Scientists have discovered the fossils of a new species of extinct marine mollusc (Shishania aculeata) from eastern Yunnan Province in southern China, dating from the early Cambrian period approximately 514 million years ago.
The astonishing discovery reveals that the most primitive molluscs were flat, shell-less slugs covered in protective spiny armour.
The findings, made by a team of researchers, including scientists from the University of Oxford, were published in the journal Science.
Mollusc evolution
Present-day molluscs have a vast array of forms, and include snails and clams, as well as highly intelligent groups, such as squids and octopuses. This diversity of molluscs evolved very rapidly during an event known as the Cambrian Explosion. But there are very few fossils from before this period of evolutionary change that chronicle the early evolution of molluscs, explains co-author of the study Professor Luke Parry.
"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 new species was found in exceptionally well-preserved fossils from eastern Yunnan Province in southern China. The specimens show that the bottom of the animal was naked, with a muscular foot much like that of present-day slugs.
Shishania would have used this foot to creep around the seafloor over half a billion years ago.
Unlike most molluscs, Shishania did't have a shell that covered its body, suggesting that it represents a very early stage in molluscan evolution.
" I called it 'the plastic bag' initially because it looks like a rotting little plastic bag." Guangxu Zhang
Soft-tissue organisms don't typically preserve well in the fossil record, and Shishania is no different, explains lead author Guangxu Zhang, who discovered the specimens.
"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."
What did Shishania look like?
The researchers found that the spines of Shishania show an internal system of canals less than a hundredth of a millimetre in diameter – the hard spikes or cones were secreted at their base by microvilli.
Hard spines and bristles are known in some present-day molluscs, providing defence or facilitating locomotion, but they are made of the mineral calcium carbonate rather than organic chitin, as shown in Shishania.
"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," explains Professor Parry.
"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."
"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." Professor Luke Parry
The new discovery provides an exciting insight into the very early stages of mollusc evolution before the shell appeared, concludes co-author Xiaoya Ma, highlighting "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."
Read more about the study: Guangxu Zhang, Luke A. Parry, Jakob Vinther, Xiaoya Ma. A Cambrian spiny stem mollusk and the deep homology of lophotrochozoan scleritomes
Main image: Shishania aculeata. Credit: Credit: G Zhang/L Parry
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