Surprising fossil discovery sets back the evolution of complex animals

A huge collection of beautifully preserved fossils discovered in China casts doubt on the idea that complex life flourished dramatically during a rapid evolutionary process known as the Cambrian Explosion.

This event, which spanned approximately 541 to 513 million years ago, is thought to be the first appearance of most of the animal groups alive today, as well as a series of bizarre evolutionary experiments that have since become extinct.

In the previous era, known as the Ediacaran period, life was thought to be less complex. However, this idea is contradicted by a new fossil site in Yunnan known as the Jiangchuan biota, which contains more than 700 fossils dating from 554 to 537 million years ago.

“This discovery shows that Cambrian-type faunal assemblages did not suddenly appear, but already had a clear foundation and a transitional morphology by the end of the Ediacaran,” said Gaorong Li of Yunnan University in Kunming, China, who led the team behind the discovery.

Another member of the team, Ross Anderson from the University of Oxford, said the fossil’s surprising complexity raises the question whether the Cambrian explosion was more of a slow burn.

“We’re beginning to see a more complex picture of where the explosion in animal diversity began and when it happened,” Anderson said.

When Lee first started investigating the site in mid-2022, all he expected was algae.

Instead, researchers discovered a group of organisms known as bilaterians, which are animals with bilateral symmetry. Only a few examples have been found so far in the Ediacaran period. These include two new species of deuterostomes, a major group that includes vertebrates, indicating that this group was already diverse in Ediacaran times.

Some of the fossils have been identified as Cambroelniids, a group with coiled bodies and filiform tentacles not known to exist before the Cambrian. There are also fossils that resemble creatures from the Cambrian period. Margaretia, It resembles a tube with a hole in the wall, and “the whole thing looks like an animal living inside a ventilation pipe,” Lee says.

He said the most common fossils the team found were animals with tubular appendages that were anchored to the ocean floor at one end and extended outward at the other end, which reminded the team of the sandworms from the science fiction series. dunes.

“This suggests that the animals were living attached to the ocean floor and were extending these structures to find food,” Lee said. “Another form is a sausage-shaped worm with a short, thick, curved body, clearly demonstrating locomotion.”

These animals are both strange and strangely familiar, he says, and may represent an “evolutionary experiment” at a time when life was exploring different body designs and ecological adaptations.

“Although it already has important features found in modern animals, such as a mouth, intestine, proboscis and pharynx, the way these structures fit together is different from that of most animals alive today,” Lee said. “In other words, their overall appearance is strange, but they still have the basic body modules found in modern animals.”

Joe Moisiuk of the Manitoba Museum in Winnipeg, Canada, says the sudden appearance of most modern animal body plans in the early Cambrian fossil record has been a “persistent conundrum” for paleontologists for centuries.

“There is good reason to think that their ancestral forms should have been discovered in an earlier era, the Ediacaran, and hints of these ancestry have been accumulating over the past few decades,” Moisiuk says.

“The specimens are a little poorly preserved, so some details are missing, but there are some clearly animal-like forms inside.”

These fossils suggest that certain animal groups existed before the Cambrian, but they don’t invalidate the idea of ​​a Cambrian explosion, he says.

“Rather, the divergence of animal body plans likely occurred over just 30 million years spanning the Ediacaran-Cambrian boundary, giving us a better temporal constraint on the likely onset of this evolutionary radiation.”

Han Zeng of the Chinese Academy of Sciences, who was not involved in the study, said the discovery of complex animal remains in sediments older than the Cambrian period would be a “significant advance in paleontology.”

“Over the past few decades, a wide variety of carbonaceous fossils have been discovered in similarly dated Late Precambrian shales in southern China,” Zeng said. “Although most fossils have been identified as algae or cyanobacteria, there are others whose possible animal affinities remain ambiguous. Future studies are essential to elucidate the biological affinities of these fossils. If these fossils prove to be animals, they could reshape our understanding of early animal evolution.”

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