A groundbreaking discovery has been made by a team of researchers from the University of Oxford and the University of Bristol. They have uncovered a new species of mollusc that thrived 500 million years ago. Known as Shishania aculeata, this remarkable fossil provides evidence that the earliest molluscs resembled shell-less slugs adorned with spiky armor.
These well-preserved fossils were unearthed in eastern Yunnan Province, China, dating back to the early Cambrian Period, around 514 million years ago. The tiny Shishania specimens, only a few centimeters in length, are adorned with chitinous spines similar to those found in the shells of modern crabs, insects, and some mushrooms.
The discovery of upside-down preserved specimens reveals fascinating insights into the ancient mollusc, Shishania. This creature, lacking a protective shell, provides valuable evidence of an early stage in the evolution of mollusks.
The Cambrian Explosion, a period of rapid diversification, led to the astonishing variety of mollusks we see today. Despite the scarcity of fossils, the study of Shishania sheds light on the intriguing evolutionary history of this diverse group of animals.
Specimens that were preserved upside down reveal that Shishania, half a billion years ago, had a naked underside with a muscular foot similar to that of a slug, which it used to move along the seafloor. Shishania, unlike most molluscs, lacked a protective shell, indicating that it represents an early stage in molluscan evolution.
Modern-day molluscs come in a wide variety of forms, including snails, clams, and highly intelligent creatures such as squids and octopuses. This rapid diversification of molluscs occurred during the Cambrian Explosion, an event in which all major animal groups underwent rapid evolution. Few fossils have been preserved that document the initial development of molluscs due to the swift period of evolutionary change.
“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,” said co-author Dr Jakob Vinther, based in Bristol’s School of Earth Sciences. “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.”
“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 paleontologists – one that can’t be solved by studying only species alive today,” said corresponding author Associate Professor Luke Parry, Department of Earth Sciences, University of Oxford. “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 body of Shishania was quite soft and composed of tissues that do not usually preserve well in the fossil record, making the specimens difficult to study due to poor preservation.
“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,” first author Guangxu Zhang said.
Associate Professor 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 reveal an internal system of canals that are less than a hundredth of a millimeter in diameter. These features indicate that the cones were secreted at their base by microvilli, tiny protrusions of cells that increase surface area, similar to those found in our intestines aiding in food absorption.
This method of secreting hard parts is comparable to a natural 3D printer, allowing many invertebrate animals to produce hard parts with a wide variation of shape and function, serving purposes from defense to facilitating locomotion.
Some modern mollusks, like chitons, have hard spines and bristles, but these are composed of calcium carbonate instead of organic chitin, as seen in Shishania. Obscure animal groups such as brachiopods and bryozoans also have similar organic chitinous bristles, and they, along with mollusks and annelids, belong to the Lophotrochozoa group.
Professor Parry added: “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, as soon as they diverged from common ancestors.”
Co-corresponding author Xiaoya Ma (Yunnan University and University of Exeter) said: “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.”
Journal reference:
- Guangxu Zhang, Luke A. Parry, Jakob Vinther, Xiaoya Ma. A Cambrian spiny stem mollusk and the deep homology of lophotrochozoan scleritomes. Science, 2024; DOI: 10.1126/science.ado0059
