Chengjiang Biota: China's Early Cambrian Fossil Treasure Trove

The Chengjiang biota is a collection of exceptionally preserved fossils from rocks about 518 million years old near Chengjiang in Yunnan Province, southern China. Discovered in 1984, it records soft-bodied worms, jellyfish-like animals, arthropods and some of the earliest known fishes in remarkable detail. As one of the oldest windows onto the Cambrian Explosion, it was listed as a UNESCO World Heritage Site in 2012.

Background: why soft-bodied fossils matter

Most fossils are hard parts, such as shells, bones and teeth. Soft-bodied animals usually decay before they can be preserved, so ordinary fossil sites show only part of an ancient community. Places where soft tissues survive are called Lagerstätten, a German word meaning roughly “storage places”.

Chengjiang is one of the most important of these sites. It preserves not only shells but also guts, gills, legs, eyes, muscles and sometimes nervous tissue. Because it dates from the early Cambrian, it shows animal life at a stage when many major groups had only recently appeared.

Discovery

On 1 July 1984, the Chinese palaeontologist Hou Xianguang, of the Nanjing Institute of Geology and Palaeontology, was searching for fossils of small bivalved arthropods on the hill of Maotianshan, near the town of Chengjiang, east of Kunming. Splitting the mudstone, he found the outline of a soft-bodied arthropod, which turned out to be Naraoia, a relative of the trilobites without a hardened shell.

Hou and his colleagues soon realized that the site was full of similar fossils. Through the late 1980s and 1990s, researchers from Nanjing, from Northwest University in Xi’an and from overseas, including the Swedish palaeontologist Jan Bergström, described a stream of new species. Fossil-bearing outcrops were found across a wide area of eastern Yunnan, including sites near Haikou and Anning.

The rocks and their age

The fossils come from the Maotianshan Shale, part of the Yu’anshan Formation, which was laid down in the early Cambrian, in what geologists call Cambrian Stage 3. Dating places it at about 518 million years ago, around 10 million years older than the Burgess Shale of Canada.

At that time, the area lay on a shallow sea at the edge of the South China continental block. The animals appear to have been buried rapidly by pulses of fine mud, perhaps stirred up by storms or flowing down gentle slopes, in water with little oxygen that slowed decay. Their soft tissues were preserved as thin films and later partly replaced by minerals, which weathering has turned into the reddish and yellow colours seen in many specimens.

What the fossils show

A rich community

More than 200 species have been described from Chengjiang, belonging to most major animal groups that existed then and to some that are hard to place. UNESCO’s evaluation noted that the fossils span at least 16 phyla as well as many enigmatic forms.

Arthropods are the most diverse group. They include trilobites such as Eoredlichia, the soft-shelled Naraoia, great-appendage arthropods such as Leanchoilia and the many small bivalved arthropods that are among the most common fossils. Large radiodonts, the group that includes Anomalocaris, were among the top predators.

Lobopodians, worm-like animals with stubby legs, are well represented. They include Microdictyon, Luolishania and a species of Hallucigenia, and they help scientists understand the ancestry of arthropods and velvet worms. Priapulid worms, sponges, brachiopods, comb jellies and many other groups also occur.

Early fishes and chordates

Some of the most celebrated finds are early chordates, the group that includes vertebrates. In 1999 Shu Degan and colleagues described Myllokunmingia and Haikouichthys in the journal Nature. These small, fish-like animals, only a few centimetres long, show gill pouches, fin folds and possibly a simple skull, and they are often counted among the earliest known vertebrates.

Other animals, such as Yunnanozoon and Haikouella, have been interpreted as early chordates or close relatives, though their exact position is debated. Together with Pikaia from the younger Burgess Shale, they show that the lineage leading to fish, and eventually to humans, was already present in Cambrian seas.

Brains and nervous systems

Chengjiang has also preserved traces of nervous tissue. In 2012 Ma Xiaoya, Hou Xianguang, Gregory Edgecombe and Nicholas Strausfeld reported a brain and optic lobes in the arthropod Fuxianhuia protensa. The structure resembled the brains of modern crustaceans and insects, suggesting that complex brains evolved early in arthropod history. The finding prompted debate about how such tissues could survive, and similar finds have since been reported in other Chengjiang arthropods.

Puzzling animals

Some Chengjiang animals are still hard to classify. The vetulicolians, for example, had a front section with openings that look like gill slits and a segmented tail, and they have been placed at different times among the chordates, among early deuterostomes (the wider group that includes chordates and echinoderms) and elsewhere. Debates like this are a normal part of studying animals so close to the origins of modern groups.

Chengjiang and the Burgess Shale compared

Chengjiang is often compared with the Burgess Shale, which was found 75 years earlier. The two sites share many kinds of animals, including Naraoia, Leanchoilia, Hallucigenia and radiodonts. This shows that similar communities lived in seas on opposite sides of the world and persisted for at least 10 million years.

There are also differences. Chengjiang is older and records a stage when some groups were still at an early point in their evolution. It has a larger number of early chordates and other deuterostomes, while the Burgess Shale has a wider range of some arthropod groups. The style of preservation differs too: Burgess Shale fossils are usually dark, shiny films on black shale, while Chengjiang fossils often appear as reddish or yellowish impressions in pale mudstone.

Taken together, the two sites, along with Sirius Passet in Greenland and the Emu Bay Shale in Australia, allow scientists to follow changes in Cambrian marine life over time and across different regions.

Protection and recognition

As the importance of the site became clear, the Chinese authorities established a protected area to control quarrying and fossil collecting. The Chengjiang Fossil Site was inscribed on the UNESCO World Heritage List in July 2012, covering a core area of about 512 hectares around Maotianshan and nearby hills. A museum near Chengjiang displays many of the finds.

Research continues. Large numbers of new specimens are collected by research teams, new species are still being named, and modern imaging methods such as CT scanning reveal hidden details of legs, guts and nervous systems inside the rock.

Timeline

• c. 538.8 million years ago: The Cambrian Period begins.
• c. 518 million years ago: The Chengjiang animals are buried in fine mud on a shallow sea floor.
• c. 508 million years ago: The younger Burgess Shale community is preserved in Canada.
• 1984, 1 July: Hou Xianguang finds the first soft-bodied fossil at Maotianshan.
• 1999: Myllokunmingia and Haikouichthys are described as early vertebrates.
• 2012: The brain of Fuxianhuia protensa is described in Nature.
• 2012, July: The Chengjiang Fossil Site is inscribed as a UNESCO World Heritage Site.

Why it matters

Chengjiang offers a detailed snapshot of animal life only about 20 million years after the start of the Cambrian. Its soft-bodied fossils show how quickly complex body plans, nervous systems and food webs arose, and its early fishes record some of the first steps in the history of vertebrates.