TheJehol Biota includes all the living organisms – theecosystem – of northeasternChina between 133 and 120 million years ago. This is theLower Cretaceous ecosystem which left fossils in theYixian Formation andJiufotang Formation. These deposits are composed of layers oftephra and sediment.[1] It is also believed to have left fossils in theSinuiju series ofNorth Korea.[2] The ecosystem in the Lower Cretaceous was dominated by wetlands and numerous lakes (not rivers, deltas, or marine habitats). Rainfall was seasonal, alternating betweensemiarid andmesic conditions. The climate wastemperate. The Jehol ecosystem was interrupted periodically by ash eruptions from volcanoes to the west. The word "Jehol" is a historical transcription of the formerRehe Province.
Some scientists have argued that the Jehol Biota evolved directly from the precedingDaohugou Biota without any strongly defined division. However, the absolute dating of the Daohugou beds has been the subject of divergent opinion: in 2006, Wanget al.[3] found an overall similarity between the fossil animals found in the Daohugou Beds and the "Jehol Biota" from the Yixian Formation. Several other research teams, including Liuet al., have attempted to disprove this reasoning by using Zircon U-Pb dating on the volcanic rocks overlying and underlying salamander-bearing layers (salamanders are often used asindex fossils). Liuet al. found that the Daohugou beds formed between 164 and 158 million years ago, in the Middle to LateJurassic.[4] Later, Jiet al. argued that the key indicator of the Jehol biota are the index fossil fishesPeipiaosteus andLycoptera. Under this definition, the earliest evolutionary stage of the Jehol Biota is represented by theHuajiying Formation.[5]
TheYixian andJiufotang Formations are consideredLagerstätte, meaning that they have exceptionally good conditions for fossil preservation. The fossils are numerous, but also very well preserved – often including articulated skeletons, soft tissues, colour patterns, stomach contents, and twigs with leaves and flowers still attached. Zhonghe Zhouet al.. (2003) deduced two things from this. The first is that the land animals and plants were washed into the lakes very gently, or were already in the lakes when they died. They do not show the damage seen in fossils formed by large floods. Secondly, volcanic ash is commonly inter-bedded with lake sediments, and ashfalls seem to have quickly buried the fossilized organisms, creatinganoxic conditions around them and preventing scavenging.
Zhonghe Zhouet al. (2003) noted that, for theEarly Cretaceous, the Jehol Biota includes a mixture of advanced and ancient species, and also of species found only in the Jehol and others found all around the world. It is possible that northeast Asia was isolated for part of theJurassic by theTurgai Sea which separated Europe from Asia at the time.
The Jehol Biota includes many species that were previously known only from theLate Jurassic or earlier. These "relict" species include thecompsognathid dinosaurSinosauropteryx and theanurognathidpterosaurDendrorhynchoides. It also has the earliest and most primitive known members of groups that spread all around the world by theLate Cretaceous, includingneoceratopsians,therizinosaurs,tyrannosaurs, andoviraptorids. Northeastern Asia may have been the center of diversification of these dinosaur groups.
The Jehol Biota was not entirely isolated, however, because it also includes animals which were known from all around the world at the same time, includingdiscoglossidfrogs,paramacellodidlizards,multituberculatemammals,enantiornithine birds,ctenochasmatidpterosaurs,iguanodontianornithopods,titanosauriformsauropods,nodosauridankylosaurs, anddromaeosauridtheropods.
The Jehol Biota is particularly noteworthy for the very high diversity of fossils and the very large numbers of individuals of each species that have been recovered.
The Jehol Biota has produced fossils of plant macro- andmicrofossils, includingangiosperms (the earliest known),charophytes anddinocysts, snails (gastropods), clams (bivalves), superabundant aquatic arthropods calledconchostracans,ostracods,shrimps,insects,spiders,fish,frogs andsalamanders (amphibians),turtles,choristoderes, lizards (squamates),pterosaurs, and dinosaurs includingfeathered dinosaurs, the largestmammals known from theMesozoic, and a great diversity ofbirds including the earliest advanced birds.
The forests around the lakes were dominated byconifers including members of thepodocarp,pine,araucaria, andcypress families. There were alsoginkgos,czekanowskialeans,bennettitaleans,ephedra,horsetails,ferns, andmosses. The leaves and needles of the trees show adaptations to a dry season, but some of the ferns and mosses are types that grow in very wet habitats. It is possible that the latter avoided dry conditions by growing very close to bodies of water.Archaefructus has been described as the earliest known flowering plant (Angiosperm), and it is reconstructed as an aquatic plant.
Gu (1983 and 1995) defined the following species as typifying the Jehol Biota:
The name "Jehol Biota" was first published by Gu (1962),[8] but was in use by geologists and paleontologists by 1959. This term replaced the former "Jehol Fauna", whichAmadeus William Grabau (1923)[9] defined as the fossil assemblage typified by numerous fossils of theconchostracanEosestheria, themayflyEphemeropsis, and theTeleost fishLycoptera.[10] Thus it was sometimes called "EEL".
The Jehol group was 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.[11] By now the group includes, in ascending order, the Yixian Formation (including the Jingangshan, Tuhulu, Jianchang, Lower Volcanic and Volcanic Rock formations), theJiufotang Formation (including the Shahai Formation) and the Fuxin Formation (including the Binggou, Haizhou and Upper Volcanic formations).[12] Chiappeet al. argued in 1999 that the lower beds of the Yixian were best subdivided into a separate formation, the Chaomidianzi Formation, with a type locality at the village of Sihetun, approximately 25 km south of Beipiao City.[13] However, this classification has fallen out of favor, and the Chaomidianzi Formation is disused as a synonym of the Jianshangou Bed of the Yixian Formation.[14]
In 2008, Jiet al. argued that these traditional definitions of the Jehol Biota arbitrarily excluded earlier fossil beds that clearly represent the first evolutionary stages of the later faunas, even though lower beds also had representatives ofEphemeropsis andLycoptera. They argued that the boundaries of the biota should rather be set based on the distinctive large-scale sequences of volcanism which produced the strata, with the upper boundary set at the Shahai and Fuxin formations and the lower boundary at theZhangjiakou Formation. Along with this sedimentary correlation, they noted that the best index fossils to identify the biota arePeipiaosteus andLycoptera. Under this definition, the earliest stage of the Jehol Biota is represented by theHuajiying Formation.[5]
