Surprising fossil discoveries trace back the evolution of complex animals

A large and well-preserved series of fossils discovered in China has cast doubt on the idea that complex life flourished during the rapid burst of evolution known as the Cambrian explosion.

This event, which took place about 541 million to 513 million years ago, is the time when most of the animal groups that live today are thought to have arisen, as well as a remarkable series of evolutionary experiments that ended later.

In the past, known as the Ediacaran, life was thought to be less complex. But that contradicts a new fossil site in Yunnan province, known as the Jiangchuan biota, which includes more than 700 fossils dating from 554 to 537 million years ago.

“The discovery shows that Cambrian animal communities did not appear suddenly, but already had clear foundations and transitional forms at the end of the Ediacaran,” says Gaorong Li of Yunnan University in Kunming, China, who led the team behind the discovery.

Ross Anderson of the University of Oxford, another member of the team, said the surprising complexity of the fossils raised questions about whether the Cambrian explosion was a slow burn.

“We’re really revealing a more complex picture of how the biodiversity explosion started and when that happened,” says Anderson.

When Li first looked at the area in mid-2022, all he expected to find was algae.

Instead, the researchers found several types of creatures known as bilaterians – animals with bilateral symmetry – of which only a few examples from the Ediacaran have been found before. They include two new species of deuterostomes – a large group that includes insects – indicating that this group had already diverged during the Ediacaran period.

Some of the fossils have been found as cambroernids, a group with rounded bodies and filamentous tentacles, which were not known to exist before the Cambrian. There are also fossils that resemble the Cambrian creature Margaret, which resembles a pipe with holes in the wall, “which makes it generally look like an animal living in a wind pipe”, says Li.

He says the most common fossil the team found was an animal that was wedged on the seabed on one side and had an expandable tubular structure on the other, reminding the team of a sandworm from a science fiction series. Dune.

Li says: “This suggests an animal that was always attached to the sea floor and stretched this structure to feed itself. The other worm is a sausage-shaped worm with a short, thick, curved body, which clearly shows that it can move.”

He says, these animals are strange and strange, and they can represent “evolutionary experiments” from the time when life was exploring different physical goals and adapting to the environment.

They already have the main features seen in modern animals, such as the mouth, stomach and throat, but the way these structures are put together is not the same as in most animals living today. “In other words, although their overall appearance is strange, they still have the basic physical features seen in modern animals.”

Joe Moysiuk at the Manitoba Museum in Winnipeg, Canada, says the sudden appearance of modern animal plans in the ancient Cambrian fossil record has been a “persistent surprise” to paleontologists for centuries.

Moysiuk says: “There is a good reason to think that their ancestral species should be found in the past, the Ediacaran, and the data of these ancestors has been increasing in the last few decades.

“The preservation of the specimens is relatively rough, so we are missing some good data, but there are some definite animal species.”

Although these fossils suggest that some animal groups existed before the Cambrian period, they do not rule out the idea of ​​a Cambrian explosion, he says.

“Instead, they give us a better time constraint on the beginning of this evolutionary radiation, and the divergence of animal body plans can occur 30 million years from the Ediacaran-Cambrian boundary.”

Han Zeng of the National Academy of Sciences of China, who was not part of the research, said the discovery of fossils of complex animals in deposits older than the Cambrian would be “a major breakthrough in paleontology”.

“Several decades ago, fossils of various types of carbon were found from recent Precambrian shales of the same age in South China,” says Zeng. Although most of the fossils have been identified as algae or cyanobacteria, some are still unclear as to how they are related to animals.

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