Fossils and Coal Formation

The Jarrow Fossils

While digging out the coal, miners occasionally came across plant fossils, the remains of ancient forests. Then in 1864, coal miners working the Jarrow pit near Clogh started to find strange animal fossils among the coal. William Booking Brownrigg was a scholar who had heard about the plant fossils and came to examine them. When he came down to the area, he found something much more significant, the remains of ancient amphibians. He started to describe them and persuaded the Geological Survey to pay for their illustration.

Room 2. Audiovisual

Nowadays if we want to find out about the history of the Earth and the plants and animals that once lived here, we can pick up a book, consult the internet or visit an exhibition but where did all that knowledge come from? At the time when the Castlecomer fossils were discovered, scientists were collecting rock and fossil specimens from all over the world and trying to piece together skeletons and plant fragments in order to find out about the past. The theory of evolution was very new and most scientists were still trying to equate fossil findings with living plants and animals.

The work of 19th Century scientists provided valuable insights into the way the world works. Their studies on the origin of rocks and landforms and the ecology and adaptations of plants and animals have provided the information we need to understand where our coal deposits came from and the conditions that led to their formation. Their work has also helped us to understand the plants and animals of the distant past.

Charles Darwin (1809 - 1882)

Darwin published his book ‘On the Origin of Species’ in 1859 just 6 years before the discovery of the amphibian fossils near Castlecomer. In it he outlined his theory of the evolution of animals through natural selection. He was strongly criticised by many scientists because he dared to contradict the teachings of the church, and it was many years before his teachings were widely accepted.

Sir Charles Lyell (1797 - 1875)

Many 19th century scientists believed that geological and landscape features were a result of Catastrophes such as Noah’s flood. Lyell argued that these features were the result of gradual change over very long periods of time. In 1830 he published his ‘Principals of Geology’. Lyell became a private supporter of Darwin’s theories on evolution but never publicly declared his support.

Professor E.P. Wright

Wright was a professor of Geology at Trinity College Dublin. He heard about the amphibian finds in Clogh and came to study them himself. After studying the amphibians, he realised their importance and arranged for further excavation of the mine, which yielded more fossils. Wright then contacted Thomas Henry Huxley to come and examine the fossils and together they published a paper on them in 1867.

Thomas Henry Huxley (1825 - 1895)

Huxley was a very vocal supporter of Darwin’s theories on evolution which earned him the nickname ‘Darwin’s Bulldog’. He was an important scientific figure at the time and an expert on vertebrate fossils. Having previously published papers on Scottish amphibians, he was called in to examine the amphibian fossils found near Castlecomer, which he, along with Professor E.P. Wright, described in a paper published in 1867.

Huxley described at least ten genera of amphibian fossils in Castlecomer, five of which were previously unknown.

Room 3: Rocks

Over time, the shape of the Earth’s crust has changed with continents moving and changing shape over millions of years. These changes in the shape of the Earth and subsequent changes in climate have led to a variety of different conditions which produced the different rock types of the Earth and led to the evolution of many different forms of plants and animals.

Ireland’s position on the Earth has varied over geological time and as a result our climate has changed many times. The main sedimentary rocks found in Ireland formed in very different ways under very different conditions.

A large portion of Ireland is covered in sandstone deposits, formed during the Devonian age approximately 400 million years ago. During this time, Ireland was situated just south of the equator and was part of a large continent known as the Old Red Sandstone Continent, which was mostly desert and was subject to occasional flash floods. Large amounts of sand were transported by rivers to be deposited in layers which over time were compressed down to form sandstone rocks. At this time, life was beginning to emerge from the oceans. Archaeopteris species were some of the first trees to grow on the Earth and examples of these have been found in a quarry at Kiltorcan in south-east Kilkenny. Early amphibians had also started to evolve and walk on land. A set of footprints from a four-legged creature dated at about 385 million years old were found on the shoreline of Valentia Island in County Kerry.

At the end of the Devonian period, there was a large mass extinction event that wiped out over 70% of all animal species on the Earth. The Archaeopteris trees also became extinct. At this time, the sea level rose dramatically and most of the Old Red Sandstone Continent was covered by sea.

This warm tropical sea was populated by a variety of sea creatures including the sharks which had survived the Devonian extinction. The bones and shells of these sea creatures and the remains of underwater plants eventually formed the Limestones of the early Carboniferous , which cover most of Ireland today.

