Yixian Formation

The Yixian Formation (simplified Chinese: 义县组; traditional Chinese: 義縣組; pinyin: Yìxiàn zǔ) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans 11 million years during the early Cretaceous period. It is known for its exquisitely preserved fossils, and is mainly composed of basalts interspersed with siliciclastic sediments. The Yixian Formation (simplified Chinese: 义县组; traditional Chinese: 義縣組; pinyin: Yìxiàn zǔ) is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans 11 million years during the early Cretaceous period. It is known for its exquisitely preserved fossils, and is mainly composed of basalts interspersed with siliciclastic sediments. 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 First battle of Hopei 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. 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 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. See Jehol Biota. For some time, the formation was believed to be from the Late Jurassic–Early Cretaceous boundary, some 145 mya (million years ago). Radiometric dating has since resolved it to be younger; it is now considered to have been deposited in the Barremian to early Aptian, some 125–121 mya. The Yixian Formation forms the lowest part of the Jehol 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. The Yixian Formation is preceded by the older Daohugou 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 younger Jiufotang Formation and the Fuxin Formation. Chiappe et al. argued in 1999 that the oldest beds of the Yixian (those bearing a fauna dominated by confuciusornithid birds) were best separated as a distinct formation, called the Chaomidianzi Formation, with a type locality at the village of Sihetun, approximately 25 km south of Beipiao City. 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. The Dabeigou Formation in Fengning, Hebei Province may immediately precede the Yixian, or it may be equivalent to the oldest Yixian beds. The Yixian also overlies the Tuchengzi Formation in places. The Yixian Formation is divided into the following subunits, from most recent to oldest: (ages when available from Chang et al., 2009) New high-precision 40Ar/39Ar dating provided ages of 125.8 ± 1.0 Ma and 126.0 ± 0.8 Ma for two basaltic samples from the Lujiatun Unit. 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).