For more than a century, paleontologists have looked to the Cambrian Period as the moment when complex animal life erupted onto the scene — a roughly 20-million-year window beginning around 535 million years ago when most of the major animal body plans appeared in the fossil record with startling suddenness. The “Cambrian explosion,” as it is known, has long been one of biology’s great set pieces: a drama with a clear opening act and an astonishing cast of characters.
A new discovery from the hills of southwestern China suggests the opening act started much earlier than anyone realized.
Researchers from Yunnan University and Oxford University have described a remarkable fossil assemblage from the Jiangchuan Biota in Yunnan Province that pushes the origins of complex animal life back by at least four million years, deep into the late Ediacaran Period. Their findings, published April 2 in Science, describe more than 700 exceptionally preserved specimens — including what appear to be the oldest known deuterostomes, the animal lineage that would eventually give rise to starfish, acorn worms, and ultimately vertebrates like humans — preserved not as the ghostly sandstone impressions typical of Ediacaran sites, but as detailed carbonaceous films that reveal guts, feeding structures, and locomotory organs.
“Our discovery closes a major gap in the earliest phases of animal diversification,” said lead author Dr. Gaorong Li, now at Oxford’s Museum of Natural History. “For the first time, we demonstrate that many complex animals, normally only found in the Cambrian, were present in the Ediacaran period, meaning that they evolved much earlier than previously demonstrated by fossil evidence.”
A World Between Worlds
The Ediacaran Period, spanning roughly 635 to 539 million years ago, is named for the Ediacara Hills of South Australia, where strange, soft-bodied organisms were first described in the mid-twentieth century. These creatures — discs, fronds, and quilted blobs — bear little resemblance to anything alive today, and their relationship to modern animals has been debated ever since. Some appear to be early relatives of animals; others may represent entirely extinct lineages with no living descendants.
What has long puzzled scientists is the apparent chasm between this Ediacaran menagerie and the sophisticated fauna that appears in Cambrian rocks. Molecular clocks — genetic analyses that estimate when lineages diverged — have consistently suggested that many major animal groups split from common ancestors well before the Cambrian, yet physical fossil evidence for them in the Ediacaran has been frustratingly sparse. The new Jiangchuan discoveries begin to close that gap.
“This discovery is extremely exciting because it reveals a transitional community: the weird world of the Ediacaran giving way to the Cambrian, the following time period where the animals are much easier to place in groups that are alive today,” said Associate Professor Luke Parry, Oxford University.
A Treasure Trove of Transitional Forms
The Jiangchuan Biota had previously yielded diverse macroalgae and a handful of problematic Ediacaran-style fossils. The new study reports a trove of previously undiscovered soft-bodied animals from two fossil localities, preserved in subtidal mudstones and siltstones dated to between 554 and 539 million years ago.
Among the most scientifically significant specimens are four fossils showing anatomical features consistent with cambroernids — a group of stem-group ambulacrarians previously known almost exclusively from the Cambrian. One resembles Herpetogaster, a stalked, tentacled animal that represents among the earliest-diverging members of the group. Another shares features with Phlogites, a related cambroernid taxon. Additionally, tubular fossils with distinctive oval perforations along their length resemble Margaretia dorus, a Cambrian structure now interpreted as a dwelling tube built by enteropneust hemichordate worms — distant relatives of vertebrates.
The presence of these ambulacrarian relatives in an Ediacaran context has cascading implications. “The discovery of ambulacrarian fossils in the Jiangchuan biota also means that the chordates — animals with a backbone — must also have existed at this time,” said co-author Dr. Frankie Dunn of Oxford’s Museum of Natural History.
The assemblage also includes an array of bilaterian worms. The most abundant — represented by 185 specimens — is an elongate vermiform animal with a basal holdfast disc and an eversible feeding structure that could be extended to roughly twelve times the width of the body. One of the researchers compared it, perhaps inevitably, to the fictional sandworms of Dune. The authors argue it is likely a total-group bilaterian, based on the musculature required to evert and retract such a structure. Alongside these, a sausage-shaped worm with a preserved terminal mouth and visible gut track adds to a picture of diverse free-living animals inhabiting the Ediacaran seafloor.
