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.

Life’s big bang?

by Harriet Drage and Scott Billings

You may have heard of the Cambrian Explosion, an ‘event’, starting roughly 540 million years ago, when all the major animal groups suddenly appear in the fossil record, an apparent explosion of life and evolution.

But was there really an evolutionary explosion of all these animal groups, or is the lack of evidence from earlier periods due to some peculiarity of the fossilisation process? The debate has rumbled on for a number of years.

Now, a new study from our research team, the University of Oxford’s Department of Zoology, and the University of Lausanne, claims that the early Cambrian saw the origins and evolution of the largest and most important animal group on Earth – the euarthropods – in a paper which challenges two major pictures of animal evolution.

Euarthropoda contains the insects, crustaceans, spiders, trilobites, and a huge diversity of other forms alive and extinct. They comprise over 80 percent of all animal species on the planet and are key components of all of Earth’s ecosystems, making them the most important group since the dawn of animals over 500 million years ago.

Exceptionally preserved soft-bodied fossils of the Cambrian predator and stem-lineage euarthropod Anomalocaris canadensis from the Burgess Shale, Canada. Top left: Frontal appendage showing segmentation similar to modern-day euarthropods. Bottom right: Full body specimen showing one pair of frontal appendages (white arrows) and mouthparts consisting of plates with teeth (black arrow) on the head. Images: A. Daley.

A team based at the museum, and now at Lausanne, conducted the most comprehensive fossil analysis ever undertaken on early euarthropods, to try and establish whether these animals really did emerge in the early Cambrian period, or whether fossilisation just didn’t occur in any earlier periods.

In an article published today in the Proceedings of the National Academy of Sciences they show that, taken together, the total fossil record does show a gradual radiation of euarthropods during the early Cambrian, 540-500 million years ago, challenging other ideas that suggest either a rapid explosion of forms, or a much slower evolution that has not been preserved in the fossil record.

Each of the major types of fossil evidence has its limitation and they are incomplete in different ways, but when taken together they are mutually illuminating
Professor Allison Daley

Reconstruction of the Cambrian predator and stem-lineage euarthropod Anomalocaris canadensis, based on fossils from the Burgess Shale, Canada. Reconstruction by Natalia Patkiewicz.

By looking at a huge range of fossil material the researchers ruled out the possibility that Pre-Cambrian rocks older than around 541 million years would not have preserved early euarthropods. The only plausible explanation left is that the origins of this huge animal group didn’t evolve until about 540 million years ago, an estimate which also matches the most recent molecular dating.

The timing of the origin of Euarthropoda is very important as it affects how we view and interpret the evolution of the group and its effects on the planet. By working out which groups developed first we can trace the evolution of physical characteristics, such as limbs.

Exploring all the evidence like this allows us to make an informed estimate about the origins of key animal groups, leading to a better understanding of the evolution of early life on Earth.

Model of the Cambrian stem lineage euarthropod Peytoia, based on fossils from the Burgess Shale. Top left: Closeup of the mouth parts and frontal appendages. Bottom right: Overall view of the body. Model and image: E. Horn.