Huge fossil bonanza preserves 512-million-year-old ecosystem

An extraordinary 512 million-year-old fossil site has been discovered in southern China, preserving in great detail almost an entire ecosystem dating from a time shortly after Earth’s first mass extinction.

The fossils date from the Cambrian period, which began 541 million years ago. The early Cambrian saw an explosion of diversity in animal life that gave rise to most of the major groups alive today.

But that boom stopped with the Sinsk Event, about 513.5 million years ago, when oxygen levels in the ocean fell, killing several groups of animals.

Han Zeng of the Nanjing Institute of Geology and Paleontology in China and colleagues began discovering fossils in a quarry in the mountainous region of Huayuan County, Hunan Province, in 2021.

So far, they have analyzed 8,681 fossils from 153 species, almost 60% of which are new to science. The team named this ancient ecosystem the Huayuan biota and say the site is comparable and perhaps superior to the most famous Cambrian fossil site, the Burgess Shale in Canada.

The assemblage consists of 16 large groups of animals that are thought to have lived in the deep ocean and appear to have been less affected by the Sinsk event.

“Our previous knowledge about the Sinsk extinction came only from the fossil record of skeletal animals such as archaeocyathid sponge reefs, trilobites and small shell fossils,” says Zeng.

The Huayuan biota also includes many different species of soft-bodied animals. “We found that the extinction mainly destroyed the shallow-water environment, and the deep-water environment at the edge of the continental shelf, where the Huayuan biota is found, was less affected,” says Zeng.

Most of the fossils discovered are arthropods, related to current insects, spiders and crustaceans. The fossils also include molluscs, shelled creatures called brachiopods, and cnidarians – relatives of jellyfish.

An 80 centimeter long arthropod named Guanshancaris kunmingensis is the largest animal recovered from the quarry and would have been the top predator in the Huayuan ecosystem.

Another arthropod, Helceia, is one of two genera that were previously found only in the Burgess Shale in Canada, but have now been found in Huayuan, which was then, as now, “on the other side of the world,” says Zeng. “This indicates that early animals were capable of spreading over a very long distance, which was most likely due to the transport of animal larvae in ocean currents,” he says.

Zeng explains that the reason for the exquisite preservation found at the site is that the animals were buried very quickly under fine mud. The soft parts of animals are preserved in extraordinary detail, including walking legs, antennae and tentacles, respiratory organs such as the gills, pharynx and intestines of many animals and even eyes and neural tissues.

Joe Moysiuk of the Manitoba Museum in Canada says the diversity of species and quality of preservation “place Huayuan in the forefront of Cambrian fossil sites.”

We know that the Sinsk event in the mid-Cambrian led to major declines in some groups of sponges, trilobites and others, he says, but we have very little information about its impact on most animal groups.

“Discoveries such as the Huayuan biota give us critical snapshots of this soft-bodied biodiversity in the Cambrian, filling in the missing frames in the proverbial tape of Earth history,” says Moysiuk.

Tetsuto Miyashita of the Canadian Museum of Nature in Ottawa says the two most famous Cambrian fossil sites to date are the 520-million-year-old Chengjiang Biota in China and the 508-million-year-old Burgess Shale in Canada.

“But it’s like comparing the entire court of Bach and the Beatles: we have to understand where the differences come from before we know what story they are telling us as a whole,” says Miyashita. “A new biota like this is important because it helps paleontologists distinguish the effects of geography, mass extinction, ocean depths and chemistry.”

A large group is noticeably absent from Huayuan. “Where are the fish?” Miyashita said. “Have they suffered a global squeeze and are very rare, or is there another ecological reason why we don’t find fish chasing as many species of soft-bodied animals?”

Zeng says his team hasn’t yet sifted through all the fossils collected. “New species will appear. There may be fish there, and we’ll wait and see,” he says.