Exploring a Middle Ordovician Burgess Shale-type fauna

Major new fossil sites don't turn up every day. This is the story behind the discovery and early investigation of Castle Bank (Wales, UK), by Joe Botting and Lucy Muir: a married couple who have an academic background but work mostly as amateurs.
Published in Ecology & Evolution

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Much of what we know about the history of life comes from relatively few sites: the flukes of chemistry and environment that preserved entire communities, rather than just the hard parts like shells. These sites of exceptional preservation (Konservat-Lagerstätten) can include entirely soft-bodied animals in major groups that would normally not fossilize at all. As a result, these deposits are crucial to our understanding of palaeobiology, evolutionary histories and palaeoecology, even though they each offer only a small glimpse into the full history of life. Each new site reveals something we've never seen before.

   There are many sites of exceptional preservation known, but they are not all equal. Some represent a very particular type of environment, with a limited assemblage of plants and animals; this is often the case because the chemical conditions necessary to preserve soft tissues occur in restricted settings. Others only preserve a particular type of tissue, or fossils within a certain size limit (amber, for example, can only preserve things that could become trapped in it). They all add to our knowledge, but are often only giving us half a perspective.

   Some of the most important exceptional preservation deposits are referred to as Burgess Shale-type faunas, after the eponymous site in Canada. These are mostly from the early and middle Cambrian (around 525-500 million years ago [Ma]), with two from the early part of the Ordovician, at around 480 Ma. These deposits have been revolutionary for our understanding of early animals, revealing the body plans and biology of the ancestors of modern phyla. In many cases, these fossils have helped to reveal links between phyla, showing how evolutionary pathways led to the origin of arthropods, for example, or molluscs. However, the Burgess Shale window wasn't open forever; in fact, it closed just when life started to diversify most spectacularly. At least, it was thought to have closed then... but a new fossil assemblage called Castle Bank (Fig. 1) gives us an unprecedented new perspective on the evolution of early animal life.

Two happy palaeontologists (the authors of the article) holding their hammers in the quarry. It's a very small rock face with the beds (including a white volcanic ash layer) dipping down to the left.

Fig. 1. The authors of this blog post in the Castle Bank quarry.

   Revealing a site like this takes a combination of luck, persistence, and the ability to understand what you're seeing. It's a mixture of old-fashioned fieldwork skills and modern techniques that help to reveal the detail of the fossils. This blog isn't so much about the deposit itself, but rather the process of uncovering it... and the peculiarities of our largely amateur circumstances that made it possible.

Hunting Lagerstätten

Discovering and developing a site like this takes a combination of luck and the right circumstances. Castle Bank is unusual among Burgess Shale-type faunas in several aspects: (a) the exposure is very restricted, being (so far) primarily a 20-cm-thick band of rock in a single small quarry; (b) the fossils are generally minute (often 1–3 mm), and many can only be identified with a microscope; (c) the fossils are also only clearly visible under good light conditions. All these features made finding the fauna extremely difficult, and explains why it took years for the importance of the site to become obvious!

We didn't expect to find such a remarkable fauna when we first began working at this site, but there were features that suggested further investigation was worthwhile. Initially, we found (while out walking) some loose rock used to make a platform for feeding sheep on. It was obvious that it contained abundant graptolite fossils, and also dark blobs that resembled sponges. These were confirmed under a microscope, and that made a second visit a priority. We like sponges, after all. A conversation with the farmer who was using the stone revealed the source of the quarry, and a visit to a nearby property resulted in permission to visit the quarry and explore more. This was in 2013. For the next six years we would occasionally visit the quarry, collecting more sponges, and sometimes other fossils, and building up a significant assemblage of new species (Fig. 2).

A selection of sponges from the Castle Bank fauna, from large rounded forms with complex walls, to delicate vase-like species.

Fig. 2. A variety of sponges from the Castle Bank Lagerstätte (scale bars 1 mm)

   These sponges were delicate animals, and would have disintegrated rapidly after death; but at this site, we found numerous complete skeletons and traces of their soft tissues. Aside from being fascinating in themselves, complete sponges are one of the signs we look for when hunting for exceptional preservation. Sponges are also, in some cases, tolerant of low-oxygen conditions, and sometimes there simply wasn't much else living with them. In this case, we found occasional very interesting fossils of other types (an alga, a beautiful bryozoan, some complete trilobites), but it was almost exclusively sponges and planktonic graptolites. The sponges were certainly well worth describing, but wouldn't be revolutionary.

   We had collected several large boxes of sponges by the time the Covid-19 pandemic arose in 2020. As the first lockdown was imposed, we saw an opportunity to finally finish writing up the paper. Collecting is addictive, though, so Joe went out for one final day of fieldwork, to collect another lot of specimens and answer a few remaining questions about their distribution in the quarry.