Rounding out the picture are a putative ctenophore — a comb jelly — with preserved locomotory comb rows consistent with Cambrian forms, and several Haootia-like organisms interpreted as early cnidarians, the group that includes modern jellyfish and sea anemones. Traditional Ediacaran-type forms are present as well, but scarce.
The Preservation Problem
One reason such a community had not been found before may have less to do with evolution than with geology. “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,” said co-author Associate Professor Ross Anderson. “Carbonaceous compressions like those at Jiangchuan are rare in rocks of this age, meaning that similar communities may simply not have been preserved elsewhere.”
Most Ediacaran fossil sites preserve organisms as impressions on sandstone surfaces — the “Ediacara-style” preservation that captures form but little anatomical detail. The Jiangchuan Biota fossils, by contrast, are preserved as carbonaceous films consistent with Burgess Shale-type preservation, the same mode responsible for the exquisite anatomical fidelity of famous Cambrian sites in Canada and China. The authors suggest that rapid burial by sediment events, combined with a chemical environment conducive to carbon film preservation, created an exceptional window into a community that may have been widespread but left almost no trace elsewhere.
This insight reframes a long-standing interpretive problem. A modified version of what paleontologists call the “Cheshire Cat” hypothesis has proposed that Ediacaran macrofossils only appear to vanish before the Ediacaran-Cambrian boundary because the distinctive seafloor mat environments they occupied — so-called “matgrounds” — diminished as burrowing animals disrupted sediment layers. The Jiangchuan assemblage, dominated by bilaterians with rarer Ediacaran-type forms, suggests the transition may have been more biological than previously appreciated — but also that taphonomic bias has been concealing Cambrian-style communities hiding in plain Ediacaran sight.
Closing the Gap Between Rocks and Clocks
The study’s larger significance lies in resolving the persistent tension between genetic and fossil evidence for animal origins. Molecular phylogenetic studies have long implied that major animal lineages — including deuterostomes — diverged during the Ediacaran, yet physical fossils to support those estimates have been elusive. The Jiangchuan discoveries bring the two lines of evidence into closer alignment.
As Science editor Sacha Vignieri noted in the journal’s summary, the new site “fills a critical gap in our understanding about the transition between these two enigmatic faunas.”
For scientists who have spent careers piecing together life’s most consequential diversification event, the Jiangchuan Biota offers something rarer still: a snapshot of the world as it was becoming, when ancient forms and new ones coexisted on the seafloor, and the animal kingdom was just beginning to reveal what it would become.
Endnotes
Liu, A.G. et al. “Haootia quadriformis n. gen., n. sp., interpreted as a muscular cnidarian impression from the Late Ediacaran period.” Proceedings of the Royal Society B 281, 20141202 (2014).
Li, G. et al. “The dawn of the Phanerozoic: A transitional fauna from the late Ediacaran of Southwest China.” Science 392, 63–68 (2026). DOI: 10.1126/science.adu2291
EurekAlert! press release: “Fossil site in southwest China fills in critical gap before Cambrian period.” American Association for the Advancement of Science, April 2, 2026.
Oxford University Museum of Natural History press release: “Spectacular fossil treasure trove pushes back origins of complex animals.” April 3, 2026.
Nanglu, K. et al. “Cambrian origin of the phylum Entoprocta and the evolution of a key lophotrochozoan body plan.” BMC Biology 14, 56 (2016).
Caron, J.B., Conway Morris, S., Shu, D. “Tentaculated worms from the Cambrian of China and the evolution of lophotrochozoan body plan.” PLOS ONE 5, e9586 (2010).
Mángano, M.G., Buatois, L.A. “Decoupling of body-plan diversification and ecological structuring during the Ediacaran-Cambrian transition.” Proceedings of the Royal Society B 281, 20140038 (2014).
Wood, R. et al. “Integrated records of environmental change and evolution challenge the Cambrian Explosion.” Nature Ecology & Evolution 3, 528–538 (2019).
IMAGE CREDIT: Xiaodong Wang.
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