The first mollusks were slugs with spines

Aug. 2 () –

A new species of marine creature that lived 500 million years ago reveals that the most primitive mollusks were flat, shell-less slugs, covered by a protective armor of spines.

The new species, named Shishania aculeata, was found in exceptionally well-preserved fossils from China’s eastern Yunnan province that date back to a geological period called the Early Cambrian, about 514 million years ago.

Described in the journal Science, the Shishania specimens They are only a few centimeters long and are covered with small pointed cones. (sclerites) made of chitin, a material also found in the shells of crabs, insects, and some modern fungi.

The specimens preserved upside down show that the animal’s underside was bare, with a muscular, slug-like foot that Shishania would have used to crawl along the seafloor more than 500 million years ago. Unlike most mollusks, Shishania did not have a shell covering its body, suggesting that it represents a very early stage in the evolution of mollusks.

Modern mollusks come in a dizzying variety of shapes, including snails and clams and even highly intelligent groups such as squid and octopus.

This diversity of mollusks evolved very rapidly long ago, during an event known as the Cambrian Explosion, when all major groups of animals were rapidly diversifying. This rapid period of evolutionary change This means that few fossils remain that account for the early evolution of mollusks.

Corresponding author Associate Professor Luke Parry, from the Department of Earth Sciences at the University of Oxford, said it’s a statement“Trying to unravel what the common ancestor of animals as different as a squid and an oyster was like is a major challenge for evolutionary biologists and paleontologists, one that can’t be solved by studying only the species alive today.

“Shishania gives us a unique insight into a time in mollusc evolution for which we have very few fossils, and tells us that the earliest ancestors of molluscs They were armored spiny slugsprior to the evolution of the shells we see in modern snails and clams.”

Because Shishania’s body was very soft and made of tissues not normally preserved in the fossil record, the specimens were difficult to study, as many of them were poorly preserved.

LIKE A PLASTIC BAG

First author Guangxu Zhang, a recent PhD graduate from Yunnan University in China who discovered the specimens, said: “At first I thought the fossils were the size of my thumb and unnoticeable, but I saw with a magnifying glass that they looked strange, spiny and completely different from any other fossils I had ever seen.

“At first I called him ‘The plastic bag’ “because it looks like a small rotten plastic bag. When I found more fossils of this type and analyzed them in the laboratory, I realized that it was a mollusk.”

Associate Professor Parry added: “We found microscopic details inside the conical spines covering Shishania’s body that show how they were secreted in life. This kind of information is incredibly rare, even in exceptionally preserved fossils.”

Shishania spines display an internal system of canals that are less than one-hundredth of a millimeter in diameter. These features show that the cones were secreted at their base by microvilli, tiny protrusions of cells that increase the surface area, as in our intestineswhere they help in the absorption of food.

This method of segregating hard parts is similar to a natural 3D printer, allowing many invertebrate animals to secrete hard parts with a huge variety of shapes and functions, from providing defense to facilitating locomotion.

Hard spines and bristles are known in some living mollusks (such as chitons), but They are made of the mineral calcium carbonate instead of organic chitin as in Shishania. Similar organic chitinous bristles are found in lesser-known animal groups such as the brachiopods and bryozoans, which together with the molluscs and annelids (earthworms and their relatives) form the group Lophotrochozoa.

Professor Parry added: “Shishania tells us that the spines and spicules we see on modern chitons and aplacophoran molluscs actually evolved from organic sclerites like those in annelids. These animals are very different from each other today and so fossils like Shishania tell us what they were like in the deep past, shortly after they separated from their common ancestors.”

Co-author Jakob Vinther from the University of Bristol said: “Molluscs alive today are extraordinarily disparate and diversified very rapidly during the Cambrian Explosion, meaning we struggle to reconstruct their early evolutionary history. We know that the common ancestor of all molluscs alive today would have had a single shell, so Shishania tells us about a very early moment in the evolution of mollusks before the evolution of a shell.”