'Completely different' – Discovery of bizarre 514-million-year-old snail changes our understanding of marine life

Newly discovered fossil Chechenia aculetaan early mollusk without a shell but covered in protective spines, offers new insights into the evolution of mollusks, pointing to a primitive snail-like form during the Cambrian.

Researchers have discovered something new. Class Of the mollusks that lived 500 million years ago. The new fossil, called Chechenia aculeta, It reveals that the most primitive molluscs were flat, shellless snails covered in a protective, spiny shell. The findings were recently published in the journal sciences.

Uncovering the life of prehistoric mollusks

Chechenia aculeta Exceptionally well-preserved fossils have been found from eastern Yunnan Province in southern China that date to a geological period called the Early Cambrian, about 514 million years ago. Chechen They are all only a few centimetres long and are covered with small, spiny cones (sclerotia) made of chitin, a substance also found in the shells of modern crabs, insects and some types of fungus.

Specimens preserved upside down show that the lower part of the animal was naked, with a muscular foot resembling that of a snail. Chechen This type of mollusk could have been crawling around the sea floor for more than half a billion years. Unlike most mollusks, Chechen It did not have a shell covering its body, indicating that it represented a very early stage in the evolution of mollusks.

Conical spines covering the body of Chechenia aculeta (left). Electron microscope image of a conical spine showing the microscopic channels preserved inside (right). Copyright: Jie Zhang/El Bari

Modern molluscs come in a dizzying array of forms, including snails, clams, and even highly intelligent groups like squid and octopuses. This diversity of molluscs evolved very rapidly long ago, during an event known as the Cambrian explosion when all major groups of animals were diversifying rapidly. This rapid period of evolutionary change means that only a handful of fossils have survived to document the early evolution of molluscs.

“Trying to unravel what the common ancestor of animals as diverse as squid and clams looked like is a major challenge for evolutionary biologists and paleontologists – one that cannot be solved by studying only living species today. Chechen “This gives us a unique look at a time in mollusc evolution where we have very few fossils, telling us that the earliest ancestors of molluscs were armoured spiny molluscs, before the evolution of the shells we see in modern snails and clams,” said corresponding author Associate Professor Luke Barry, Department of Earth Sciences, University of Oxford.

Artist's reconstruction of the Chechenia aculeata plant as it would have appeared in life when viewed from above, side and below (left to right). Image credit: M. Cawthorne

Detailed analysis of the structure of Chechnya

Because the body Chechen The bones were very soft and made of tissues not usually preserved in the fossil record, and the specimens were difficult to study, with many specimens poorly preserved.

“At first, I thought the fossils, which were only about the size of my thumb, were unremarkable, but under a magnifying glass I saw that they looked strange, spiky and completely different from any other fossils I had seen,” said first author Guangxu Zhang, a recent PhD graduate from Yunnan University in China who discovered the specimens. “I initially called them ‘plastic bag’ because they looked like a small, moldy plastic bag. When I found more of these fossils and analyzed them in the lab, I realized they were mollusks.”

“We found microscopic details inside the conical spines that cover the body,” Barry added. Chechen “This information shows how it was produced in life. This kind of information is extremely rare, even in exceptionally preserved fossils.”

vertebral column Chechen These images show an internal system of canals less than a hundredth of a millimeter in diameter. These features suggest that the cones were secreted at their base by microvilli, tiny projections of cells that increase surface area, much like in our intestines, helping to absorb food.

Evolutionary insights from Chechen fossils

This method of secreting hard parts is similar to nature's 3D printer, allowing many invertebrates to secrete hard parts with wide variations in shape and function from providing defense to facilitating movement.

Hard spines and bristles are known to exist in some extant molluscs (such as chitons), but they are made of the mineral calcium carbonate rather than organic chitin as in Chechen. Similar chitinous bristles are found in lesser-known groups of animals such as brachiopods and annelids, which together with mollusks and annelids (earthworms and their relatives) form the group Lavotrochozoa.

Barry added:Chechen He tells us that the spines and thorns we see in chitons and aplakophores today actually evolved from hard, organic structures like those found in annelids. These animals are very different from each other today, so fossils like Chechen Tell us what they looked like in the distant past, shortly after they diverged from their common ancestors.

Co-author Jacob Winter in University of Bristol “Molluscillus species today are extraordinarily diverse and diversified very rapidly during the Cambrian explosion, which means we struggle to piece together their early evolutionary history,” he said. “We know that the common ancestor of all living molluscs today had a single shell, so Chechen It tells us about a very early time in the evolution of mollusks before the development of the shell.

“This new discovery highlights the treasure trove of early animal fossils preserved in Cambrian rocks in Yunnan Province,” said co-author Xiaoya Ma (from Yunnan University and the University of Exeter). “Soft-bodied molluscs have a very limited fossil record, so these extremely rare finds tell us a lot about these diverse animals.”

Reference: “Cambrian stem-spiny mollusks and deep similarity to hard mosses” by Guangxu Zhang, Lucky A. Barry, Jacob Vinther, and Xiaoya Ma, August 1, 2024, sciences.
DOI: 10.1126/science.ado0059