The discovery comes from well-preserved fossils found in eastern Yunnan Province, southern China, from the early Cambrian period, approximately 514 million years ago. Shishania specimens, only a few centimeters long, are covered in small spiky cones (sclerites) made of chitin, a material present in modern crab shells, insects, and some mushrooms.
Fossils preserved upside down show a naked underside with a muscular foot, similar to modern slugs, which Shishania likely used to move across the seafloor. Unlike most molluscs, Shishania lacked a shell, indicating an early stage in molluscan evolution.
Today's molluscs, including snails, clams, squids, and octopuses, evolved rapidly during the Cambrian Explosion, leaving few fossils to trace their early development. Shishania provides critical insights into this formative period.
Associate Professor Luke Parry from Oxford's Department of Earth Sciences stated, "Trying to unravel what the common ancestor of animals as different as a squid and oyster looked like is a major challenge for evolutionary biologists and palaeontologists - one that can't be solved by studying only species alive today. Shishania gives us a unique view into a time in mollusc evolution for which we have very few fossils, informing us that the very earliest mollusc ancestors were armoured spiny slugs, prior to the evolution of the shells that we see in modern snails and clams."
The soft body of Shishania, made of tissues that rarely fossilize, posed challenges for study due to poor preservation.
Guangxu Zhang, the first author and recent PhD graduate from Yunnan University, who discovered the specimens, noted, "At first I thought that the fossils, which were only about the size of my thumb, were not noticeable, but I saw under a magnifying glass that they seemed strange, spiny, and completely different from any other fossils that I had seen. I called it 'the plastic bag' initially because it looks like a rotting little plastic bag. When I found more of these fossils and analysed them in the lab I realised that it was a mollusc."
Parry added, "We found microscopic details inside the conical spines covering the body of Shishania that show how they were secreted in life. This sort of information is incredibly rare, even in exceptionally preserved fossils."
The spines of Shishania contain internal canals less than a hundredth of a millimeter in diameter, indicating they were secreted by microvilli, cell protrusions that increase surface area, similar to those in human intestines aiding food absorption.
This secretion method, akin to a natural 3D printer, allows invertebrates to create varied hard parts for defense or locomotion.
Modern molluscs, such as chitons, have calcium carbonate spines, unlike Shishania's chitinous ones. Similar structures are found in brachiopods and bryozoans, which, along with molluscs and annelids, form the Lophotrochozoa group.
Parry continued, "Shishania tells us that the spines and spicules we see in chitons and aplacophoran molluscs today actually evolved from organic sclerites like those of annelids. These animals are very different from one another today and so fossils like Shishania tell us what they looked like deep in the past, soon after they had diverged from common ancestors."
Jakob Vinther from the University of Bristol commented, "Molluscs today are extraordinarily disparate and they diversified very quickly during the Cambrian Explosion, meaning that we struggle to piece together their early evolutionary history. We know that the common ancestor of all molluscs alive today would have had a single shell, and so Shishania tells us about a very early time in mollusc evolution before the evolution of a shell."
Xiaoya Ma from Yunnan University and the University of Exeter added, "This new discovery highlights the treasure trove of early animal fossils that are preserved in the Cambrian rocks of Yunnan Province. Soft bodied molluscs have a very limited fossil record, and so these very rare discoveries tell us a great deal about these diverse animals."
Research Report:A Cambrian spiny stem mollusk and the deep homology of lophotrochozoan scleritomes
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University of Oxford
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