|The Jehol biota|
A series of papers in leading international journals, such as Nature and Science, astonished the palaeontological world in the 1990s. In these, ever-more amazing fossils were announced from the Jehol beds in NE China: examples of early birds, feathered dinosaurs, pterosaurs, early mammals, amphibians, pollinating insects and angiosperms. The specimens came from a time interval, the Early Cretaceous, whose faunas and floras were relatively poorly known from other locations and yet these specimens tended to be complete and they were often remarkably well preserved.
So far, the Jehol Group has yielded some 60 species of plants, nearly 1000 species of invertebrates, and 140 species of vertebrates (fishes and tetrapods). The tetrapods are listed in more detail here. This page reviews the history, geology, and fossils of the Jehol Group of China, and further detail may be found in reviews by Zhou et al. (2003) and Benton et al. (2008).
HistoryThe Jehol beds are so extensive and so rich in fossils that it seems amazing that the remarkable birds, dinosaurs, and other fossils were not reported earlier. The first report in the 1880s announced a small teleost fish that was later assigned to Lycoptera by Arthur Smith Woodward at the Natural History Museum in London.
The first broad overview of the faunas was made by the German-American palaeontologist, Amadeus William Grabau (1870-1946) in the 1920s. He began his career in North America, and then became Professor of Geology at Peking University in 1919, one of a small number of Americans and Europeans who were brought in at that time to develop and Westernize the leading universities in China. He wrote numerous papers, and seven books, about the geology of China. Among his many studies, Grabau (1923) gave the first description of what he called the 'Jehol fauna', consisting of abundant examples of the conchostracan Eosestheria, the mayfly Ephemeropsis and the teleost fish Lycoptera (Fig. 1). These three dominant fossils gave rise to an alternative name, the 'EEL' fauna. Grabau dated the Jehol fauna as Jurassic and Cretaceous, although predominantly Cretaceous, based on comparison of the fossils with those from elsewhere.
Figure 1. The classic triumvirate of fossils that define the Jehol Biota: (a) the conchostracan Eosestheria, (b) the mayfly Ephemeropsis and (c) the teleost fish Lycoptera. (Photographs courtesy of IVPP.)
Through the 1930s and 1940s, additional isolated fossils were reported, including new invertebrates, fishes, plants and occasional vertebrates. The next substantial work on the Jehol fossils was by the Chinese geologist and palaeontologist Gu Zhi-wei (born 1918). Because the Jehol fauna included plants, Gu (1962) renamed it the Jehol Biota. Work continued at a modest level until, in the late 1980s and early 1990s, a series of papers on fossil birds (Sereno et al. 1988; Sereno & Rao 1992; Zhou et al. 1992; Hou & Zhang 1993; Hou 1994; Hou et al. 1995; Zhou 1995), mammals (Hu et al. 1997) and dinosaurs (Ji & Ji 1996; Chen et al. 1998; Ji et al. 1998) began to draw enormous international attention.
GeologyThe Jehol Group, comprising the Dabeigou, Yixian and Jiufotang formations, crops out in western Liaoning, northern Hebei and SE Inner Mongolia (Nei Mongol), all in NE China (Fig. 2a). Comparable deposits of similar age occur across northern China and adjacent areas of eastern and central Asia, including Kazakhstan, Mongolia, Siberia, Japan and Korea, and many of these have yielded fossils very like those of the Jehol Biota (Zhou et al. 2003).
The Yixian and Jiufotang formations are a nearly conformable sequence of weakly laminated to finely bedded siliciclastic sediments, mainly low-energy sandstones and shales that are interpreted as having been deposited in ancient lakes. There is no evidence of other kinds of continental deposits, such as channel infills, palaeosols or aeolian dune beds. The lake sediments preserve fantastic numbers of fossils, many of them showing soft tissues.
These finely bedded sediments are interrupted by volcanic beds, largely conformable tuffs and basalts, as well as occasional cross-cutting dykes and sills (Zhou et al. 2003). The Jehol beds were deposited on the Eurasian landmass (Fig. 2b), a large continental area that was fully emergent from the oceans in the Early Cretaceous and composed of numerous tectonic blocks.
The volcanic beds throughout the Jehol Group resulted from igneous activity around the then Pacific rim, and it was largely contemporaneous, as indicated by the fact that most volcanic beds are conformable with the sediments. Volcanic activity peaked at the time of deposition of the Yixian Formation and dwindled through the time of deposition of the Jiufotang Formation.
StratigraphyThe Jehol Group (Fig. 4) consists of two major formations, the Yixian and Jiufotang, with the Dabeigou below in some regions. The Yixian Formation includes the Lujiatun bed, Jianshangou bed, Dawangzhangzi bed and Jingangshan bed; the Jiufotang Formation includes the Shahai bed. A further unit, the Dabeigou Formation in Fengning, northern Hebei Province, has been added as the lowest part of the Jehol Group below the Yixian Formation, although it has also been considered as equivalent to the lower part of the Yixian Formation in Liaoning Province.
