Spines covering the body of Shishania aculeata
Image credit: G Zhang/L Parry
Palaeontology has a habit of throwing up surprises.
Sadly for creationists, these surprises never show us that science is fundamentally wrong and creationism is fundamentally right. It invariably shows us exactly the opposite.
The surprise today is that an early mollusc resembled a slug with a spiky armour, not the smooth shell we normally associate with snails which were believed to have evolved in the Early Cambrian when this species lived.
The confirmation that creationism is fundamentally wrong comes from the fact that this strange slug lived in that very long pre-'Creation' period of Earth history, 500 million years before creationists think the Universe was magicked up out of nothing by some magic words spoken in a language that no-one else spoke because, so the story goes, there was no-one else for the magician to speak to.
The mollusc, named Shishania aculeata, was probably an early evolutionary dead-end; an early response to the threat to soft-bodies creatures from the emerging predators that gave the impetus for the rapid radiation of forms that characterised the Cambrian.
What information do you have on the evolution of molluscs? The evolution of mollusks (or molluscs) is a fascinating story of adaptation and diversification that dates back over 500 million years. Mollusks are one of the most diverse groups of animals, comprising more than 85,000 known species, including snails, clams, squids, and octopuses. Their evolution is characterized by a wide range of body forms, ecological niches, and survival strategies.It was discovered in eastern Yunnan Province in southern China in a rock formation dating from the early Cambrian, approximately 514 million years ago, by Guangxu Zhang, a recent PhD graduate from Yunnan University.
Origins
- Cambrian Explosion (~541 million years ago): Mollusks are believed to have originated during the Cambrian Explosion, a period of rapid evolutionary diversification. Early mollusks were likely simple, soft-bodied animals with a basic shell, similar to modern-day chitons or monoplacophorans.
- Fossil Record: The earliest mollusk fossils date back to the Cambrian period. Some of the earliest known mollusks include genera like Wiwaxia and Odontogriphus, which show a mix of characteristics seen in later mollusks.
Major Evolutionary Groups
Mollusks are divided into several major classes, each of which evolved distinct characteristics:
Evolutionary Adaptations
- Gastropoda (Snails and Slugs):
- Evolution: Gastropods likely evolved from an early mollusk with a simple, uncoiled shell. The coiling of the shell, a key feature of gastropods, provided structural strength and reduced the size of the opening, offering better protection.
- Diversification: Gastropods are the most diverse class of mollusks, adapting to a wide range of environments from deep-sea vents to terrestrial habitats. They exhibit a range of feeding strategies, from herbivory to carnivory.
- Bivalvia (Clams, Oysters, and Mussels):
- Evolution: Bivalves evolved from early mollusks with a single shell that later split into two hinged parts. This adaptation allowed for a more efficient burrowing lifestyle and protection against predators.
- Adaptations: Bivalves developed filter-feeding mechanisms and became highly specialized for life in marine and freshwater environments. Their evolution includes the development of siphons, which allow them to feed while buried in sediment.
- Cephalopoda (Squids, Octopuses, and Nautiluses):
- Evolution: Cephalopods evolved from early mollusks with a straight, conical shell, like the extinct *Orthoceras*. Over time, some cephalopods lost their external shells (e.g., octopuses), while others evolved complex internalized shells (e.g., squids and cuttlefish).
- Complexity: Cephalopods are known for their high intelligence, advanced nervous systems, and complex behaviors. They have also evolved remarkable adaptations such as jet propulsion, ink production, and highly developed eyes.
- Polyplacophora (Chitons):
- Evolution: Chitons are considered to be among the most primitive of the mollusk classes, retaining many features of early mollusks, such as a simple, segmented shell made up of eight plates.
- Adaptations: They are well adapted to life on hard substrates, such as rocks in intertidal zones, where they scrape algae with their radulae.
- Scaphopoda (Tusk Shells):
- Evolution: Scaphopods are a smaller class of mollusks that evolved from early burrowing mollusks. Their tubular shells are adapted to a burrowing lifestyle in marine sediments.
- Feeding: They use tentacle-like structures called captacula to capture small particles of food from the surrounding water.
- Shells: The evolution of the shell is a key feature of mollusks. The shell provides protection from predators, prevents desiccation, and offers structural support. Over time, shells have evolved into various forms, from the coiled shells of gastropods to the two-part shells of bivalves and the internalized or lost shells of cephalopods.
- Radula: Most mollusks possess a radula, a unique feeding organ that consists of a ribbon-like structure covered with tiny teeth. The radula has adapted to various feeding strategies, from scraping algae off rocks to drilling into the shells of other mollusks.
- Locomotion: Mollusks have evolved different methods of locomotion, such as the muscular foot of gastropods and bivalves, the jet propulsion of cephalopods, and the ciliary movement in some smaller mollusks.
