The Maotianshan Shales (simplified Chinese: 帽天山页岩; traditional Chinese: 帽天山頁岩; pinyin: Màotiānshān yèyán) are a group of Early Cambrian sedimentary layers found in the Chiungchussu Formation or Heilinpu Formation. These layers are famous for their Konservat Lagerstätten, which are special deposits that preserve fossils and traces of ancient life in remarkable detail. The Maotianshan Shales are one of about 40 Cambrian fossil sites worldwide, known for preserving soft tissues that are rarely found in fossils, similar to the fossils in the Burgess Shale of British Columbia, Canada. They are named after Maotianshan Hill (Chinese: 帽天山; pinyin: Màotiānshān; meaning "Hat Sky Mountain") in Chengjiang County, Yunnan Province, China. The site is located in the "Eoredlichia-Wutingaspis Zone" of South China. A 512-hectare (1,270-acre) area within this formation, called the Chengjiang Fossil Site, was recognized as a UNESCO World Heritage Site in 2012.
The most well-known group of fossils from this area is called the Chengjiang biota, named after the many fossil sites in Chengjiang. The age of these fossils is locally called the Qiongzhusian stage, which matches the late Atdabanian Stage in Siberian rock layers from the Early Cambrian period. The shales are dated to be about 518 million years old.
Along with the Burgess Shale, the Maotianshan Shales are considered the best way to study the Cambrian "explosion," especially the origins of chordates.
History and scientific significance
Fossils from the Chengjiang region have been known since the early 10th century, but the area was first noted for its excellent preservation in 1984 with the discovery of Misszhouia, a soft-bodied relative of trilobites. Since this discovery, scientists worldwide have studied the site extensively, leading to many new findings and scientific discussions. Over time, some species have been reclassified, and the relationships between groups have been refined. These studies also led to the creation of a new group called Vetulicolia, which includes early deuterostomes.
The Chengjiang biota contains all the animal groups found in the Burgess Shale. However, because Chengjiang is 10 million years older, it provides stronger evidence that animals diversified earlier or more quickly during the early Cambrian period than the Burgess Shale alone suggests. The well-preserved variety of life at Chengjiang makes the Maotianshan shale one of the most important sites for understanding the evolution of early multi-cellular life, especially the group Chordata, which includes all vertebrates. Chengjiang fossils are the oldest and most diverse collection of animal life found after the boundary between the Proterozoic and Phanerozoic eras, making them a key source for studying the rapid diversification of life known as the Cambrian Explosion.
A special area within Chengjiang, called the Haiyan Lagerstätte, is famous for containing hundreds of juvenile fossils. This site offers valuable information about the development of many animal groups, making it a unique and important deposit from the Cambrian period.
In recognition of Chengjiang fossils as an unmatched record of the rapid diversification of animal life in the early Cambrian, the International Union of Geological Sciences (IUGS) added the "Cambrian Chengjiang fossil site and lagerstätte" to its list of 100 global "geological heritage sites" in October 2022. The IUGS defines these sites as places with geological features or processes of international scientific importance, used as references, or that have greatly contributed to the development of geological sciences.
In 2012, UNESCO designated a 512-hectare (1,270-acre) area within this formation, known as the Chengjiang Fossil Site, as a World Heritage Site.
Preservation and taphonomy
The fossils are found in a layer of mudstone that is 50 meters (160 feet) thick in the Yuanshan Member of the Qiongzhusi Formation. The Yuanshan Member covers a large area, spanning over 10,000 square kilometers (3,900 square miles) in eastern Yunnan Province. This region has many scattered rock outcrops that contain fossils. Studies of the rock layers show that the area was once a tropical environment with changes in sea level and tectonic movement. Scientists believe the region was a shallow sea with a muddy seafloor. The preserved animal life was mostly bottom-dwelling, and the fossils were likely buried by periodic underwater mud flows, as most fossils show no signs of being moved after death. Similar to the younger Burgess Shale fossils, the environment helped preserve soft body parts that were not made of minerals. The fossils are found in thin layers less than one inch thick. The soft parts are preserved as thin films of aluminosilicate, often containing high levels of oxidized iron and showing detailed features.
