A surprisingly well-preserved collection of tiny fossils that made up a “marine dwarf world” in central Wales some 462 million years ago has been unearthed by palaeontologists. Among the many unusual species found in the rocks were armoured worm-like creatures and a primitive example of a horseshoe crab. The site in Castle Bank, Powys, is an example of a “Konservat-Lagerstätte” — a site of extraordinary fossil preservation that can contain soft tissues and even complete organisms. Accordingly, they can provide some of the best snapshots from the history of life.

Some of the most highly-regarded Konservat-Lagerstätten are known as “Burgess Shale-type” faunas — after 508 million year old deposits exposed in the Canadian Rockies of British Columbia, in western Canada.

Most of the Burgess Shale-type deposits are found in rocks that date back to the Cambrian Period of 542–485 million years ago — a time known for an explosion in the diversity of life and the emergence of the major animal groups.

In contrast, precious few Burgess Shale-type fauna are known from younger rocks. Because of this, palaeontologists know a lot about marine life in the Cambrian, but a lot less about exactly how it evolved afterwards.

Castle Bank’s fossil deposits are special, therefore, for having been laid down during the middle of the Ordovician, the geological period directly following the Cambrian.

The Ordovician was also a critical time in the history of life. It saw a diversification in the array of animals that produced hard skeletons — which are more readily preserved in the fossil record — and saw the emergence of familiar ecosystems like coral reefs.

Paper author and palaeontologist Dr Lucy Muir of Amgueddfa Cymru–Museum Wales said that the Castle Bank fauna: “coincides with the ‘Great Ordovician Biodiversification Event’, when animals with hard skeletons were evolving rapidly.

“For the first time, we will be able to see what the rest of the ecosystem was doing as well.”

In fact, the researchers said, the quality of the preservation and the diversity of the species found at Castle Bank rivals that of the Cambrian-aged Lagerstätte — with one other Ordovician site (the “Fezouata Biota” of Morocco) standing alongside it.

The new fossil assemblage was first discovered in 2020 by Dr Muir and her colleague, Dr Joe Botting of the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences.

To date, the researchers have recovered examples of more than 150 species from the rocks, almost all of which are new to science.

They include species of arthropods, like crustaceans and horseshoe crabs, through to worms, spongers, starfish and many more besides.

Many are also very small, at around one–three millimetres long, but nevertheless their minute details are well preserved.

In some of the specimens, this even includes internal features like digestive systems and a possible primitive brain, while others preserve fragile external structures like tiny limbs and filter-feeding tentacles — fossilised features previously only seen in Cambrian Lagerstätten.

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The Castle Bank fauna also included several unusual finds — including unexpectedly late examples of animals that emerged in the Cambrian, such as opabiniids (weird proto-arthropods with a long, clawed proboscis) and wiwaxiids (scaled, slug-like molluscs).

The researchers also report the discovery of specimens that resemble modern goose barnacles, examples of horseshoe shrimp (previously unknown from the fossil record) and what may even be a marine relative of the insects.

By dissolving away parts of the rocks in hydrofluoric acid, researchers in Sweden were able to extract minute fragments of organic remains.

Astonishingly, analysis of these under the microscope revealed them to have preserved details down to even the cellular level.

With their initial study complete, the researchers will be dividing deeper into the world of Castle Bank 462 million years ago.

Dr Muir explained: “Most fossil deposits of this importance are studied for many decades, and this is likely to be no different.”

Dr Botting concluded: “Every time we go back, we find something new, and sometimes it’s something truly extraordinary.

“This is just the beginning, and we’re excited to see what comes next.”

The full findings of the study were published in the journal Nature Ecology & Evolution.

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