Jan Bartek – AncientPages.com – A recent study by scientists from Durham University and Yunnan University has led to a significant reclassification of a peculiar spiny fossil, initially believed to be one of the earliest molluscs.
Specimens of Shishania. A and B, specimen in different lighting conditions, showing how the uneven breakage of the specimen reveals the distinct upper and lower surfaces. E-G, specimen showing three-dimensionality of its spines and their arrangement around an apical orifice. H, two individual spines: on the left, preserved in top-down view, revealing the circular cross-section; on the right, preserved side-on. Credit: Zhang Xiguang.
This fossil, named Shishania aculeata, originates from 500-million-year-old Cambrian deposits in Yunnan Province, China. Previously thought to be an ancestor of modern snails, slugs, and clams due to its supposed mollusc-like features such as a muscular foot and mineralised spines, Shishania has now been identified as a distant relative of chancelloriids—sponge-like creatures with defensive spines.
This groundbreaking discovery is poised to alter our understanding of early animal evolution. The new classification emerged from an international study published in Science that utilized better-preserved specimens and advanced imaging techniques. These methods revealed that many features previously attributed to molluscan characteristics were actually misleading artefacts caused by the fossilisation process—a phenomenon described as ‘taphonomic illusion.’
Consequently, structures once interpreted as a ‘foot’ were found to result from distortion during preservation. This revelation underscores the importance of continuous research and technological advancements in paleontology for refining our knowledge of ancient life forms.
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 mollusc ancestor.
“But as it dawned on us that the mollusc-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 chancelloriids holds particular importance due to their enigmatic nature, as they are known solely from Cambrian rocks and vanished approximately 490 million years ago. Although they superficially resemble sponges, their bodies feature star-shaped spicules with intricate microstructures that suggest potential links to more complex animals.
The discovery of Shishania, with its extremely simple spines, indicates that chancelloriids developed their ornate spicules independently rather than adapting them from existing skeletal structures.
This insight provides a profound understanding of how complex body plans evolved during the Cambrian explosion—an evolutionary surge that led to the emergence of all modern animal groups.
Hollow triangular spines protrude from the margin of a Shishania specimen. Credit: Zhang Xiguang.
Dr Smith admitted the fossils initially seemed to confirm his own long-standing theories about early mollusc 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 has revealed that patterns previously believed to reflect molluscan biology, such as a ‘paintbrush-like’ arrangement in the spines, were actually preservation artefacts. This conclusion was drawn because the same patterning appeared randomly across the fossil. Additionally, compression and deformation during fossilization made these simple cylindrical animals seem more anatomically complex than they truly were.
This reinterpretation holds significant implications for understanding chancelloriids and identifying other ambiguous Cambrian fossils. It prompts a reevaluation of early mollusc evolution and advises caution against over-interpreting unclear fossil features.
Simultaneously, it strengthens our understanding of chancelloriid origins and provides new insights into how evolutionary novelty emerged. The study highlights the essential role of China’s fossil-rich strata and demonstrates how international collaboration continues to enhance our comprehension of life’s earliest chapters.
The study was published in the journal Science
Written by Jan Bartek – AncientPages.com Staff Writer
