Initially interpreted as a primitive Cambrian mollusk, Shishania aculeata — a 500-million-year-old spine-covered fossil found in China’s Yunnan province — is in fact a distant relative of sponge-like creatures known as chancelloriids, according to a team of paleontologists from Yunnan University, Yuxi Normal University and Durham University.
Shishania aculeata. Image credit: Yang et al., doi: 10.1126/science.adv463.
Shishania aculeata was previously believed to display mollusk-like features including a muscular foot and specialized mineralized spines.
However, the new fossils show that the ancient animal closely resembles chancelloriids — bag-like animals covered in defensive spines, anchored to the Cambrian sea floor.
“We discovered that many of the features previously thought to indicate molluskan affinity were in fact misleading artifacts of fossilization,” said Durham University paleontologist Martin Smith and colleagues.
“For example, structures taken to be a ‘foot’ were revealed to be the result of distortion during the fossil’s preservation, a process described as a taphonomic illusion.”
“These ancient fossils turned out to be masters of disguise. Shishania aculeata seemed to show all the hallmarks we might expect of an early mollusk ancestor.”
“But as it dawned on us that the mollusk-like outlines of the fossil material represented a work of fossil origami, we were led to re-examine each other part of the interpretation in turn.”
“The mystery started to unfurl once we found chancelloriids preserved in a very similar way in the same rock unit.”
The reclassification of Shishania aculeata is particularly significant because chancelloriids are an enigmatic group known only from Cambrian rocks, disappearing around 490 million years ago.
Though superficially resembling sponges, their bodies are adorned with star-shaped spicules whose intricate microstructure hints at possible connections to more complex animals.
With its extremely simple spines, Shishania aculeata suggests that chancelloriids developed their ornate spicules from scratch, rather than adapting them from pre-existing skeletal structures.
That tells something profound about how complex body plans evolved during the Cambrian explosion — the evolutionary burst that gave rise to all modern animal groups.
“When Shishania aculeata was first described last year, I was thrilled — it seemed to match the early ‘slug-like’ animals I’d always imagined,” Dr. Smith said.
“But the new material forced me to re-evaluate everything.”
The further analysis revealed patterns once thought to reflect molluskan biology such as a ‘paintbrush-like’ arrangement in the spines were actually preservation artifacts, as the same patterning occurred randomly across the fossil.
Compression and deformation during fossilization had also made the simple cylindrical animals appear more anatomically complex than they truly were.
This reinterpretation has implications not only for understanding chancelloriids, but also for identifying other ambiguous Cambrian fossils.
It reopens questions about early mollusk evolution and cautions against over-interpreting ambiguous fossil features.
“At the same time, it helps solidify our picture of chancelloriid origins and gives us fresh insight into how evolutionary novelty emerged,” the researchers said.
Their paper was published today in the journal Science.
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Jie Yang et al. 2025. Shishania is a chancelloriid and not a Cambrian mollusk. Science 388 (6747): 662-664; doi: 10.1126/science.adv463
