Oceans before the Dawn of the Phanerozoic: The Jiangchuan Biota Reveals a Transitional Fauna between the Ediacaran and Cambrian

The fossil record has long preserved a puzzling gap. The Ediacaran Period contains an array of strange organisms unlike most animals alive today, whereas the Cambrian Period suddenly reveals many forms that seem much closer to the ancestral members of modern animal groups. How these two worlds were connected has remained difficult to resolve, largely because clear body-fossil evidence from this critical interval has been scarce. The Jiangchuan Biota of eastern Yunnan, China, opens a rare window into this obscure chapter of evolutionary history, allowing us to glimpse a marine world that was already undergoing profound change before the celebrated Cambrian explosion.

Reconstruction of the Jiangchuan Biota（Courtesy of Xiaodong Wang ）

The research team discovered more than 700 exceptionally preserved fossils from the middle to upper Jiucheng Member of the Dengying Formation, at the Qingshuigou and Shanglijiao sections. These fossils include macroalgae as well as a diverse range of animal remains. They come from subtidal mudstones and siltstones, and many occur at the bases of event beds, suggesting that organisms were rapidly buried and, in some cases, preserved in place. Most are preserved as carbonaceous films, with additional pyritization and phosphatization. This style of preservation is closer to Cambrian Burgess Shale–type preservation than to the more familiar Ediacara-style preservation. For that reason, the Jiangchuan Biota is not merely another late Ediacaran fossil assemblage. It appears to capture a seafloor community with distinctly Cambrian-like qualities before the Cambrian had even begun.

Typical late Ediacaran animals are relatively uncommon in this assemblage. The researchers identified only a small number of radialomorph fossils, along with specimens resembling Haootia quadriformis. Another fossil was interpreted as a putative ctenophore. It is about 1.8 centimeters long and 1 centimeter wide, and it preserves possible ctenes, the comb-like locomotory structures characteristic of ctenophores. Two other elongate cylindrical fossils resemble Cambrian mackenziids. Their longitudinal stripes and circular apical apertures argue against a simple algal interpretation.

Haootia-like fossil（Courtesy of Gaorong Li ）

Reconstruction of Haootia quadriformis（Image source：McIlroy D et al. (2024)， CC BY 4.0 ）

Putative ctenophore fossil（Courtesy of Gaorong Li ）

The real significance of the Jiangchuan Biota lies in its variety of bilaterian body fossils. The most abundant form is an elongate vermiform animal represented by 185 specimens. The most complete individual is about 6.1 centimeters long and 0.2 to 0.27 centimeters in diameter, with a posterior holdfast disc about 1.5 centimeters across. At the anterior end, it bears an eversible feeding structure. In some specimens, this structure is contracted into a blunt or trumpet-shaped oral region; in others, it is extended into a narrow, tongue-like projection. The longest preserved example reaches about 4 centimeters, roughly 12 times the width of the body. The ability to evert and retract such an anterior feeding organ may indicate the presence of mesodermal musculature, a key feature associated with bilaterian animals. Although the researchers do not assign these fossils to any living bilaterian clade, they consider them likely members of total-group Bilateria.

An elongate vermiform animal（Courtesy of Gaorong Li ）

A second vermiform animal has a sausage-shaped body and preserves a circular terminal opening, interpreted as a mouth. A dark, slender band running along the length of the body most likely represents a gut. The specimen is strongly curved, which may reflect contraction after death or deformation during burial.

Sausage-shaped vermiform fossil（Courtesy of Gaorong Li ）

The Jiangchuan Biota also preserves evidence for early deuterostomes. Four specimens show features compatible with cambroernids, a group otherwise known mainly from the Cambrian. Some resemble Herpetogaster, with a coiled body, a slight invagination or constriction at one end, and simple, unbranched to minimally branched tentacles around the head region. Another specimen has a shorter, thicker stalk comparable to that of Phlogites. Most of these fossils are no more than 2 centimeters long, smaller than many Cambrian Herpetogaster specimens, although similar in size to examples from the Balang Formation of South China. If this interpretation is correct, it would mean that stem-group ambulacrarians were already present by the terminal Ediacaran, and that the deuterostome body plan had been established earlier than the previous unambiguous fossil record suggested.

Cambroernid-like fossil（Courtesy of Gaorong Li ）

Reconstruction of Herpetogaster collinsi（Image source：Marianne Collins， CC BY 2.5 ）

With a stalk region similar to that of Phlogites（Courtesy of Gaorong Li ）

Reconstruction of Phlogites longus (left)（Image source：Apokryltaros， CC BY-SA 4.0 ）

The researchers also found elongate tubular fossils with regular oval-shaped perforations arranged along their length. These fossils resemble the Cambrian Margaretia dorus, which was once interpreted as an alga but has more recently been regarded as a dwelling tube for an enteropneust hemichordate worm. The Jiangchuan specimens are about 2.5 to 3 centimeters long and 0.2 to 0.4 centimeters wide. Their perforations are 0.05 to 0.45 centimeters long and 0.03 to 0.07 centimeters wide, with proportions comparable to related Cambrian material from North America.

Fossil similar to the Cambrian Margaretia dorus（Courtesy of Gaorong Li ）

The study also emphasizes the difference between the preservation of macroalgae and animal fossils. Macroalgal fossils from Jiangchuan usually have thicker carbonaceous films with sharp boundaries, and the films often crack into polygonal fragments as a result of shrinkage. Animal fossils generally show weaker carbonaceous preservation, yet they retain structures such as tentacles, eversible feeding organs, guts, and flexible soft bodies capable of deformation. These features are difficult to explain as algae and support their interpretation as animals.

The Jiangchuan Biota therefore changes how we understand the boundary between the Ediacaran and Cambrian worlds. Cambrian-type animal communities may have begun taking shape in certain late Ediacaran environments, but were rarely preserved in a way that allowed us to see them clearly. This also fits a modified version of the Cheshire Cat hypothesis, which suggests that some Ediacaran macroscopic organisms may appear to disappear before the Cambrian partly because the unusual seafloor environments they inhabited were diminishing. The carbonaceous compression fossils of Jiangchuan reveal another side of that hidden world: an animal community already moving toward the ecological and evolutionary patterns that would soon define the Phanerozoic.

Author: Shui-Ye You

Reference:

Li G et al. (2026). The dawn of the Phanerozoic: A transitional fauna from the late Ediacaran of Southwest China. Science.