A ‘rotting plastic bag’ was actually a 514 million year old fossil

A newly discovered extinct species of mollusk that roamed the ocean floor half a billion years ago offers new insights into the early days of this diverse group of animals. Fossils out Shishania aculeata indicate that some early molluscs were flat, armored, slug-like creatures that lacked the distinctive shells seen in today’s snails, and bivalves. This species was also covered in hollow cone-shaped spines called sclerites. The findings are detailed in a study published Aug. 1 in the journal Science.

Shishania was discovered thanks to some well-preserved fossils discovered in Yunnan Province in southern China. The newly named species dates to the early Cambrian, about 514 million years ago. The copies of Shishania which the team studied is a few centimeters long and from which the spiny cones are made chitin. This crunchy material is also found in the shells of modern insects, crabs and even some mushrooms.

The fossils preserved upside down indicate that it probably had a muscular foot similar to a slug. Shishania would have used that leg to crawl across the seabed. Unlike most molluscs, it lacked a shell covering its body.

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Living molluscs come in a wide variety of forms: snails, mussels and highly intelligent cephalopods such as squids and octopuses. All this biodiversity developed very quickly during the Cambrian explosion. This event about 530 million years ago was when all major groups of animals diversified rapidly. However, this accelerated pace of change has left few fossils left to tell the story of molluscs’ early evolution. The team believes that Shishania represents a very early stage in mollusk evolution.

“Trying to unravel what the common ancestor of animals as different as an octopus and an oyster looked like is a major challenge for evolutionary biologists and paleontologists – one that cannot be solved by studying only the species that survive today. life,” says co-author and university. from Oxford in England paleontologist Luke Parry said in a statement. “Shishania gives us a unique look at a time in mollusc evolution for which we have very few fossils, and informs us that the very first ancestors of mollusks were armored spiny slugs, before the evolution of the shells we see in modern snails and mussels.

Shishanias body was made of soft tissues that are generally not well preserved in the fossil record. This made the specimens a bit challenging to study, as several were 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,” Guangxu Zhang, co-author of the study and recent PhD from Yunnan University in China, who discovered the fossils, said in a statement. “I initially called it ‘the plastic bag’ because it looks like a rotting plastic bag. When I found more of these fossils and analyzed them in the laboratory, I realized it was a mollusk.”

Shishanias spines show an internal system of channels less than a hundredth of a millimeter in diameter. The cones were separated at the base by microvilli: small protrusions of cells that increase their surface area. Microvilli are found on the human tongue and in the intestines, where they help the body absorb food.

“We found microscopic details in the conical spines that covered the body Shishania showing how they were hidden in life,” Parry said. “This kind of information is incredibly rare, even in exceptionally preserved fossils.”

The team compares Shishanias method for separating hard parts from a natural 3D printer which can change its body parts depending on what the animal needs. This method allows various invertebrates to secrete hard parts that do everything from providing defense to helping them crawl around.

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Chitons – the hard spines and bristles of some modern molluscs – are made of the mineral calcium carbonate instead of the organic chitin found in Shishania. Similar chitinous bristles are found in some more obscure groups of animals, including brachiopods and bryozoans. These animals, together with molluscs and annelids (modern earthworms and their relatives), form the group Lophotrochozoa.
“Shishania tells us that the spines and spicules we see today in chitons and aplacophoran molluscs actually evolved from organic sclerites like those of annelids,” Parry said. “These animals are now very different from each other and therefore also from fossils Shishania tell us what they looked like deep in the past, shortly after they diverged from common ancestors.”