FOSSILS
Half a billion-year-old spiny slug reveals the origins of mollusks
University of Oxford
image:
Conical spines that cover the body of Shishania aculeata (left). Electron microscope image
of a conical spine showing the microscopic channels preserved inside (right). Credit: G Zhang/L Parry.
Credit: G Zhang/L Parry.
- Exceptional fossils with preserved soft parts reveal that the earliest molluscs were flat, armoured slugs without shells.
- The new species, Shishania aculeata was covered with hollow, organic, cone-shaped spines.
- The fossils preserve exceptionally rare detailed features which reveal that these spines were produced using a sophisticated secretion system that is shared with annelids (earthworms and relatives).
A team of researchers including scientists from the University of Oxford have made an astonishing discovery of a new species of mollusc that lived 500 million years ago. The new fossil, called Shishania aculeata*, reveals that the most primitive molluscs were flat, shell-less slugs covered in a protective spiny armour. The findings have been published today in the journal Science.
The new species was found in exceptionally well-preserved fossils from eastern Yunnan Province in southern China dating from a geological Period called the early Cambrian, approximately 514 million years ago. The specimens of Shishania are all only a few centimetres long and are covered in small spikey cones (sclerites) made of chitin, a material also found in the shells of modern crabs, insects, and some mushrooms.
Specimens that were preserved upside down show that the bottom of the animal was naked, with a muscular foot like that of a slug that Shishania would have used to creep around the seafloor over half a billion years ago. Unlike most molluscs, Shishania did not have a shell that covered its body, suggesting that it represents a very early stage in molluscan evolution.
Present-day molluscs have a dizzying array of forms, and include snails and clams and even highly intelligent groups such as squids and octopuses. This diversity of molluscs evolved very rapidly a long time ago, during an event known as the Cambrian Explosion, when all the major groups of animals were rapidly diversifying. This rapid period of evolutionary change means that few fossils have been left behind that chronicle the early evolution of molluscs.
Corresponding author Associate Professor Luke Parry, Department of Earth Sciences, University of Oxford, said: ‘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.’
Because the body of Shishania was very soft and made of tissues that don’t typically preserve in the fossil record, the specimens were challenging to study, as many of the specimens were poorly preserved.
First author Guangxu Zhang, a recent PhD graduate from Yunnan University in China who discovered the specimens said: ‘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.’
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 show an internal system of canals that are less than a hundredth of a millimetre in diameter. These features show that the cones were secreted at their base by microvilli, tiny protrusions of cells that increase surface area, such as in our intestines where they aid food absorption.
This method of secreting hard parts is akin to a natural 3D printer, allowing many invertebrate animals to secrete hard parts with huge variation of shape and function from providing defence to facilitating locomotion.
Hard spines and bristles are known in some present-day molluscs (such as chitons), but they are made of the mineral calcium carbonate rather than organic chitin as in Shishania. Similar organic chitinous bristles are found in more obscure groups of animals such as brachiopods and bryozoans, which together with molluscs and annelids (earthworms and their relatives) form the group Lophotrochozoa.
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, soon after they had diverged from common ancestors.’
Co-author Jakob Vinther at the University of Bristol said: ‘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.’
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.’
*Etymology of Shishania aculeata: Shishan refers to Shishan Zhang, for his outstanding contributions to the study of early Cambrian strata and fossils in eastern Yunnan; aculeata (Latin), having spines, prickly.
Complete specimen of Shishania aculeata seen from the dorsal (top) side (left). Spines
covering the body of Shishania aculeata (right). Credit: G Zhang/L Parry.
Artist’s reconstruction of Shishania aculeata as it would have appeared in life as viewed
from the top, side and bottom (left to right). Reconstruction by M. Cawthorne.
Notes:
For media enquiries and interview requests, contact Associate Professor Luke Parry luke.parry@seh.ox.ac.uk
Images relating to the study which can be used in articles can be found at https://drive.google.com/drive/folders/1eLiDU7bDWpVyxNJK9IzevPGL2ORud86n?usp=drive_link These images are for editorial purposes only and MUST be credited (see captions document in file). They MUST NOT be sold on to third parties.
