Half a Billion Years

Fossils are the bodies of living organisms that fell into mud, sand or soil in the past and then over the millennia were turned into sedimentary rocks preserving the shapes of those bodies. Fossils give us an unsurpassed view of the creatures that existed on our planet right back, close to the earliest forms of life well over three billion years ago.

At the beginning of the Cambrian period of rock formation about 570 million years ago a burgeoning range of new animal types can be seen in the fossils such as sponges, corals, trilobites and other even more exotic forms that all arose in a relatively short time. This speedy transformation in animal life has been called the “Cambrian Explosion”.

Some of our fossil specimens are from the next, more recent geological era, the Ordovician, which lasted from 500 until 440 million years ago. These fossils are on the surface of a layered slate-like slab of shale about 30 cm across. We have no written details about the provenance of the piece of rock but the distinctive fossils give us very powerful clues. Fossils can often act as a type of “brand-mark” that allows a particular sedimentary rock to be identified and aged.

This rock is thickly strewn with imprints of individuals of a group of animals called graptolites that were characteristic of ancient seas. Graptolites were marine animals made from linked colonies of tiny individuals sitting in pockets in a tubular external skeleton. Each tube had tentacles allowing the individual organism to feed by filtering plankton from seawater. Graptolites are thought to be related to early echinoderms, modern forms of which include starfish, and to the major group of animals that includes vertebrates.

The last examples of graptolites went extinct about 320 million years ago in the Carboniferous Age. They were abundant and diverse in the Ordovician.

Our characteristic graptolite comes from Ordovician seas from 480 million years ago. It is called the “Tuning Fork Graptolite” or Didymograptus murchisoni.

The “Tuning Fork Graptolite” Didymograptus murchisoni Wales

As the name suggests, these 3 cm long colonial organisms have a forked shape like a tuning fork. It is thought that they floated in a planktonic way in the ancient seas. When they died their tubular skeletons fell in their trillions to the muddy ocean floor where they produced layers that were covered by more mud sediment and later petrified to produce the fossils that have lasted hundreds of millions of years to be collected today. The inner edge of each arm of the “fork” is decorated with regular teeth so it looks more like a hacksaw blade than a tuning fork. Each tooth is the remains of the skeleton pocket in which a single graptolite individual in the colony lived.

The structure and size of the colonies on our rock enable us to identify it as Didymograptus murchisoni and we may be able to discover the collection location. There is only one good source of fossils like this in the UK and that is Abereiddy Bay on the SW coast of Wales. The beach there is made up of dark blue-grey flattened boulders of Ordovician shale and the dark sand there comes from the same rocks. Every boulder when split reveals the characteristic graptolites.

Ordovician rocks at Abereiddy Bay, SW Wales, a classic D.muchisoni location

It is a sobering and awesome fact to recognise that this single piece of rock in our museum is an ossified image of the thriving, biodiverse life that flourished almost half a billion years ago in seas that were located in what is now the coast of Wales. It connects us back to some of the earliest stages in the development of complex animal life on our planet.

Both images licensed under CC