Fossil Record and Molecular Data
- Fossil Record: The fossil record of mollusks is rich and provides important insights into their evolutionary history. Fossils of ancient mollusks show a progression from simple to more complex forms, with significant diversification occurring during the Paleozoic era.
- Molecular Phylogenetics: Advances in molecular biology have allowed scientists to study the evolutionary relationships between different mollusk groups. DNA sequencing has helped clarify the evolutionary tree of mollusks, revealing that some traditional classifications based on morphology may need revision.
Evolutionary Significance
Mollusks have played a significant role in the evolution of marine ecosystems. Their diversification has contributed to the complexity of food webs, the development of coral reefs, and the structure of benthic (seafloor) communities. The evolutionary history of mollusks also provides valuable insights into the processes of speciation, adaptation, and extinction.
In summary, the evolution of mollusks is a story of incredible diversity and adaptation, with different groups evolving unique features to thrive in a wide range of environments. Their long evolutionary history and the variety of forms they exhibit make them a key group for understanding the broader patterns of life on Earth.
The new species is described in a paper in the journal Science, which, sadly is behind an expensive paywall. It is also described in a Bristol University News release:
Half a billion-year-old spiny slug reveals the origins of molluscs
A team of researchers including scientists from the University of Oxford and the University of Bristol 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.
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
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.
Dr Jakob Vinther, co-author
Schools of Earth Sciences and Biological Sciences
University of Bristol, Bristol, UK.
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.
Associate Professor Luke Parry, co-corresponding author
Department of Earth Sciences
University of Oxford.
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.
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.
Dr. Guangxu Zhang, first author
Yunnan Key Laboratory for Palaeobiology
and MEC International Joint Laboratory for Palaeobiology and Palaeoenvironment
Institute of Palaeontology
Yunnan University, Kunming, China.
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.
Associate Professor Luke Parry.
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.
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.
Associate Professor Luke Parry.
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.
Xiaoya Ma, Co-corresponding author
Yunnan Key Laboratory for Palaeobiology and MEC International Joint Laboratory for Palaeobiology and Palaeoenvironment
Institute of Palaeontology
Yunnan University, Kunming, China. And Centre for Ecology and Conservation
University of Exeter, Penryn, UK.
*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.
Paper:Guangxu Zhang, Luke A. Parry, Jakob Vinther and Xiaoya Ma (2024)
Cambrian spiny stem mollusk and the deep homology of lophotrochozoan scleritomes
Science
AbstractNot only does this find fill a gap in our understanding of the early origins of the molluscs, it also fills the gap between the annelids (segmented worms) and the molluscs in the structure of the spines which is ancestral to the chitinous shells (and limbs) of many extant molluscs (and arthropods) which is the same as that of some chelae of annelids.
Mollusks encompass enormous disparity, including familiar clams and snails alongside less familiar aculiferans (chitons and vermiform aplacophorans) with complex multicomponent skeletons. Paleozoic fossils trace crown mollusks to forms exhibiting a combination of biomineralized shells and sclerites (e.g., scales, spines, and spicules). We describe a shell-less, Cambrian stem mollusk, Shishania aculeata gen. et sp. nov., with conical, hollow chitinous sclerites and a smooth girdle, broad foot, and mantle cavity. The sclerites have a microstructure of narrow canals consistent with the impressions of chaetal microvilli found in annelids and brachiopods. Shishania sclerites provide a morphological stepping stone between typical chaetae (chitinous bristles) and the external organic part of aculiferan sclerites that encloses a mineralized body. This discovery reinforces a common origin of lophotrochozoan chaetae and the biomineralized aculiferan sclerites, suggesting that the mollusk ancestor was densely covered with hollow chitinous chaetae.
Guangxu Zhang et al.
A Cambrian spiny stem mollusk and the deep homology of lophotrochozoan scleritomes.
Science 385, 528-532 (2024). DOI:10.1126/science.ado0059
© 2024 The Authors.
Published by American Association for the Advancement of Science
Reprinted with kind permission under license #5846750531591
This supports the idea that the rapid radiation of body plans over a few million years that characterised the Cambrian, was driven initially by the evolution of motility, which created the opportunity for predation to exploit the ready supply of organic nutrients, which in turn drove the proliferation of defensive structures, sensory systems, and strategies such as burrowing.
Creationists might like to note the complete dependence on the Theory of Evolution to explain these findings with no hint that it needs to be replaced by creationism's magical superstition with unproven entities and child-like guesses on which creationism depends, because the current biology isn't up to the task.
And then of course there is the evidence of evolved organisms living in that vast expanse of time before creationists believe the Universe was created, which would normally cause rational people to revise their belief even if their mummy and daddy had thought Earth was just a few thousand years old and had been created as is by magic, and magically populated with millions of unrelated species, none of which had any ancestors.
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