Researchers from the University of Oxford have unearthed a fossil that reshapes our understanding of the early evolution of mollusks.

This remarkable find, a new species named Shishania aculeata, dates back 500 million years and provides insights into the origins of one of the most diverse and widespread groups of animals on the planet.

The study, published in the journal Science, reveals that the earliest mollusks were slug-like creatures, lacking shells, and covered in a protective armor of spines.

Discovery and Significance

The fossilized remains of Shishania aculeata were found in the exceptionally well-preserved Cambrian deposits of eastern Yunnan Province in southern China. These fossils date to approximately 514 million years ago, during the Early Cambrian period, a critical time in the history of life known as the Cambrian Explosion. During this period, most major animal groups rapidly diversified, laying the foundation for modern biodiversity.

Thorns that cover the body of Shishania aculeata
Thorns that cover the body of Shishania aculeata. Credit: G. Zhang / L. Parry

The Shishania fossils are small, only a few centimeters in length, and are covered in tiny spiny cones, known as sclerites, made of chitin. Chitin is a versatile organic material found in the shells of crabs, insects, and even some fungi.

Unlike modern mollusks, Shishania lacked a protective shell, suggesting it represents an early stage in mollusk evolution before the development of the hard shells seen in today’s snails and clams.

The new fossil sheds light on the evolutionary history of mollusks, a group that includes snails, clams, squids, and octopuses. This discovery suggests that mollusks started as flat, shell-less slugs armored with chitinous spines, which provided protection and possibly aided in locomotion.

Insights from the Fossil Record

The preservation of the Shishania fossils is exceptional, allowing scientists to study their intricate details. The lower part of the fossilized creature reveals a muscular foot similar to that of a modern slug, which it likely used to move along the ancient sea floor.

The lack of a shell in Shishania implies that this creature represents an early evolutionary branch of mollusks before they developed the diverse body forms seen today.

Associate Professor Luke Parry, the corresponding author from Oxford’s Department of Earth Sciences, highlighted the importance of this discovery. He stated, Unraveling how the common ancestor of diverse animals like squids and oysters looked is a significant challenge for evolutionary biologists and paleontologists. Shishania provides us with a unique window into a time in mollusk evolution where we have very few fossils, indicating that the earliest ancestors were spiny and armored slugs before the evolution of the shells we see in modern mollusks.

Challenges in Studying Shishania

Studying Shishania proved challenging due to its soft body, which is rarely preserved in the fossil record. Guangxu Zhang, the lead author and recent Ph.D. graduate from Yunnan University, initially found the fossils unimpressive.

Artistic reconstruction of Shishania aculeata with the appearance that would have been seen from above, side and below
Artistic reconstruction of Shishania aculeata with the appearance that would have been seen from above, side and below. Credit: M. Cawthorne

However, closer examination revealed their unique, spiny appearance, distinguishing them from any other known fossils. Zhang recalled, At first, I thought these thumb-sized fossils were unremarkable, but under magnification, they appeared strange, spiny, and completely different from any other fossils I had seen. I initially nicknamed it ‘the plastic bag’ because it resembled a small, decayed plastic bag.

Detailed analysis of the spines covering Shishania revealed a complex internal structure. The spines contain a network of microscopic channels, less than a hundredth of a millimeter in diameter, indicating that they were secreted by microvilli.

These small cellular extensions increase surface area, similar to those in our intestines that aid in nutrient absorption. This secretion method is akin to a natural 3D printer, allowing invertebrates to create hardened parts with various shapes and functions, from defense to movement facilitation.

Evolutionary Connections

While some modern mollusks, like chitons, possess hard spines and bristles made of calcium carbonate, Shishania‘s spines were composed of organic chitin.

Similar chitinous bristles are found in other animal groups like brachiopods and bryozoans, which, along with mollusks and annelids, form the Lophotrochozoa group.

Professor Parry explained, Shishania informs us that the spines and spicules seen in modern chitons and aplacophoran mollusks evolved from organic sclerites akin to those of annelids. These animals appear vastly different today, and fossils like Shishania illuminate their past appearances shortly after they diverged from common ancestors.

SOURCES

University of Oxford

Guangxu Zhang et al., A Cambrian spiny stem mollusk and the deep homology of lophotrochozoan scleritomes. Science385, 528-532(2024). DOI:10.1126/science.ado0059

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