Figure 4. A stratigraphic column of the Jehol Group showing the five main fossil-bearing beds (I-V). The lithological key is: 1, basalt and andesite with volcanic breccia (lava); 2, congolomerate with volcanic breccia; 3, sandstone and conglomerate; 4, tuffaceous sandstone and tuff; 5, shale and tuff; 6, silt and silty sandstone; 7, subvolcanic rock. (Courtesy of Xu Xing.)
AgeIn many earlier papers (e.g. Hou & Liu 1984; Hou et al. 1995; Sun et al. 1998; Ji et al. 1999), the Jehol Group as a whole, or particular fossiliferous horizons, were claimed to be Late Jurassic in age. This added to the apparent significance of the fossils and would have suggested that several major groups, including angiosperms, various clades of birds, and placental mammals had had much earlier origins than otherwise understood.
Subsequent work has shown that the early claims for a Late Jurassic age, based mainly on comparisons of faunas, but also on some radiometric dates, were flawed (Zhou et al. 2003). For example, dates of 137 ± 7 Ma and 143 ± 4 Ma from the Yixian Formation were interpreted as Late Jurassic on the basis of an older dating that placed the Jurassic-Cretaceous boundary at 135 Ma. When this is corrected to the current boundary date (145 Ma), these Yixian dates become Cretaceous, although still rather old.
The dating has now become clearer as a result of further comparisons of the fossils, but particularly because there are now high-quality radiometric dates throughout the Dabeigou and Yixian formations and, to a lesser extent, the Jiufotang Formation. As a result of sporadic eruptions throughout the time of deposition of the Jehol Group, tuffs are commonly interbedded with the shales or mudstones, even distal lacustrine sediments may contain tuffaceous horizons.
Published dates for the Yixian Formation range from 121 Ma to 147.1 Ma (reviewed Zhou et al. 2003; Zhou, 2006; Chang et al. 2009), but Swisher et al. (1999, 2002) dated the base of the formation at about 125 Ma (and the top at something younger than 121.6 Ma. Chang et al. (2009) presented further revised dates for the Yixian Formation, 129.7 ± 0.5 Ma for a basaltic lava from the bottom of the Yixian Formation and 122.1 ± 0.3 Ma for a tuff from the lowermost part of the overlying Jiufotang Formation. The top of the overlying Jiufotang Formation was dated at older than 110.6 Ma, but a direct date is 120.3 Ma (He et al. 2004). The Dabeigou Formation may date to 131 Ma.
Zhou et al. (2003) estimated that the Jehol Group spanned some 18 million years, based on a date of 128.4 Ma from a basalt overlying the Lujiatun Bed (basal Yixian Formation), and the date of 110.6 Ma from the upper part of the Jiufotang Formation. The 128.4 Ma date is now considered (Zhou 2006) as probably too old, and so the total duration of the Jehol Group is recalculated as 11 Ma (131-120 Ma), but this might be extended. The Yixian Formation lasted c. 7 Ma (Chang et al. 2009). A tentative timescale for the Jehol Group is:
The Jehol plants and animalsThe Jehol Biota has produced fossils of plants, including early angiosperms and microscopic charophytes and dinocysts; aquatic snails, bivalves and hugely abundant aquatic arthropods (conchostracans, ostracods, shrimps), fishes, frogs, salamanders, turtles and choristoderes; terrestrial and flying insects, spiders, lizards, pterosaurs and dinosaurs, including feathered dinosaurs, birds and mammals. Many of these fossils are exceptionally preserved.
Plants. The forests around the lakes were dominated by conifers, including members of the podocarp (Podocarpites), pine, araucaria (Araucarites) and cypress families. There were also ginkgos, czekanowskialeans, bennettitaleans, gnetaleans (Ephedrites, Gurvanella), horsetails (Equisetites), ferns and mosses (Zhou et al. 2003). The leaves and needles of the trees show adaptations to a dry season, and these were presumably derived from plants living on higher land, while the ferns and mosses normally grow in wet habitats, presumably around the waters' edge. Archaefructus (Fig. 5) was described as the earliest known angiosperm (Sun et al. 1998), and it is reconstructed as a modestly-sized water plant (Friis et al. 2003). Revision of the age of the deposits (see above) means that Archaefructus is certainly an early angiosperm, but by no means the oldest. More details of Archaefructus, with photographs of specimens and reconstructions, are here.
Animals. Many authors, including Amadeus Grabau and Gu Zhi-wei, have described invertebrate fossils from the Jehol Group, and Chen (1988) showed there were three phases of evolutionary radiation, corresponding to the Dabeigou, Yixian and Jiufotang formations. The majority of the fossils come from the lower Yixian Formation and the Jiufotang Formation. Methods of collection and study have changed over the years.
The Dabeigou Formation assemblage includes the Nestoria-Keratestheria conchostrachan assemblage, the Luanpingella-Eoparacypris-Darwinula ostracod assemblage, the Arguniella bivalve assemblage, the Lymnaea websteri gastropod assemblage, and the Peipiaosteus fengningensis-Yanosteus longidorsalis acipenseriform fish assemblage. Species and specimens of these fossils are rare, and the only birds known from this assemblage are Protopteryx fengningensis, the most primitive enantiornithine bird known (Zhang & Zhou 2000) and Eoconfuciusornis zhengi, the most primitive confuciusornithid (Zhang et al. 2008; Fig. 6).