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The Jehol Biota – Early Cretaceous

Wang & Zhou have compared the formation of the site of the Jehol Biota, the “Mesozoic Pompeii”, to the situation at the time of the Pompeii disaster when the town of Pompeii was buried beneath volcanic ash and the people and animals were buried where they fell.  

The development of the Jehol Biota occurred over a relatively short time, geologically speaking, rapidly spreading into a large area of East Asia, and it was one of the largest terrestrial vertebrate radiations of the Cretaceous. The fossils of the Jehol Biota are exquisitely preserved, revealing many evolutionary events of terrestrial life.

The discovery in the western part of Liaoning Province in the late 1980s and 1990s of birds and feathered dinosaurs attracted worldwide attention of palaeontologists. In this region there are several dozen localities where major discoveries of birds and dinosaurs have been made, mostly by people from the Institute of Vertebrate Palaeontology and Palaeoanthropology (IVPP) and the Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sciences. Important discoveries have also been made in the neighbouring area of Hebei Province and southeastern Inner Mongolia (Nei Mongol Autonomous Region) that are similar to those from Liaoning. About half a dozen large-scale excavations have been launched by IVPP field crews since 1997 in Beipiao, Chaoyang, Fuxin in Liaoning, Fengning in Hebei and Ningcheng in Inner Mongolia, collecting hundreds of significant specimens that included fish, amphibians, turtles, aquatic reptiles, lizards, pterosaurs, dinosaurs, birds and mammals.

The climate of the area was warm with plenty of water in most seasons in the area of the Jehol Biota. Animals and plants thrived and differentiated in this ideal climate, with lakes being widespread. Wang & Zhou suggest that some of the water bodies were probably wide and deep, plants flourishing around the margins of the lakes. At this time there were frequent volcanic eruptions, and most of the vertebrates recovered had been preserved in lake deposits in water that is believed to have been rather deep.

Volcanic activity increased throughout the Late Cretaceous in the area of the Jehol Biota, with at least 3 major eruptions occurring during the Yixian Formation deposition that were responsible for the 4 lake deposit-volcanic eruption cycles. Basalts and Andesites are the major components of the volcanic rocks in the Yixian Formation, the lake deposits in this formation being mainly formed in the intervals between the major intermediate-basic eruptions, though occasionally there were volcanic eruptions that were small-sized and of intermediate-acid type. At the time the Jiufotang Formation was deposited volcanic activity was less frequent and relatively weak.

The development and evolution of the Jehol Biota were subjected to an impact of the volcanic eruptions that was long-lasting. Large volumes of poisonous gases were produced and put in the atmosphere by the intermediate-acid eruptions which had the potential to cause the deterioration of the entire ecological system, such as occurred at the time of the deposition of several layers that were highly fossiliferous in the Sihetun locality that were preserved exquisitely indicating major mass mortality events of birds and feathered dinosaurs. Greyish black lacustrine shale and mudstone are the main rocks containing the vertebrate fossils, and these are frequently found covered by a layer of volcanic ash (tuff). Almost all the fossils preserved are articulated; and birds and dinosaurs often have feathers visible, with skins and soft tissues often in the form of imprints or impressions with organic traces. Some of the dinosaur specimens were uncovered with eggs, Gastroliths and stomach contents, the remains being identifiable as plant seeds, and the skeletons of lizards and mammals. Wang & Zhou suggest it is most likely the volcanic eruptions resulted in environmental changes and it was these that caused the mass mortality of birds and other vertebrates. It is also suggested that after floating on the surface of the lakes for a short time the bodies of the dead rapidly descended into the water where they were buried in deep water, the deposition process being speeded up by the abundant volcanic ash, the result being that the dead bodies were rapidly completely preserved, as occurred in the Pompeii eruption.

Among the better known fossil localities of the Jehol Biota are Sihetun and Lujiatun in Beipiao; Fanzhangzi and Shanzui (Dawangzhangzi) in Lingyuan; Wanfotang, Hejiaxin, Wujiatun and Xierhuqiao in Yixian, Jinzhou; Shangheshou, Dapingfang, Lianhe and Dongdadao in Chaoyang, western Liaoning Province; Schaakou Senjitu in Fengning, northern Hebei Province; and Daohugou and Xitaizi in Ningcheng, southeastern Inner Mongolia.

Knowledge of the Jehol Biota has been increased  greatly after more than 50 years of research, especially after the unusual discoveries of the last 10 years, when compared with the earlier knowledge represented by the Eosestheria-Ephemeropsis-Lycoptera (E.E.L. Fauna, as has traditionally been defined). New insights have been gained on the origin of birds and their flight by recent discoveries of fossil vertebrates in western Liaoning, and the origin of feathers, the early radiation of birds, mammals and angiosperms, as well as furthering understanding of the continental ecosystem of the Early Cretaceous.

