Paleontologists Unravel the Mystery of Cambrian Fossils Veiled by Spines

Initially thought to be a primitive Cambrian mollusk, Shishania Ashreata—a 500 million-year-old spine-covered fossil from Yunnan, China—has now been identified as a distant relative of sponge-like creatures known as chancelloriids, according to a research team from Yunnan University, Yuki Normal University, and Durham University.

Shishania Ashreata was previously thought to display characteristics similar to mollusks, including muscular legs and unique mineralized spines.

However, new fossils suggest that these ancient animals are more akin to bag-like creatures, akin to prime minister-like organisms, with spines anchored to the seabeds of Cumbria.

“Our findings reveal that many previously considered mollusk traits are misleading, artifacts of fossilization,” stated Martin Smith, a paleontologist at Durham University, and his colleagues.

“For instance, what were once thought to be ‘feet’ have turned out to be distortions from fossil preservation, a phenomenon known as taphonomy.”

“These ancient fossils have proven to be masters of disguise. Shishania Ashreata appeared to exhibit all the characteristics we expected from early mollusc ancestors.”

“Nevertheless, we discovered that the mollusk-like contours of the fossil material represent misinterpretations, prompting us to revisit our interpretations.”

“My discovery of chancelloriids in very similar conditions started to unravel the mystery.”

Reclassifying Shishania Ashreata places it among chancelloriids, an enigmatic group known solely from Cambrian rocks, significant for having vanished around 490 million years ago.

Though they may look like sponges at first glance, their bodies feature star-shaped thorns, indicating a complex microstructure that could connect them to more advanced animals.

The simple thorns of Shishania Ashreata suggest that they developed elaborate structures independently, rather than evolving from existing skeletal frameworks.

This sheds light on the evolution of complex body plans during the Cambrian explosion—a rapid evolutionary event that led to the emergence of all contemporary animal groups.

“When Shishania Ashreata was first reported last year, I was thrilled—it resembled the ‘slug-like’ creatures I had always imagined,” Dr. Smith remarked.

“However, the new findings required me to reassess everything.”

Further examinations indicated that certain patterns appeared randomly throughout the fossil, and attributes once believed to reflect mollusk biology, such as ‘paintbrush-like’ formations in the spines, were actually preserved artifacts.

The distortion and compression from fossilization made simple cylindrical animals appear more anatomically complex than they were.

This reinterpretation is crucial not only for understanding primeval organisms but also for clarifying other ambiguous Cambrian fossils.

We are revisiting questions about early mollusk evolution, being cautious of overinterpretation regarding ambiguous fossil characteristics.

“Simultaneously, it contributes to solidifying our understanding of chancelloriid origins and offers fresh insights into the emergence of evolutionary novelty,” the researchers concluded.

Their study was published today in the journal Science.

____

Ziyan et al. 2025. Shishania is not a Cambrian mollusk; it is a prime minister. Science 388 (6747): 662-664; doi: 10.1126/science.adv463