   Inevitably, that was the day that the first soft-bodied animals appeared. The first was what we are currently referring to as a problematic tubicolous species: a strange creature with two long tentacles extending from a 3-mm-long organic tube (Fig. 3). This is tiny, and the tentacles, whatever the animal was, must have been extremely delicate and prone to decay. The first few specimens weren't perfect, but they were enough to transform our understanding of the site. Suddenly it was catapulted from a sponge fauna to something with wider soft tissue preservation, and it is these sites that can potentially reveal all manner of new wonders.

A strange creature with a narrow conical tube, from which a soft body emerges, bearing two long tentacles.

Fig. 3. The first soft-bodied fossil found at the site: a tube-dwelling animal with two long tentacles.

   The secrets don't just present themselves, though; it takes persistence, dedication, and a lot of time. For the Chengjiang Biota, a celebrated Burgess Shale-type deposit in China, teams of local villagers are hired to find the fossils. Here, it was just us. We've spent over 100 days in the quarry, having worked out exactly where the soft-bodied fossils are preserved, and how to split the rocks minutely, scouring every surface with a lens for millimetre-sized flecks of carbon. Another factor is that we are both very short-sighted; this makes it possible to focus on the tiniest fossils that many others would miss entirely. Even so, often we don't know whether a fossil is worthwhile until we have it under a microscope, so we collect all of the right sort of material. Sometimes a new discovery leaps out and renews the enthusiasm, but other times you can find nothing good for hours. Being sure not to miss important fossils is a real challenge, after hours of squinting in the hot Welsh sun (yes, that does exist).

Freedom and challenges

If we were employed as palaeontologists, none of this would likely have happened. Firstly, the type of exploratory fieldwork we can do here would never have received funding, and the initial discovery of the site was simply through knowing the local area, and automatically stopping to look at any new rocks. Collecting for 100 days over 2 or 3 years would also be impossible as an expedition, as good weather is vital for finding the fossils. The nature of the fossils (their tiny size, especially) means that we can't work on an industrial scale: working through a fist-sized piece of the right bed is a slow process, and taking out large lumps of outcrop would only destroy the fossils through rapid weathering. This fieldwork is only really suited to people who live nearby and have flexible work.

Being effectively amateurs also has problems, though. The exquisite detail in these fossils required far better photomicroscopy equipment than we could afford, and we needed those microscopes on a daily basis. The only solution, suggested by the site owners, was crowdfunding. This turned out to be amazingly successful, and allowed us to buy the equipment that most of the paper's images were taken with. In return, we have made the microscopes available to anyone who needs to come and use them, in the hope of encouraging amateur science across Wales.

   We still needed other aspects that were beyond our home laboratory, though, such as electron microscopy, elemental mapping, and dissolution in hydrofluoric acid. Although we are Honorary Research Fellows at Amgueddfa Cymru—National Museum Wales, we couldn't use their equipment during the pandemic. For these aspects, we were blessed with a wonderful group of collaborators, in the UK, Sweden and China, who all worked seamlessly together. Collaborations are not always beautiful, but this group is a joy to work with. Some of our colleagues have visited us here, to use the microscopes and work on particular fossils, and to see the locality.

An early arthropod, only 1.2 mm long, with a tiny carapace and a long, thin body ending in a leaf-shaped telson. At the front (right) there are long, sturdy appendages with spines on the end. The gut is visible as a narrow dark line running along the body.

Fig. 4. One of the arthropods, resembling the Cambrian megacheiran Yohoia. The scale bars are 1 mm in the main image and 0.1 mm in the inset image, and the gut (magnified, inset) is 20 μm wide.

An Ordovician bonanza

The result of this contorted process is a major new fossil assemblage with no counterpart known anywhere else: a Burgess Shale-type fauna from the middle of the Ordovician biodiversification. Many of the most remarkable species are currently known from single specimens, but the total diversity is well over 150 species, and steadily rising. They may not look quite as beautiful as the Burgess Shale or Chengjiang specimens, but this is largely due to their minute size; and yet, the finest details within the specimens are still present, including features like a 20 μm-wide gut (Fig. 4), gill filaments, tentacles, and even nerves. Research at this site is still in the very early stages, but the range of taxa already discovered provides an entirely new perspective on Ordovician biodiversification.

   The fauna includes a complex suite of both ancient (Cambrian) survivors (or taxa that resemble them in form), and what appear to be unexpectedly advanced members of more modern groups. There are also others that have no counterpart in the known fossil record. Many of these will be fully described and interpreted in detailed papers on individual species, as the overall ecosystem is gradually unveiled. At the same time, fieldwork will continue, and more new discoveries will be made and described... probably for decades. This is just the beginning.

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