| Yixian Formation | |
|---|---|
| Stratigraphic range:Barremian ~125.8–124.1 Ma | |
 An hillside showing an outcrop of the Yixian Formation's Lujiatun Member | |
| 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″N121°14′18″E / 41.5330°N 121.2383°E /41.5330; 121.2383 |
| Approximate paleocoordinates | 44°18′N122°48′E / 44.3°N 122.8°E /44.3; 122.8 |
| Region | Liaoning |
| Country | China |
| Type section | |
| Named for | Yixian, Liaoning |
| Named by | Amadeus William Grabau |
| Year defined | 1923[1] |
  Yixian Formation (China) Show map of China Yixian Formation (Liaoning) Show map of Liaoning | |
TheYixian Formation (simplified Chinese:义县组;traditional Chinese:義縣組;pinyin:Yìxiàn zǔ; formerlytranscribed asYihsien Formation[1] orYixiang Formation[2]) is ageological formation inJinzhou,Liaoning,People's Republic of China, that spans theBarremian stage of the Early Cretaceous. It is known for its exquisitely preservedfossils, and is mainly composed ofbasalts interspersed withsiliciclastic sediments.[3]
The potential importance of the Yixian Formation was initially recognized during the time theEmpire of Japan occupied China'sRehe ("Jehol") Province after theDefense of the Great Wall in 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 II ended in 1945.[citation needed] By 1949, when administration of the area passed to theChinese Communist Party and 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 ofdinosaur fossils that have revolutionized knowledge of these animals have been found at Yixian; among them are the first known non-aviantheropods withfeathers.[citation needed]
The Yixian Formation is the middle formation of theJehol Group. It is stratigraphically positioned between the olderDabeigou Formation and the youngerJiufotang Formation. These three formations together preserve broadly similar organisms and vary only in the composition of different groups. This assemblage is generally known as theJehol 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 oflacustrine deposits interbedded withvolcanic sediment, indicating the presence of a long-lived dynamic lake system punctuated by catastrophic volcanism.[3] 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.[4][5]
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 theJurassic Period (specifically theTithonian stage). If correct, this would mean a much earlier origin forangiosperms,birds, andeutherians 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 olderZhangjiakou andTuchengzi formations in some places.[6][7] Modern estimates more confidently place the age of the Yixian Formation as being between theBarremian andAptian stages, which is corroborated by biostratigraphic correlation of theJehol biota with other biota of known ages. Specifically, the presence of fossilized pollen similar to otherEarly Cretaceous sediments as well as the genusPsittacosaurus, which is known from many other Early Cretaceous strata.[8]
Refinement of this age has been controversial in the years since study began. The interbedded volcanic sediments allow for the use ofradiometric dating oftuff,biotite, andzircons by various methods. Among these,potassium-argon dating,uranium-lead dating, andrubidium-strontium dating have been used to varying degrees of success. Most modern studies have settled on the use ofargon-argon dating ofbasalts, which is generally believed to yield more precise age estimates.[3] When originally estimated, the Yixian Formation was believed to have been deposited over a period of about 7 million years, during the laterBarremian and earlyAptian stages. However, the duration of deposition of the formation was narrowed in 2016 by analyzing continuous sedimentary cores to roughly the interval between 129.7 ± 0.5 and 125.2 ± 0.9 million years ago.[9] Recent application of radiometric estimates in 2021 has yielded a maximum age of 125.755 ± 0.061 million years and a minimum age of 124.122 ± 0.048 million years. If true, this would make the entire duration of deposition of the Yixian Formation about 1.633 ± 0.078 million years, which is considerably shorter than the initial estimates and would place it wholly within the Barremian stage of the Early Cretaceous.[10][11]
The Yixian Formation is broadly subdivided into several geological subunits. In ascending order (lowest to highest):[11]
The lower units (Lujiatun, Lower Lava, and Jianshangou) were originally grouped separately from the Yixian Formation under the names "Sihetun Formation" or "Chaomidianzi Formation" in the late 1990s. Various revisions to this arrangement have been made by several authors in the period from 2000 to 2010, including the consolidation of the Jiulongsong and Hengdaozi members into what is today called the Jianshangou Unit. Since 2010, most authors have used the same nomenclatural scheme (shown above). Although the nomenclature of the uppermost units has been inconsistent, these composition of these members have been generally agreed upon in the literature. Some authors do not consider the Dawangzhangzi and Huanghuanshan units to be distinct enough to separate, opting instead to call this interval the "Undivided Upper Yixian Formation".[9][12][11]
The relative ages of these subunits has been a matter of considerable scientific debate. In particular, the lower members of the Yixian Formation have been subject to revision multiple times over the last three decades. They were originally interpreted as a traditional chronological sequence, with the Lujiatun Unit being deposited first, followed by a major volcanic deposition event creating the Lower Lava Unit, and then the deposition of the Jianshangou Unit.[9] However, more recent measurements have suggested slightly overlapping or even fully contemporaneous deposition of the Lujiatun and Jianshangou Units.[13] Wang and colleagues made efforts to standardize the stratigraphy of the lower half of the Yixian Formation and their scheme further subdivided the Jianshangou Member into (in ascending order) the Dajianshanzi, Anjiagou, Hengdaozi, and Huangbanjigou beds.[9] The relative ages of these beds at present is unresolved and different researchers have yet to reach a consensus. Furthermore, the upper units of the Yixian Formation are generally less fossil-rich, and have received considerably less attention in the scientific literature, and so their stratigraphy remains somewhat unresolved.[11][13]
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 insubtropical andtemperate upland forests. The presence of ferns, cycads, and horsetails indicates a generally humid climate. However, evidence from thegrowth 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.[14] 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.[15] 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.[16]
The Yixian Formation represents the second of three major faunal phases that characterize the Jehol Biota, mainly based on changes ininvertebrate diversity. 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 shrimp andinsects. Insects, as a group, experienced their largest diversification of the entireMesozoic era in the Yixian. On the other hand, some invertebrate groups, such asbivalves andgastropods (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).[3]
Studies ofvertebrates have shown support for the division of the Jehol into phases, and the diversity offish in the Yixian was distinct from older and younger formations, withLycoptera as the dominant species. The Yixian preserves the first Jeholdinosaurs andpterosaurs (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)mammal radiation 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.[3]
Plant life reached its Jehol biota peak in the Yixian. Five species offlowering plant were present (three ofArchaefructus, one ofArchaeamphora and one ofHyrcantha (formerlySinocarpus), as were a variety ofhorsetails 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 asginkgoes,conifers,cycads, andseed fern trees. Ground cover plants includedlycopods, horsetails,ferns, and primitive flowering plants, which were rare compared to the others.[3]
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 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.[3]
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 sauropodDongbeititan and the ceratopsianPsittacosaurus. Other biota included the troodontidMei, the dromaeosauridTianyuraptor, the compsognathidSinosauropteryx and the tyrannosauroidDilong.[citation needed]Microraptorians, such asSinornithosaurus andGraciliraptor, also inhabit the Yixian. Alimnic eruption may have preserved many of the fossils, excludingDongbeititan.[citation needed]
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