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, Zhangjiakou Formation, Tuchengzi Formation |
| Lithology | |
| Primary | Basalt |
| Other | Siliciclastic |
| Location | |
| Coordinates | 41°31′59″N 121°14′18″E / 41.5330°N 121.2383°E |
| Approximate paleocoordinates | 44°18′N 122°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) | |
The Yixian Formation (simplified Chinese: 义县组; traditional Chinese: 義縣組; pinyin: Yìxiàn zǔ; formerly transcribed as Yihsien Formation[1]) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans the Barremian stage of the Early Cretaceous. It is known for its exquisitely preserved fossils, and is mainly composed of basalts interspersed with siliciclastic sediments.[2]
Research history
[edit]The potential importance of the Yixian Formation was initially recognized during the time the Empire of Japan occupied China's Rehe ("Jehol") Province after the Defense of the Great Wall in 1933. Many Japanese scientists had noticed fossil remains of extinct fish and reptiles, possibly the champsosaurs. These initial fossil discoveries made by Japanese scientists vanished once World War II ended in 1945.[citation needed] By 1949, when administration of the area passed to the Chinese Communist Party and its leader Mao 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 of dinosaur fossils that have revolutionized knowledge of these animals have been found at Yixian; among them are the first known non-avian theropods with feathers.[citation needed]
Stratigraphy
[edit]
The Yixian Formation is the middle formation of the Jehol Group. It is stratigraphically positioned between the older Dabeigou Formation and the younger Jiufotang Formation. These three formations together preserve broadly similar organisms and vary only in the composition of different groups. This assemblage is generally known as the Jehol biota and is widely known to preserve fossils in extreme detail, including those of small organisms. The Yixian Formation (and the Jehol Group more generally), is composed of lacustrine deposits interbedded with volcanic sediment, indicating the presence of a long-lived dynamic lake system punctuated by catastrophic volcanism.[2] In older publications, these interbedded rock strata were given various names including the Jingangshan, Tuhulu, Jianchang, Lower Volcanic, and Volcanic Rock formations, but today, these are all recognized as being synonymous with the Yixian Formation.[3][4]
The precise geologic age of the Yixian Formation has been relatively uncertain since it was originally discovered. It has been previously considered to have been deposited during the Jurassic Period (specifically the Tithonian stage). If correct, this would mean a much earlier origin for angiosperms, birds, and eutherians than other evidence tends to suggest. It is also difficult to directly determine the age by comparing it to possible lateral equivalents because in addition to overlying the Dabeigou Formation, parts of the Yixian Formation also unconformably overlies the much older Zhangjiakou and Tuchengzi formations in some places.[5][6] Modern estimates more confidently place the age of the Yixian Formation as being between the Barremian and Aptian stages, which is corroborated by biostratigraphic correlation of the Jehol biota with other biota of known ages. Specifically, the presence of fossilized pollen similar to other Early Cretaceous sediments as well as the genus Psittacosaurus, which is known from many other Early Cretaceous strata.[7]
Refinement of this age has been controversial in the years since study began. The interbedded volcanic sediments allow for the use of radiometric dating of tuff, biotite, and zircons by various methods. Among these, potassium-argon dating, uranium-lead dating, and rubidium-strontium dating have been used to varying degrees of success. Most modern studies have settled on the use of argon-argon dating of basalts, which is generally believed to yield more precise age estimates.[2] Recent application of these estimates has yielded a maximum age of 125.755 ± 0.061 million years and a minimum age of 124.122 ± 0.048 million years. This would make the entire duration of deposition of the Yixian Formation about 1.633 ± 0.078 million years, which is considerably shorter than previous estimates of 2-7 million years and would place it wholly within the Barremian stage of the Early Cretaceous.[8][9]
The Yixian Formation is informally divided into several subunits and the stratigraphy remains controversial[9] so a conservative list is give here, from most recent to oldest: (ages when available from Zhong et al., 2021[9])
- Upper Yixian (124.1 Ma)
- Upper Lava Unit
- Jianshangou Unit (125.46 Ma)
- Lower Lava Unit
- Lujiatun Unit (125.76-125.68 Ma)
Paleoclimate