The Chengjiang beds are deeply weathered, as shown by their low density (they are very lightweight). Many trace fossils are found in this area.
Chengjiang fauna
The Chengjiang biota includes a very diverse group of animals, with about 185 species identified by scientists as of June 2006. Nearly half of these are arthropods, but most did not have the hard, strong outer coverings found in later arthropods. Only about 3% of the animals from Chengjiang had hard shells. Most of these were trilobites, of which five species have been found with traces of legs, antennae, and other soft body parts, which is very rare in fossils. Other groups, such as sponges (15 species) and priapulids (16 species), are also found in large numbers. Other groups include brachiopods, chaetognaths, cnidarians, ctenophores, echinoderms, hyoliths, nematomorphs, phoronids, and chordates. Possible mollusks include Wiwaxia.
About one in eight animals from Chengjiang are difficult to classify, as their relationships to other groups are unclear. Some may have been short-lived experiments in evolution as environments changed rapidly during the Cambrian period. Chengjiang is the richest source of lobopodians, a group of early panarthropods, with six genera found there: Luolishania, Paucipodia, Cardiodictyon, Hallucigenia, Microdictyon, and Onychodictyon.
Some of the most important fossils from Chengjiang are eight possible members of the chordate phylum, which includes all vertebrates. One of these is Myllokunmingia, which may be a very early jawless fish. Another is Haikouichthys ercaicunensis, a similar early fish-like animal.
The mysterious Yunnanozoon lividum has been linked to many different groups, including stem cephalochordates, stem or crown hemichordates, craniates, stem deuterostomes, stem bilaterians, or ambulacrarians. Fossils once called Haikouella (later renamed Yunnanozoon) show features like a heart, blood vessels, gill structures, and a notochord. A 2024 study placed Yunnanozoon on the chordate evolutionary path.
Another debated group is the Vetulicolia, first discovered in 1987 with the fossil Vetulicola. Close relatives include Beidazoon and Didazoonids. Heteromorphus is related to Banffia from the Burgess Shale, but both are considered vetulicolians. Initially thought to be crustacean arthropods, Vetulicola were later classified as a new group of primitive deuterostomes. Recent studies suggest vetulicolians are stem chordates, with a 2024 study placing them as an early chordate group before Yunnanozoon. Some scientists think vetulicolians may be closely related to tunicates. These animals likely swam and may have filtered food or eaten dead matter.
About two dozen animals from Chengjiang are difficult to classify. Among these, 'Anomalocaris' saron, once thought to be a top predator in the early Cambrian, was later renamed Houcaris saron and Innovatiocaris maotianshanensis. In 2006, Shu described Stromatoveris psygmoglena as a possible link between Ediacaran fronds and Cambrian ctenophores. Cambrocornulitus had a shell that may have been made of minerals, and it shares some traits with cornulitids and lophophorates.
The Chengjiang biota is believed to have lived in a delta front area with plenty of oxygen, where high sedimentation rates and major changes in salt levels were the main environmental challenges.
Guanshan biota
The Guanshan biota are located in Yunnan Province, South China, and are found in the Cambrian Stage 4 Wulongqing Formation. These fossils are similar to the Burgess shale-type fossils but are slightly younger than the Chengjian biota, dating back 515–510 million years. Brachiopods are the most common fossils, followed by trilobites. Other fossils include sponges, chancelloriids, cnidarians, ctenophores, priapulids, lobopodians, arthropods, anomalocaridids, hyoliths, molluscs, echinoderms, algae, and vetulicolians. The Guanshan biota also contain the earliest-known eocrinoids, unidentified soft-bodied animals, and many trace fossils.
The Guanshan biota are considered to follow the Chengjian biota and share many species. Some unique species include arthropods like Guangweicaris and Astutuscaris, as well as vetulicolians such as Vetulicola gantoucunensis and V. longbaoshanensis.