Towards the end of the Carboniferous Period, about 300 million years ago, the sea levels dropped again and a large area of tropical swamp emerged, stretching around most of the equator. Trees falling onto the waterlogged forest floor formed peat deposits which over millions of years were compressed and heated by subsequent rock layers to form the coal deposits of the Castlecomer Coal-field. The extinction of so many plants and animals at the end of the Devonian had cleared the way for a wide variety of new organisms to evolve. Huge forests grew up in the swamps and these formed an ideal habitat for early amphibians.

Geological Timescale

Room 4: Swamp Scene

If we could travel back in time 300 million years, we would see a very different landscape from the one that surrounds us today. The Carboniferous swamp forest is recreated here to give us an idea of what the area would have looked like at the time.

Some of the plants and animals would have looked familiar but many were unlike anything that is found on the Earth today. The trees were very large, many of them up to 45m tall. Tree ferns, similar to those found in New Zealand and certain tropical countries today were quite common. Calamites species were hollow stemmed plants related to modern day horsetails that grew up to 20m tall.

The Lycopods, of which Lepidodendron and Sigillaria are two types, grew up to 35m tall. They had distinctive diamond patterned leaf scars on their stems, which are often preserved in the coal. It has been estimated that up to 70% of the coal may be composed of lycopod remains. Cordaites species grew to a height of up to 45m tall. They had a root system similar to modern mangroves.

Invertebrates are animals without a backbone. The group includes insects, spiders, centipedes, millipedes, worms, slugs and snails. Carboniferous invertebrates were much larger than modern day forms and one possible reason for their large size was a greater level of oxygen in the air, which made it easier for them to absorb oxygen through their skin. Insects were the first animals to develop flight and may have evolved wings in order to feed from the very tall vegetation that characterised the Upper Carboniferous Period.

Arthropleuris was similar in shape to modern millipedes but much larger, growing up to 3m long. Like its modern relatives, it would probably have fed on rotting vegetation. Dragonflies would have grown extremely large during the Carboniferous Period. Fossils have been found with wingspans of up to 75cm.

Most of the animal fossils found in the Castlecomer coal were amphibians. Amphibians were the first tetrapods to walk on land, a tetrapod being any animal that walks on four legs. There are three groups of amphibians living today: the newts and salamanders (order Caudata); the caecilians (order Gymnophiona) and the frogs and toads (order Anura). The fossil forms found near Castlecomer are closely related to modern day newts and salamanders. Some of these amphibians would have been terrestrial forms living mostly on land, while others were more aquatic, spending most of their time in water. Some were predators, feeding on fish and other animals, while some of the smaller species would themselves have been preyed upon by larger animals.

Keraterpeton galvani was similar in form to modern newts. It was about 50cm long and would probably have eaten small fish and invertebrates. The flattened tail and the wings on either side of its skull are thought to have been used as aids for swimming.

Ophiderpeton brownrigii was similar in appearance to modern snakes. Although it had lost its legs, Ophiderpeton seems to have been more terrestrial than many of the other amphibians lacking the flattened tail, which marks the more aquatic forms

Megalocephalus pachycephlus can be translated as ‘big headed thick head’. This creature seems to have been a very large aquatic predator with needle-like teeth for fish eating. Megalocephalus was first identified from a flattened skull, about 35cm long.

Dendrerpeton rugosum was a terrestrial amphibian, which grew up to a metre in length. Its large head and short squat body suggest that it probably couldn’t move fast over long periods of time, but the sharp backward-facing teeth suggest that this was a fierce predator that possibly lay in ambush waiting for its prey.

We can tell a lot about prehistoric animals by studying their fossils. By studying their teeth and the shape of their heads, we can tell whether they were predators or prey species and the shape of their bodies and the weight of their skeletons can tell us a lot about how they moved. We cannot however tell the colour of their skin from a fossil. Probably many of them used camouflage, to hide from predators. Predator species may even have used camouflage to ambush prey. It is possible however that some of the smaller species may have had poisons in their skin similar to some tree frogs and salamanders today. If this was the case, they would probably have been brightly coloured to deter predators.

Room 5: Fossil Room

Many specimens of plant fossils were found in the shale quarries around the Castlecomer area. The shale is found directly above the coal and is quarried for brick-making. These fossils can be seen in the display cabinets. Other fossils on loan from the Geological Survey of Ireland include fossil shark teeth from the Devonian Period and a specimen of Archaeopteris, from Kiltorcan, one of the first trees to colonise the Earth. The Jarrow amphibian fossils are held in the collection of the National Museum of Ireland, but replicas are on display and can be used for fossil rubbings.

For more information on Fossils and Coal Formation contact Castlecomer Discovery Park