: The Fossil Record

Part of a rock containing a fossil coral which is only just visible. Most of the fossil is hidden inside the rock.

Small fossils require large machines: the Diamond Light Source synchrotron facility at Didcot, Oxfordshire.

Fossils often need to be carefully prepared before they can be identified by palaeontologists and traditional methods can damage the specimen irreversibly. Scientists at Amgueddfa Cymru are experimenting with new technologies to study ancient fossils in minute detail with no damage to the specimen whatsoever.

The preparation of fossils for identification and study often requires the surrounding rock to be removed. A variety of tools are used for this, including specialist equipment such as pneumatic pens driven by compressed air and air-abrasive machines which work like miniature sandblasters. In some cases the surrounding rock can be dissolved away from the fossil

Destructive techniques

These preparation methods will reveal the surface of a fossil but to identify some specimens we must look inside them. Certain fossil groups are routinely examined using destructive techniques. For example, to study the internal structure of corals, bryozoans and brachiopods, specimens are cut up (sectioned) into slices so thin that we can shine light through them and examine them under a microscope. These slices are known as 'thin sections'.

Sometimes it is not desirable to remove the rock or to cut up the specimen. The fossil may be very delicate and break during preparation. Or, if the fossil is very rare, we would rather not use a destructive technique as it may be difficult or even impossible to replace the fossil.

Non destructive X-Ray tomography

In these instances, it is now possible to use X-rays to build up a virtual three-dimensional image. The technique is called X-ray tomography. As rock is much denser than living tissue, X-ray tomography of fossils requires a more powerful radiation source than a hospital X-ray machine. The Diamond Light Source Synchrotron at Didcot, Oxfordshire is one such facility, the only one in the UK. This machine is in the shape of a giant donut with a diameter of 300 meters. It accelerates charged particles (electrons) through sequences of magnets to almost the speed of light, producing the X-rays.

This method was recently used by Amgueddfa Cymru scientists to investigate a small fossil found in Ordovician rocks 462 million years old in Iran. It was half buried in the rock and appeared to be a solitary rugose coral. The conventional approach to identifying rugose corals – to slice into thin sections – was rejected because of the small size and rarity of the specimen. Instead we took it to Didcot for X-ray tomography.

The new technique worked very well and we managed to obtain spectacular 3D images, and even and virtual thin sections – without any damage to the fossil itself. From these, we determined the internal structure of the fossil and concluded that the specimen was indeed a coral, probably a species of the genus Lambelasma.

It is approximately 5 million years older than the earliest previously described rugose coral, making it a significant addition to our knowledge of early life on our planet.

Chaning Pearce [Image (c) Bristol City Museum & Art Gallery]

The fossil turtle at National Museum Cardiff, its significance previously unknown

Sir Richard Owen in 1855 by Maull & Polyblank. Founding father of the National Museum of History, London and inventor of the word 'dinosaurs'

In 1842 the famous naturalist and palaeontologist Sir Richard Owen described four new fossil turtle specimens from the Purbeck Limestone (Lower Cretaceous) of Dorset. One of these has always been in Natural History Museum in London, but the other three were held in private collections, and after 1842, effectively vanished for 150 years!

One of the missing three was discovered in the Natural History Museum several years ago by Dr Andrew Milner whilst studying turtles and other reptiles, but the whereabouts of the other two remained a mystery. However, further research led him to the National Museum Cardiff where a fossil turtle in the collections was positively identified as one of Owen's missing specimens.

This fossil turtle - originally named Chelone obovata - was owned by Joseph Chaning Pearce (1811-1847), who worked as a doctor in Bath until his early death at the age of 37. He built up one of the largest private collections of fossils outside London and had set aside part of his house as a private museum. After his death his family kept the small museum until at least 1886 when they moved to Kent. The collection is next heard of in 1915, when much of it was bought by the Bristol Museum and Art Gallery, along with its original catalogue.

Dr Milner searched the collections at Bristol Museum but did not find the Chaning Pierce specimen, and assumed that it was destroyed in 1940 when incendiary bombs landed on the exhibition hall of the Bristol Museum during the Second World War. However, in 2008 he found the original hand-written catalogue, and on page 32 is the record for Fossil no.12 Chelone obovata with a pencil annotation - Sent to Cardiff Museum, 3rd March 1933.

Dr Milner contacted the Department of Geology, National Museum Cardiff, and it transpired that in 1933 a Purbeck turtle shell was registered, although we had very little information about it. The specimen had been on display in the Evolution of Wales exhibition since 1993 as it is very well preserved and fairly complete.

Richard Owen's original description in 1842 describes this turtle as the 'type specimen' of the species Chelone obovata - meaning that this is the specimen against which all others should be checked.

Although there were no illustrations, he published a very detailed and accurate description. This description matches the specimen in National Museum Cardiff exactly, and there is no doubt that it is the same specimen.

A recent investigation shows that the turtle now belongs in the genus Hylaeochelys and the species latiscutata. This specimen has significant historical interest as it was collected prior to 1840, and described by Sir Richard Owen - the man who invented the name 'dinosaur'.

Archaeopteryx is an iconic fossil, often thought of as the ‘missing link’ between dinosaurs and birds. It was first described in 1861 by the German palaeontologist Hermann von Meyer (1801–1869). Since then Archaeopteryx has been the focus of controversy surrounding the origin of birds and their links with dinosaurs.

