A newly discovered trove of fossils in southwestern China is shifting the timeline of when complex animals evolved.

The diversity and complexity of animal life is thought to have increased rapidly beginning around 539 million years ago, in an evolutionary burst known as the Cambrian explosion. But the new fossil site suggests that some of that complexity was already present several million years before the Cambrian explosion, during the end of the Ediacaran period (roughly 635 million to 539 million years ago).

"One specimen looks a lot like the sand worm from Dune," study co-author Frankie Dunn, a researcher who studies Ediacaran organisms at the Oxford University Museum of Natural History, said in a statement.

Some simple multicellular creatures, such as sponges, first appeared during the Ediacaran period. But most modern animal phyla showed up during the subsequent 13 million- to 25 million-year-long Cambrian explosion, including chordates, the phylum that includes humans and other vertebrates.

The new fossil discovery suggests that some of that complexity had already arisen by the late Ediacaran. Uncovered as part of the Jiangchuan Biota collection of fossils in southwestern China, the collection contains more than 700 specimens of fossilized animals and algae dating to between 554 million and 539 million years ago. Researchers reported the findings Thursday (April 2) in the journal Science.

"When we first saw these specimens, it was clear that this was something totally unique and unexpected," study co-author Luke Parry, a paleobiologist at the University of Oxford, said in the statement.

The fossils from this site are mostly flat imprints of the organism on the surrounding rock, known as carbonaceous films. Unlike the three-dimensional imprints left by durable body parts, such as bones and shells, carbonaceous films capture some details of the organism's soft tissues, such as its gut and mouthparts.

This less-common method of preservation might help to explain why scientists haven't found evidence of these more complex animals in the Cambrian until now.

"Our results indicate that the apparent absence of these complex animal groups from other Ediacaran sites may reflect differences in preservation rather than true biological absence," study co-author Ross Anderson, a researcher who studies the evolution of complex life at the Oxford University Museum of Natural History, said in the statement. "Carbonaceous compressions like those at Jiangchuan are rare in rocks of this age, meaning that similar communities may simply not have been preserved elsewhere."

Skyler Ware is a freelance science journalist covering chemistry, biology, paleontology and Earth science. She was a 2023 AAAS Mass Media Science and Engineering Fellow at Science News. Her work has also appeared in Science News Explores, ZME Science and Chembites, among others. Skyler has a Ph.D. in chemistry from Caltech.