The study ‘A Cambrian spiny stem mollusk and the deep homology of lophotrochozoan scleritomes’ will be published in the journal Science at 19:00 BST / 14:00 ET Thursday 1 August 2024 at www.science.org/doi/10.1126/science.ado0059 (link will go live when embargo lifts). To view a copy of the study before this, under embargo, contact the Science editorial team at scipak@aaas.org or see the Science press package at https://www.eurekalert.org/press/scipak/
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Journal
Science
Article Title
A Cambrian spiny stem mollusk and the deep homology of lophotrochozoan scleritomes
Article Publication Date
2-Aug-2024
New fossil resembling a bristly durian fruit reveals insights into the origin of molluscan skeletons
American Association for the Advancement of Science (AAAS)
The early evolution of mollusks has been hard to pin down, but now a newly discovered fossil – of a shell-less, soft-bodied, spiny mollusk from the early Cambrian – provides crucial insights, researchers report. The findings suggest that this fossil, of a creature called Shishania aculeata, is a stem mollusk – representative of an intermediate between early members of the superphylum lophotrochozoans and more derived mollusks. Mollusks are one of the most diverse groups of animals, encompassing various well-known forms such as clams, snails, and octopuses, as well as less familiar aculiferans (i.e., chitons and vermiform aplacophorans). Despite this impressive diversity, understanding the ancestorial traits of mollusks has been difficult due to the limited view provided by existing fossils and currently living forms. Although early mollusk fossils from the Cambrian period have offered some insights, revealing forms that exhibit a combination of biomineralized shells and sclerites, few stem lineage taxa have been described, leaving gaps in the understanding of early molluscan evolution. Here, Guangxu Zhang and colleagues describe S. aculeata, an early Cambrian-age mollusk that displays a combination of features, including a broad foot, mantle cavity, and a back covered in hollow chitinous sclerites, giving it the appearance of a bristly durian fruit. Unlike the biomineralized sclerites of modern aculiferan mollusks, Shishania sclerites are non-mineralized, suggesting a position outside the molluscan crown group. Electron microscopy revealed that the sclerites have a narrow tubular microstructure, similar to the bristles of annelid worms and brachiopods. The findings shed light on the evolutionary transition from simple chitinous bristles to the more complex biomineralized skeletal forms in mollusks.
Journal
Science
Article Title
A Cambrian spiny stem mollusk and the deep homology of lophotrochozoan scleritomes
Article Publication Date
2-Aug-2024
Fossil shows how penguins’ wings evolved
Peer-Reviewed PublicationA tiny fossil penguin plays a huge role in the evolutionary history of the bird, an international study shows.
Published in the Journal of the Royal Society of New Zealand, the study describes a new species of fossil penguin which lived in Otago about 24 million years ago.
Named Pakudyptes hakataramea, the penguin was very small – about the same size as the little blue penguin, the smallest in the world – with anatomical adaptations that allowed it to dive.
Lead author Dr Tatsuro Ando, formerly a PhD candidate at the University of Otago – Ōtākou Whakaihu Waka and now at the Ashoro Museum of Palentology in Japan, collaborated with researchers from Otago, Okayama University of Science and Osaka University.
Dr Ando’s inspiration for the paper came from discussions with the late Professor Ewan Fordyce, his supervisor and mentor at Otago.
Researchers analysed three bones – a humerus, femur and ulna – found by Professor Fordyce in the Hakataramea Valley, South Canterbury.
Dr Ando says Pakudyptes fills a morphological gap between modern and fossil penguins.
“In particular, the shape of the wing bones differed greatly, and the process by which penguin wings came to have their present form and function remained unclear,” he says.
The humerus and ulna highlight how penguins’ wings have evolved.
“Surprisingly, while the shoulder joints of the wing of Pakudyptes were very close to the condition of the present-day penguin, the elbow joints were very similar to those of older types of fossil penguins.
“Pakudyptes is the first fossil penguin ever found with this combination, and it is the ‘key’ fossil to unlocking the evolution of penguin wings.”
Co-author Dr Carolina Loch, from Otago’s Faculty of Dentistry, says analysis of the internal bone structure conducted at the Faculty of Dentistry, with comparison with data on living penguins provided from the Okayama University of Science, shows these penguins had microanatomical features suggestive of diving.
Modern penguins have excellent swimming abilities, largely due to their dense, thick bones that contribute to buoyancy during diving.
In Pakudyptes, the bone cortex was reasonably thick although the medullary cavity, which contains bone marrow, was open, similar to what we see in the modern little blue penguin, which tends to swim in shallow waters.
The ability for Pakudyptes to dive and swim comes down to the distinctive combination of its bones.
Bones such as the humerus and ulna show areas for attachment of muscles and ligaments which reveal how the wings were being used to swim and manoeuvre under water.
Dr Loch says fossil penguins were usually large, about 1m in height.
“Penguins evolved rapidly from the Late Oligocene to Early Miocene and Pakudyptes is an important fossil from this period. Its small size and unique combination of bones may have contributed to the ecological diversity of modern penguins.”
Journal
Journal of the Royal Society of New Zealand
Method of Research
Case study
Subject of Research
Animals
Article Title
A new tiny fossil penguin from the Late Oligocene of New Zealand and the morphofunctional transition of the penguin wing