The rock layers need to be studied systematically to determine where the fossils are and the geological age of the rocks they are embedded in, and these rocks have been studied for many years before the recent discovery of feathered dinosaurs and birds. An American geologist, Amadeus W. Grabau was the first to propose the “Jehol Series” and the “Jehol Fauna” in the 1920s. The Jehol Group which was introduced into the literature in 1962 by Prof. Zhi-wei Gu. The Jehol Group is a set of rocks from the Late Mesozoic that consists of volcanic and lacustrine deposits, interlaced with each other, that are rich in tuffaceous materials. The Yixian Formation and the Jiufotang Formation that overlies it comprise the Jehol Group.

Northern Hebei Province, western Liaoning Province and Inner Mongolia in northeast China are the main areas in which the Jehol Group is distributed. In northeastern Asia the deposits formed in a series of northeast faulting basins during the late Mesozoic. The Fuxin-Yixian Basin, Jinlingsi-Yangshan Basin, Beipiao-Chaoyang Basin, Jianchang-Kazuo Basin, Lingyuan-Sanshijiazi Basin, Pingzhuang-Ningcheng Basin, and Chifeng-Yuanbaoshan Basin are some of the main basins. In the western rim of the Pacific vigorous plate collisions led to the intensive tectonic activities in the eastern margin of the European continent that occurred at that time. The depositional history in different basins were complicated by frequent volcanic eruptions and tectonic activities have increased the difficulty of correlating the deposits in these basins.

The Yixian Formation, Jiufotang Formation, Shahai Formation, chronologically from earliest to most recent, comprise the traditional Jehol Group. The lithographic characteristics and fossil assemblages of the Shahai and Fuxin Formation have been shown by recent studies to differ greatly from those of the Yixian and Jiufotang Formations. The Yixian Formation is mostly comprised of basalts and andesites with interbedded lacustrine sediments composed of tuffaceous sandstones, grey shales, grey-black shales, mudstones and tuffs. Within the Yixian Formation 4 beds containing fossils have been recognised. The Jiufotang Formation is mostly comprised of lacustrine sediments – greyish, grey-yellow, and grey-black sandstones, siltstones, shales and mudstones, with intercalated tuffs. Coal deposits and clastics, occasionally with volcanic contents, are the components of the Shahai and Fuxin Formations. The Yixian and Jiufotang Formations are the only formations in which the typical Jehol elements, such as Eosestheria-Ephemeropsis-Lycoptera assemblage, as well as the feathered dinosaurs, early birds, together with some other vertebrates that are distinctive are found. A major biological radiation event that occurred in the Early Cretaceous is represented by their appearance in the Yixian Formation, or possibly slightly earlier deposits, most of which continued through until the Jiufotang, the result being that a complete history of the Jehol Biota is recorded in the Yixian and Jiufotang Formations. At present the Jehol Group is generally accepted as comprising only these 2 formations.

There are 5 fossil-bearing beds, or “members”, of different ages and with distinctive vertebrate assemblages that have recently been recognised. From the bottom up they are Lujiatun Bed of the lower Yixian Formation (Jeholosaurus-Repenomamus assemblage), Jianshangou Bed of the lower Yixian Formation (Lycoptera sinensis-Confusiornis avian fauna assemblages), Dawangzhangzi Bed, middle Yixian Formation (Lycoptera davidi-Hyphalosaurus assemblage), Jingangshan Bed, the upper Yixian Formation (Lycoptera muroii-Manchurochelys manchoukuoensis assemblage), and the Boluochi Bed, Jiufotang Formation (Jinanichthys-Cathayornis avian fauna assemblage). According to Wang et al. most of the fossil localities of the Jehol Group can be referred to 1 of these 5 beds.

Jehol Biota Extension into Southern China – chronological evidence

In this paper the authors report that strata that may correlate with those of localities in which the classic Jehol Biota have been discovered are also known over much of central and eastern China, and also in Korea, Japan, Mongolia and Siberia. Based on known fossil assemblages it has been hypothesised previously that the Jehol Biota expanded progressively from the classic Jehol area, the younger representatives covering a progressively increasing geographic range. The youngest layers containing the Jehol Biota have been interpreted as being in the East Anhui Province, South China. They present 40Ar/39Ar dates of a rhyolite sample, that are considered to be robust, of 129 ± 0.2 Ma, from the lower Longwanshan Formation, and for 2 volcanic samples from the upper Honghuaqiao Formation, of 127.1 ± 1.3 Ma and 128.9 ± 0.9 Ma from outcrops of the Jehol fossils in East Anhui. The authors2 say it is indicated by their results that the fossils in these formations are time equivalent with fossils of the second phase from the lower Yixian Formation in western Liaoning, NE China, and the deposition of these beds is 6 Ma older, at least, than earlier estimates. It is therefore suggested by their results that there should be a re-evaluation of the Jehol radiation and migration hypothesis.

Sources & Further reading

1.      Wang, X-l & Zhou, Z-h, in Chang, Mee-mann (ed.), 2008, The Jehol Fossils: The Emergence of Feathered Dinosaurs, Beaked Birds, and Flowering Plants, Academic Press

2.      Chang, S.-C., et al. (2012). "Chronological evidence for extension of the Jehol Biota into Southern China." Palaeogeography, Palaeoclimatology, Palaeoecology 344–345(0): 1-5.


Author: M. H. Monroe
Last Updated 25/07/2014
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