms present in the Yixian ecosystem.[2]
Climate
[edit]With the diversity of plant life in the Yixian well known, including examples of a variety of petrified wood and growth rings, and with the help of chemical analysis, scientists have been able to determine the climate of the formation. The Yixian flora was dominated by conifers closely related to modern species that are found mainly insubtropicalandtemperateupland forests. The presence of ferns, cycads, and horsetails indicates a generally humid climate. However, evidence from thegrowth ringsof petrified wood indicates that the humidity and water supply dropped regularly. This shows that the wet, humid conditions were punctuated by dry seasons, in which the environment became more arid.[9]Evidence from the study of oxygen isotopes has shown that the average yearly temperature during this time period was 10 degreescelsius(50 degreesFahrenheit), significantly colder than once thought. This indicates a temperate climate with unusually cold winters for the generally warm Mesozoic era, possibly due to northern China's high latitude during this time.[10]A study by Wu et al. (2013) concluded thatorbital forcing,which is the effect on climate caused by shifts in the tilt of the Earth's axis and by the shape of the Earth's orbit, contributed to the climate fluctuations of this formation.[11]
Paleobiota
[edit]The Yixian Formation is well known for its great diversity of well-preserved specimens and its feathered dinosaurs, such as the large tyrannosauroidYutyrannus,the therizinosaurBeipiaosaurus,and various small birds, along with a selection of other dinosaurs, such as the iguanodontianBolong,the sauropodDongbeititanand the ceratopsianPsittacosaurus.Other biota included the troodontidMei,the dromaeosauridTianyuraptor,the compsognathidSinosauropteryxand the tyrannosauroidDilong.[citation needed]Microraptoralso hails from this formation, as well as from the youngerJiufotang Formation.However, other microraptorines, such asSinornithosaurusandGraciliraptor,also inhabit the Yixian. Alimnic eruptionmay have preserved many of the fossils, excludingDongbeititan.
See also
[edit]- List of fossil sites(with link directory)
- List of dinosaur-bearing fossil sites
References
[edit]- ^abGrabau, A. W. (1923)."Cretaceous Mollusca from North China".Bulletin of the Geological Survey of China.5(2): 183–197.
- ^abcdefZhou, Z (2006)."Evolutionary radiation of the Jehol Biota: chronological and ecological perspectives".Geological Journal.41(3–4): 377–393.Bibcode:2006GeolJ..41..377Z.doi:10.1002/gj.1045.
- ^abcZhong, Yuting; Huyskens, Magdalena H; Yin, Qing-Zhu; Wang, Yaqiong; Ma, Qiang; Xu, Yi-Gang (2021-04-12)."High-precision geochronological constraints on the duration of 'Dinosaur Pompeii' and the Yixian Formation".National Science Review.8(6): nwab063.doi:10.1093/nsr/nwab063.ISSN2095-5138.PMC8288181.PMID34691675.
- ^Gu, Z.W. (1983) "On the boundary of non-marine Jurassic and Cretaceous in China" in: "Nanjing Institute of Geology and Paleontology, Academica Sinica" "Stratigraphical Chart in China with Explanatory Text" Science Press, Beijing 1983:65–82.
- ^Sha, Jingeng (2007). "Cretaceous Stratigraphy of northeast China: non-marine and marine correlation".Cretaceous Research.28(2): 146–170.Bibcode:2007CrRes..28..146S.doi:10.1016/j.cretres.2006.12.002.
- ^Chiappe, L.M., Ji, S.A., Ji, Q., and Norell, M.A. (1999). "Anatomy and systematics of the Confuciusornithidae (Aves) from the Mesozoic of North-eastern China."Bulletin of the American Museum of Natural History,1999.
- ^Chen, P.; Wang, Q.; Zhang, H.; Cao, M.; Li, W.; Wu, S.; Shen, Y. (2005). "Jianshangou Bed of the Yixian Formation in west Liaoning, China".Science in China Series D: Earth Sciences.48(3): 298–312.Bibcode:2005ScChD..48..298C.doi:10.1360/04yd0038.S2CID130825449.
- ^Wang, Y.; Ken, S.; Zhang, W.; Zheng, S. (2006). "Biodiversity and palaeoclimate of the Middle Jurassic floras from the Tiaojishan Formation in western Liaoning, China".Progress in Natural Science.16(1): 222–230.doi:10.1080/10020070612330087.
- ^Wang, Y., Zheng, S., Yang, X., Zhang, W., and Ni, Q. (2006). "The biodiversity and palaeoclimate of conifer floras from the Early Cretaceous deposits in western Liaoning, northeast China."International Symposium on Cretaceous Major Geological Events and Earth System,56A.
- ^Amiot, R.; Wang, X.; Zhou, Z.; Xiaolin Wang, X.; Buffetaut, E.; Lécuyer, C.; Ding, Z.; Fluteau, F.; Hibino, T.; Kusuhashi, N.; Mo, J.; Suteethorn, V.; Yuanqing Wang, Y.; Xu, X.; Zhang, F. (2011)."Oxygen isotopes of East Asian dinosaurs reveal exceptionally cold Early Cretaceous climates".Proceedings of the National Academy of Sciences.108(13): 5179–5183.Bibcode:2011PNAS..108.5179A.doi:10.1073/pnas.1011369108.PMC3069172.PMID21393569.
- ^Wu, Huaichun; Zhang, Shihong; Jiang, Ganqing; Yang, Tianshui; Guo, Junhua; Li, Haiyan (2013). "Astrochronology for the Early Cretaceous Jehol Biota in Northeastern China".Palaeogeography, Palaeoclimatology, Palaeoecology.385:221–228.Bibcode:2013PPP...385..221W.doi:10.1016/j.palaeo.2013.05.017.