Coloured digital models of animals in strange shapes

Revealing Exceptional fossils, one layer at a time

Around 120 years ago, William Sollas, Professor of Geology at the University of Oxford, developed a special technique for grinding down and imaging certain kinds of fossils. Sollas was based at the Museum at the time, and the process he pioneered is still used here today, as our Palaeobiology Technician Carolyn Lewis explains to mark the anniversary of Sollas’ birthday on 30 May.

Rock face with geologists hammer
Site of the Herefordshire Lagerstätte, showing the nodules embedded in soft volcanic ash.

Here at the Museum, I work on a collection of exceptionally well-preserved fossils from the Silurian Herefordshire Lagerstätte. They were deposited on the seabed 430 million years ago when the animals were buried by a volcanic ash flow. The fossils range in size from less than a millimetre up to a few centimetres, and represent a diverse collection of marine invertebrates that includes sponges, echinoderms, brachiopods, worms, molluscs and a wide variety of arthropods.

These Herefordshire Lagerstätte fossils are unusual in that many of them have preserved soft tissues in remarkable detail, including eyes, legs, gill filaments, and even spines and antennae only a few microns in diameter. The key to this extraordinary preservation is that as the fossils developed, calcium carbonate nodules formed around them, protecting and preserving the fossils since the Silurian Period.

Usually, only the hard parts of fossil invertebrates are preserved – the carapace of trilobites or the shells of brachiopods, for example – so the Herefordshire material provides us with a great opportunity to work out the detailed anatomy of these early sea creatures.

Split rock nodule showing fossil of Offacolus kingi inside.
Close-up of the fossil of Offacolus kingi

But the problem we face is how to extract the specimen from the rock nodule without losing the information it contains. The fossils cannot be separated from the surrounding rock by dissolution, because both fossil and nodule are made mainly of calcium carbonate, so would dissolve together. And they are too delicate to be extracted mechanically by cutting and scraping away the surrounding nodule. Even high resolution CT scans cannot, at present, adequately distinguish between the fossils and the surrounding rock material.

To get round this problem we use a method of serial grinding and photography based on the technique developed by William Sollas in the late 19th century. We grind the fossils in increments of 20 microns then photograph each newly ground surface using a camera mounted on top of a light microscope. This generates hundreds of digital images of cross sections through the specimen.

Then, using specially developed software we convert the stack of two-dimensional images into a 3D digital model that can be viewed and manipulated on screen to reveal the detailed form of the animal. These 3D models are artificially coloured to highlight different anatomical structures and can be rotated through 360o, virtually dissected on screen, and viewed stereoscopically or in anaglyph 3D.

Although our method of serial grinding is still fairly labour intensive, it is far less laborious and time-consuming than the process used by William and his daughter Igerna Sollas. Compared to the photographic methods of the early 20th century, where each photographic plate required long exposure and development times, digital photography is almost instant, enabling us to grind several specimens simultaneously.

Grid of images show a fossil at different stages of grinding down
Sequential serial grinding images of an ostracod

Computer software also allows us to create 3D virtual models rather than building up physical models from layers of wax. Yet despite our modern adaptations, we are using essentially the same technique that William Sollas developed here at the Museum 120 years ago. And using this technique to study the fossils of the Silurian Herefordshire Lagerstätte has yielded a wealth of new information that opens up a unique window into the evolution and diversification of early life in our oceans.

How the sea cucumber lost its armour

By Imran Rahman, Research Fellow

You have probably heard of sea cucumbers. If you’re lucky, you might have seen one, if not in the wild, then perhaps in a nature documentary like Blue Planet or the children’s cartoon Octonauts. If you’re less lucky, you might have eaten one – they are most commonly described as slippery and bland in taste!

Despite their appearance, sea cucumbers are actually marine animals most closely related to sea urchins, rather than to worms or slugs. Over the past century palaeontologists have uncovered a range of ancient fossil relatives of modern sea cucumbers that allow us to piece together the story of how they evolved from armoured ‘tanks’ into the naked slug-like forms we see today. One such fossil is described in a new paper by my colleagues and I, just published in the journal Proceedings of the Royal Society B.

The fossil in question is 430-million-years-old, and it comes from a site of exceptionally-preserved fossils in England called the Herefordshire Lagerstätte. Herefordshire has produced many exciting discoveries over the years, from prehistoric parasites to an ancient ‘kite runner’. The new fossil is the first of its kind from this deposit.

Like all fossils from Herefordshire, the specimen was preserved in an egg-shaped nodule of rock. Because the rock has the same chemical composition as the fossil, it could not be studied with modern imaging methods such as CT scanning. Instead, it had to be studied by painstakingly grinding away the fossil, a few hundredths of a millimetre as a time, with photographs taken of each exposed surface using a digital camera. This allowed us to build up a dataset of hundreds of slice images through the fossil, which were digitally reconstructed as a 3-D ‘virtual fossil’ on a computer.

The 3D computer reconstruction revealed a very peculiar animal, about 3 cm wide, with 45 tentacle-like ‘tube feet’ and a large mouth surrounded by five teeth. The animal had a skeleton made up of numerous hard plates, which were composed of the mineral calcite. After studying this fossil and comparing it to other similar ones from the same time period, we were able to identify it as a species new to science. We named the species Sollasina cthulhu, for its resemblance to monsters from the Cthulhu universe created by author H.P. Lovecraft.

One of the most useful things about our 3D computer reconstruction was that it enabled us to study the inner features of the fossil, as well as the parts visible on the outer surface. This revealed internal soft parts that had never previously been described in this group of fossils. In particular, it allowed us to see an internal ring-like structure within the main body cavity.

3D reconstruction of Sollasina cthulhu. Left-hand image shows part of lower surface. Right-hand image shows same view with outer surface partly transparent to reveal inner ring (in red). Credit: Imran Rahman, Oxford University Museum of Natural History

We interpreted this inner ring as part of the water vascular system – the system of fluid-filled canals used for feeding and movement in modern sea cucumbers and their relatives, such as sea urchins and starfish. In life, the ring was connected to the large tube feet, which were filled with seawater. Most of these tube feet were used for crawling over the seafloor, with those nearest the mouth used for capturing food. The teeth could cut and crush food items, which were then eaten by the animal.

Life reconstruction of Sollasina cthulhu. Credit: Elissa Martin, Yale Peabody Museum of Natural History

To work out the evolutionary relationships of Sollasina cthulhu, we assembled a list of characteristics for various fossil and modern sea cucumbers and sea urchins. We analysed this matrix using several computational methods to determine how these different animals were related to one another. The results confirmed that Sollasina cthulhu and closely-related forms were ancient relatives of modern sea cucumbers. This allowed us to reconstruct the early evolution of sea cucumbers, back to their shared common ancestor with sea urchins, over 450 million years ago. Our study demonstrates this was a story of loss, with fossil sea cucumbers becoming progressively less armoured as they evolved into modern forms.

This discovery has greatly improved our understanding of sea cucumber evolution, but several questions remain. One intriguing question is when and how did sea cucumbers lose their teeth, and did these evolve into any features seen in living sea cucumbers? Future study of existing and new fossil sea cucumbers and sea urchins will help to answer this and other intriguing questions.