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Scientists solve 500-million-year fossil mystery

A peculiar spiny fossil, once thought to represent one of the earliest mollusks, has now been conclusively reclassified by scientists from Durham University and Yunnan University as something entirely different—a distant relative of sponge-like creatures known as chancelloriids.

This striking revelation is set to reshape our understanding of early animal evolution.

The fossil, named Shishania aculeata, hails from 500-million-year-old Cambrian deposits in Yunnan Province, southern China, a region world-renowned for its remarkably preserved snapshots of ancient life.

Initially interpreted as a primitive mollusk, potentially an ancestor of modern snails, slugs, and clams, Shishania was believed to display mollusk-like features, including a muscular foot and specialized mineralized spines.

However, a new international study led by Durham University and in Science paints a dramatically different picture.

The researchers said the new fossils show that Shishania doesn't fit with mollusks at all; instead, it closely resembles a group of ancient creatures called chancelloriids (bag-like animals covered in defensive spines, anchored to the Cambrian sea floor).

The researchers, using better-preserved specimens and advanced imaging techniques, discovered that many of the features previously thought to indicate molluscan affinity were in fact misleading artifacts of fossilization.

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."

Study co-author, Dr. Martin Smith of Durham University said, "These ancient fossils turned out to be masters of disguise. Shishania 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 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 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.

Dr. Smith admitted the fossils initially seemed to confirm his own long-standing theories about early mollusk ancestors: "When Shishania was first described last year, I was thrilled—it seemed to match the early 'slug-like' animals I'd always imagined. But our Chinese colleagues' new material forced me to re-evaluate everything."

Further analysis revealed patterns once thought to reflect molluscan 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 study underscores the crucial role of China's fossil-rich strata and exemplifies how international collaboration continues to transform our understanding of life's earliest chapters.