Surprise fossil discoveries push back the evolution of complex animals

A remarkable collection of fossils found in China challenges the notion that complex life suddenly thrived during the Cambrian explosion, a rapid evolutionary period. This explosion, occurring between approximately 541 million and 513 million years ago, is believed to mark the emergence of most of today’s animal groups, along with various evolutionary experiments that eventually vanished.

Previously, it was thought that life in the preceding Ediacaran period was significantly less complex. However, this view is questioned by the discovery at the Jiangchuan biota site in Yunnan province, which contains over 700 fossils from 554 to 537 million years ago.

“The findings indicate that Cambrian-type animal communities did not emerge abruptly but had established foundations and transitional forms by the Ediacaran’s end,” states Gaorong Li of Yunnan University in Kunming, China, who led the research team.

Ross Anderson from the University of Oxford, another team member, notes that the fossils’ unexpected complexity suggests the Cambrian explosion might have been more gradual.

“We are uncovering a more intricate picture of the origins of animal diversity and when it occurred,” Anderson explains.

Initially, when Li began examining the site in mid-2022, he anticipated finding only algae.

Instead, the team discovered a variety of bilaterians—animals with bilateral symmetry—previously rare in Ediacaran fossils. Among these were two new deuterostome species, a group including vertebrates, hinting at their diversity during the Ediacaran period.

Among the fossils are cambroernids, a group with coiled bodies and filamentous tentacles, which were not thought to exist before the Cambrian. Other fossils resemble the Cambrian organism Margaretia, described as tube-like with holes, akin to an animal in a ventilation pipe, according to Li.

Li describes the most common fossil as an animal anchored to the seabed with a tubular appendage extending outward, reminiscent of the sandworm from the science-fiction series Dune.

“This indicates an animal that lived attached to the seafloor, extending this structure to feed,” Li explains. “Another type resembles a sausage-shaped worm with a short, thick, curved body, suggesting it could move.”

Li observes that these animals, though peculiar, may represent evolutionary trials from a time when life was experimenting with different body structures and ecological roles.

“They already possess essential features found in modern animals, like a mouth, gut, and proboscis or pharynx, yet their combination is unlike most animals today,” Li points out. “So, despite their unusual appearance, they have the basic body components seen in contemporary animals.”

Joe Moysiuk from Manitoba Museum in Winnipeg, Canada, says the sudden emergence of modern animal body plans in the early Cambrian fossil record has puzzled palaeontologists for centuries.

“There’s strong evidence suggesting their ancestors existed in the Ediacaran period, and clues of these ancestors have been accumulating for decades,” Moysiuk notes.

“The specimens’ preservation is somewhat coarse, so fine details are missing, but some forms distinctly resemble animals,” he adds.

These fossils indicate that certain animal groups existed before the Cambrian period, yet they do not dismiss the concept of the Cambrian explosion, Moysiuk explains.

“Instead, they provide better timing constraints on the likely start of this evolutionary radiation, with the divergence of animal body plans probably occurring over about 30 million years spanning the Ediacaran-Cambrian boundary,” he states.

Han Zeng from the Chinese Academy of Sciences, not involved in the research, believes finding complex animal fossils in deposits older than the Cambrian would be a significant palaeontological breakthrough.

“Over the decades, various carbonaceous fossils have been found in late Precambrian shales of similar age in South China,” Zeng says. “While most are identified as algae or cyanobacteria, some are ambiguous with possible animal connections. Future studies will be crucial to determine these fossils’ biological relationships. If confirmed as animals, these fossils could transform our understanding of early animal evolution.”

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