Only eleven specimens and an isolated feather have so far been found, all coming from a few quarries near the Bavarian town of Solnhofen in southern Germany.

Almost all of the specimens are from the Solnhofen Limestone, fine muddy limestones deposited in tropical lagoons about 150 million years ago near the end of the Jurassic Period. One was found in the overlying Mörnsheim Formation and is younger by perhaps half a million years.

Discovery

In 1861 Hermann von Meyer published a description of a single fossil feather found in the Solnhofen Limestone and named it Archaeopteryx lithographica.

Archaeopteryx means ‘ancient wing’. Von Meyer also mentioned that ‘an almost complete skeleton of an animal covered in feathers’ had been found. After competition from other museums this skeleton was eventually bought by the British Museum along with other Solnhofen fossils for £700, then a huge sum of money.

In 1863 Richard Owen, Superintendent of the natural history collections at the British Museum, described and illustrated the specimen, declaring it be a bird with ‘rare peculiarities indicative of a distinct order’. The discovery of this remarkable fossil came just two years after the publication of Charles Darwin's book On the Origin of Species, which changed people’s perception of the natural world.

Archaeopteryx seemed to fit well with Darwin’s theory as it showed features of both birds and reptiles.

What did Archaeopteryx look like?

Archaeopteryx was a primitive bird with feathers, but its fossilised skeleton looks more like that of a small dinosaur. It was about the size of a magpie.

Unlike modern birds it had a full set of teeth, a long bony tail and three claws on its wing which may have been used for grasping branches. It lacked the fully reversed toes which enable many modern birds to perch. However, Archaeopteryx did have a wishbone, wings and asymmetrical ‘flight’ feathers, like a bird. It is likely that Archaeopteryx could fly, although perhaps not strongly.

The world of Archaeopteryx

Archaeopteryx lived on land near a series of stagnant and salty lagoons within a shallow tropical sea. Life in the lagoons was concentrated in the surface waters, as most of the lower levels were extremely toxic. It is possible that the only animals living in the lagoons were small floating crinoids (sea-lilies) and some fish.

Ammonites, shrimps, lobsters and starfish lived in the open sea nearby and were occasionally washed into the lagoons during storms. They did not survive for long in the lagoon waters. Horseshoe crabs have been found preserved at the end of a short trail of their own footprints. Very occasionally marine reptiles, such as ichthyosaurs and crocodiles, were also washed in.

Flying over the sea were pterosaurs and large insects such as dragonflies. These were blown into the lagoonal waters during these storms. A juvenile specimen of a small theropod dinosaur called Compsognathus has also been discovered in the same deposit, which must have been washed in from the land.

Reconstruction of Archaeopteryx (© J. Sibbick)

How did Archaeopteryx die and become preserved?

Although Archaeopteryx lived on land, occasionally some would have been caught up in storms as they flew or glided over the water. Waterlogged and unable to take off again, they would have drowned and sunk to the floor of the lagoon.

All of the known specimens display various characteristics of immaturity, indicating that none of the specimens was fully adult. This may have been the reason for their inability to survive storm events.

The carcasses were quickly buried by fine lime muds deposited on the lagoon floors. It is thought that one metre of rock today represents 5,000 years of deposition. The fossils of Solnhofen are exceptionally well preserved, due to the lack of disturbance from both predators and water movement. Within the fine-grained limestones, delicate features such as dragonfly wings or the feathers of Archaeopteryx can be found.

A limestone rock, from Southerndown in south Wales, containing fossil shells. (Gryphaea arcuata — Devil's toenails).

Fossil shells (Gryphaea arcuata – Devil's toenails) after the rock has been dissolved in acid.

A fossil bivalve shell that has been partially prepared using acid. The two parts of the shell (the valves) are still attached to each other, and the bottom valve is still attached to the rock.

We rarely find fossils in perfect condition. When we collect them, most of them are partially or almost completely concealed in rock. To study them in detail, or to prepare them to be displayed, we must carefully remove the rock.

It's not easy to remove the rock from the fossils, which are sometimes extremely delicate. It can be a long and painstaking process. Sometimes we use hand tools, like pins, scrapers or little chisels. We can also use specialist tools, like pneumatic pens similar to engraving devices. A third option is to use an air-abrasive machine that is little like a miniature sandblaster.

An alternative is to dissolve the rock in acid. Most fossil shells were composed originally of calcium carbonate, and many are preserved in

Vinegar, no salt please

The acid we commonly use is acetic acid — the same acid that is in vinegar. We use highly concentrated acetic acid, 80%, but dilute it with water to about 5%.

We immerse the rock sample containing fossils in the diluted acetic acid, which almost immediately starts to fizz as it reacts with the limestone.

The rock can take anything from a few days to many months to dissolve completely, depending on its specific composition. The reaction with the rock gradually neutralizes the acid, which needs to be refreshed from time to time. At the end of the process all that remains is some slushy rock residue, and the silicified fossils.

Many silicified fossils are very delicate; they have to be handled with extreme care, and we often need to strengthen them with adhesives.

The fossils are now ready for research or for displaying. Many are exquisitely preserved: 450-million-year-old silicified shells sometimes look as if they have just been picked up from a modern beach!