When Sponges Ruled the World

When most people think about the geology of Wales, they think there is nothing new to be found, as some of the founding ideas about Geology were developed here in the early 19th Century. In fact, this couldn't be further from the truth! Although Wales was studied by the early geologists, it has had surprisingly little attention since, and recently some colleagues and I discovered a rather exciting fossil deposit.

First discovery of the new fossil deposit, in the snow in Wales

The rocks which we studied are around 480 million years old, and they were deposited during a period of time known as the Early Ordovician. At this time, Wales was in the southern hemisphere, at about 60 degrees south, approximately where Argentina is now. However, during this time the world  was much warmer than it is now, so the seas covering Wales would also have been warmer.

The Early Ordovician is an exciting time in respect to evolution, with the Great Ordovician Biodiversification Event (GOBE) just starting to kick off; during GOBE the complexity and structure of ecosystems completely changed, and animal diversity greatly increased, but scientists are still not sure exactly what triggered it as there are very few well preserved fossil deposits.

During the preceding period of time, in the Cambrian, there are a number of really good fossil deposits, including famous sites such as the Burgess Shale. The fossils from these sites suggest that arthropods, including trilobites (extinct animals a bit like underwater woodlice), were the dominant organisms. In the modern, arthropods, including animals such as crabs and lobsters, are still dominant in the seas, so scientists have suggested that arthropods have always been dominant. However, in this new fossil deposit, arthropods are not dominant!

Worm from the Afon Gam deposit with close up of its plates. Scale bar 1 mm

Our Welsh deposit, which we have named the Afon Gam deposit, contains fossils of animals which are not usually preserved because they rot easily; creatures such as worms, sponges and algae have all been fossilised, their bodies turned into a mineral called pyrite (also known as fool's gold), and that pyrite has now weathered into black and orange films on the rocks.  This means the fossils are only visible when the rocks are wet.

Algae fossil, scale bar 5 mm. Sponge fossil, scale bar 10 mm. Both from the Afon Gam deposit

To be able to preserve animals which rot easily, they have to be buried really quickly after death, or buried alive. In these rocks, the fossils have been preserved because the animals were buried rapidly by mudslides which came down the slope they were living on in the sea.  This is quite unusual for a fossil deposit, and so the fossils have the potential to tell us a lot about the time when they were alive.

Although the rocks do contain fossils of arthropods, including trilobites, these fossils are in the minority when compared to the number of sponges found - this is not what is expected, as usually arthropods are dominate. These fossils suggest that at this time in Wales, at the very least, sponges ruled, not arthropods!  This has the potential to change how scientists view evolution during this period of time.

Trilobite from the Afon Gam deposit. Scale bar 5 mm

So in conclusion, the Welsh rocks are still holding many secrets waiting to be discovered, and you never know what you may find when you look at a rock hard enough!

If you would like to hear more, there is a podcast available through Imperial College London at: http://wwwf.imperial.ac.uk/imedia/content/view/4746/when-sponges-ruled-the-earth

If you are interested in reading the paper, which is open access, and so accessible to everyone, please find it here: http://www.nature.com/srep/2015/150424/srep09947/full/srep09947.html

Studying the rocks of the Afon Gam deposit, Snowdonia, Wales

Photos courtesy of Joe Botting and Lucy Muir.