[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 in subtropical and temperate upland forests. The presence of ferns, cycads, and horsetails indicates a generally humid climate. However, evidence from the growth rings of 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.[10] Evidence from the study of oxygen isotopes has shown that the average yearly temperature during this time period was 10 degrees celsius (50 degrees Fahrenheit), 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.[11] A study by Wu et al. (2013) concluded that orbital 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.[12]
Fossil content
[edit]The Yixian Formation represents the second of three major faunal phases that characterize the Jehol Biota, mainly based on changes in invertebrate diversity. In the Yixian, ostracods (seed shrimp) had diversified considerably, despite a very low diversity in the earlier Dabeigou Formation. Other major invertebrate groups in the Yixian include clam shrimp and insects. Insects, as a group, experienced their largest diversification of the entire Mesozoic era in the Yixian. On the other hand, some invertebrate groups, such as bivalves and gastropods (snails and slugs), were numerous but low in diversity, being mainly represented by one or two dominant species (Arguniella in the case of the bivalves).[2]
Studies of vertebrates have shown support for the division of the Jehol into phases, and the diversity of fish in the Yixian was distinct from older and younger formations, with Lycoptera as the dominant species. The Yixian preserves the first Jehol dinosaurs and pterosaurs (which have not been found in the older Dabeigou Formation), and the first major radiation of birds (only one bird species is known from the Dabeigou). The Yixian also preserves the largest (and only) mammal radiation so far known from the Jehol group. Most vertebrates showed a tendency to climb trees or become arboreal, including many tree-dwelling birds, and climbing mammals and lizards.[2]
Plant life reached its Jehol biota peak in the Yixian. Five species of flowering plant were present (three of Archaefructus, one of Archaeamphora and one of Hyrcantha (formerly Sinocarpus), as were a variety of horsetails that 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 as ginkgoes, conifers, cycads, and seed fern trees. Ground cover plants included lycopods, horsetails, ferns, and primitive flowering plants, which were rare compared to the others.[2]
This plant life grew around a series of freshwater lakes, provided with abundant minerals thanks to periodic volcanic eruptions. Volcanic activity, along with periodic wildfires, and noxious gasses released from the lake bottoms caused 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]
The Yixian Formation is well known for its great diversity of well-preserved specimens and its feathered dinosaurs, such as the large tyrannosauroid Yutyrannus, the therizinosaur Beipiaosaurus, and various small birds, along with a selection of other dinosaurs, such as the iguanodontian Bolong, the sauropod Dongbeititan and the ceratopsian Psittacosaurus. Other biota included the troodontid Mei, the dromaeosaurid Tianyuraptor, the compsognathid Sinosauropteryx and the tyrannosauroid Dilong.[citation needed]Microraptor also hails from this formation, as well as from the younger Jiufotang Formation. However, other microraptorines, such as Sinornithosaurus and Graciliraptor, also inhabit the Yixian. A limnic eruption may have preserved many of the fossils, excluding Dongbeititan.[citation needed]
See also
[edit]- List of fossil sites (with link directory)
- List of dinosaur-bearing fossil sites
References
[edit]- ^ a b Grabau, A. W. (1923). "Cretaceous Mollusca from North China". Bulletin of the Geological Survey of China. 5 (2): 183–197.
- ^ a b c d e f g Zhou, 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.
- ^ 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.
- ^ Liu, Yong-Qing; Kuang, Hong-Wei; Jiang, Xiao-Jun; Peng, Nan; Xu, Huan; Sun, Hui-Yi (2012). "Timing of the earliest known feathered dinosaurs and transitional pterosaurs older than the Jehol Biota". Palaeogeography, Palaeoclimatology, Palaeoecology. 323–325: 1–12. Bibcode:2012PPP...323....1L. doi:10.1016/j.palaeo.2012.01.017.
- ^ Qin, Zuohuan; Xi, Dangpeng; Shi, Zhongye; Xu, Yankang; Wei, Feng; Yu, Zhiqiang; Wu, Baoxu; Wan, Xiaoqiao (2019). "Lagerstätte fossils from the Lower Cretaceous Yixian Formation of the Pingquan Basin, North China: Stratigraphical correlation and palaeoenvironmental implications". Lethaia. 52 (3): 335–349. Bibcode:2019Letha..52..335Q. doi:10.1111/let.12315.
- ^ Barrett, Paul M. (2000). "Evolutionary consequences of dating the Yixian Formation". Trends in Ecology & Evolution. 15 (3): 99–103. doi:10.1016/S0169-5347(99)01782-6. PMID 10675924.
- ^ Chang, Su-Chin; Zhang, Haichun; Renne, Paul R.; Fang, Yan (2009). "High-precision 40Ar/39Ar age for the Jehol Biota". Palaeogeography, Palaeoclimatology, Palaeoecology. 280 (1–2): 94–104. Bibcode:2009PPP...280...94C. doi:10.1016/j.palaeo.2009.06.021.
- ^ a b c Zhong, 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. ISSN 2095-5138. PMC 8288181. PMID 34691675.
- ^ 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. PMC 3069172. PMID 21393569.
- ^ 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.