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Landscape Fossils in the rocks

The Burren Fossils

8 Fossils in LimestoneDuring the Carboniferous Period of Earth’s history, the land that would eventually form North Clare was situated 10ºS of the Equator. It was covered by a shallow, warm tropical sea which teemed with marine life. When these marine animals and plants completed their natural life cycle and died, their remains floated down to the sea floor. Their soft body parts rotted away over time, leaving the hard skeletons.

The hard skeletons of the marine plants and animals became embedded in layers of calcium carbonate. Over a long time these layers compacted to form limestone and the skeletons of the organisms became fossilised within those rocks.

The most common fossils found in the Burren limestones are • crinoids (relatives of starfish), • corals (very similar to those alive today), • brachiopods (a filter-feeding animal with

two shells that is found today on the ocean floor in the deep sea), and

• gastropods (snails) – all former inhabitants of that ancient tropical sea.

Common fossils in the limestone, (A) corals, (B) brachiopods, (C) gastropods, (D) crinoids.

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Skeleton gets replaced by minerals and pressure forms rock

Erosion and excavation reveals the fossil and evidence of body parts

A

C

B

D

Fossils are the remains or traces of animals and plants that lived long ago, preserved in rock. They can be bits of shells, skeletons, stems or even footprints or tracks made by the animal when it was alive.

Remains get buried by sediment

Decomposing remains of an organism

Fossils in ShaleBetween 326 and 318 million years ago when particles of clay (the sediments that hardened and formed the black coloured Clare Shales) were being washed into the sea, on top of the limestones, the deep sea floor supported very little animal and plant life. There was no light and very low levels of oxygen in the deep water. The only fossils we find in the shales are therefore the remains of animals that lived higher up, close to the water surface e.g. goniatites.

Goniatite shell

Fossils in the Siltstones and Sandstones

The sediments that were washed into the sea, 318 million years ago, along a massive river delta created the siltstones and sandstones of North Clare (Information Sheet No. 7).

These sandstones and siltstones were laid down during floods. In between the floods, the sea was relatively calm and current ripples formed on the sea floor. During these quiet periods, marine organisms, living in and on the sea floor, left many sinuous trails that we can still see today. The best place to see these ‘trace fossils’ is on flagstones at the Cliffs of Moher. These trails were not made by worms; instead, we know that they were made by woodlouse-like creatures and by snails.

All the fossils found in the rocks of North Clare are broken to some degree and very few are found intact. This indicates that the fossils were moved around and that the floor of the shallow tropical sea was agitated by waves.

Goniatites are extinct sea

creatures. They had a spiral

shell with an open end. The

animal which resembled an

octopus or squid had tentacles

which protruded from the open

end of the shell.

Trace fossils are

impressions made on

the sediment by an

organism that lived in

the sea long ago e.g.

tracks and burrows.

The fossilised remains of a Goniatite

Current ripples in sandstone. The ripples

are symmetric, which tells us that water was

flowing in two directions, i.e. the marine

environment was tidal

Sinuous trails left by a woodlouse-like creature or snail

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9Identifying coral fossils

Landscape Fossils in the rocks

Corals are still in existence today and form great reefs in shallow tropical waters. They can grow in colonies, or live independently as a solitary coral.

Most of the corals found in the Burren limestones lived in a colony, but there are some solitary corals also.

Colonial corals often have a branching growth pattern. Each branch is usually 4-8 mm wide. They have a hard skeleton made from calcium carbonate that gradually grows upwards. The living animal (the polyp) sits at the top of the skeleton and gathers food using tentacles.

The fossil corals of the Burren appear differently depending on the way the piece of limestone has broken. If the corals are cut in cross section, each branch appears as a small circle with white lines radiating out from the centre, like the spokes of a bicycle wheel. If the corals are cut in long section, they appear as long pencil-like features.

Corals in the Burren. A: colonial coral. B: close-up view of coral branches in cross-section. C shows a cut face of limestone exposed on a step. You can see how the coral’s branches look circular on top of the step (where they are in cross section) but look like long pencils on the side of the step (where they are in long section).

In some corals in the Burren, the branches touch each other. These corals are called cerioid corals and are a different species to the others. They are very important for dating the age of the Burren limestones. Here you can see a cross-section of one of these corals.

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A B

C

Branching growth pattern of colonial coral

Cerioid (shared-wall) coral

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Identifying brachiopod fossils

Landscape Fossils in the rocks

10

Brachiopods, or “lamp shells”, are very common fossils in the Burren limestones. They are a type of shellfish with two valves and are still in existence today. They live attached to the sea floor by a tough ligament-like stalk and feed by filtering tiny particles of food (e.g. plankton) out of seawater.

Like the corals and crinoids, fossil brachiopods can look different depending on the way the block of limestone has been broken. In cross section, the valves of the shell look

like large white circles (A). Collections of brachiopod valve fossils may be visible where wave action dumped many shells together after a storm (B). In long section, the valves of the shell look like white semicircles (C). Most of the time, you only see one valve of the shell. This is because the two valves are held together in life by ligaments, which decay after the animal dies. With nothing holding the two valves together, the valves are easily separated by any currents passing over the sea floor.

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A

B

C

Modern brachiopods.

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Landscape Fossils in the rocks

11Identifying crinoid fossils

Crinoids have a radial array of feathery arms on top of a vertical stem. They are animals that use their feathery arms to pick or pluck food particles from the seawater and place it in their mouth. Their skeleton is composed of many tiny circular pieces, held together during life by ligaments and skin.

After the animal dies, these soft tissues decay and the pieces of the skeleton fall apart. The pieces are then scattered by currents. Hence the only remains of crinoids that we see in the limestones of the Burren are scattered white circular structures, about 2-10 mm wide.

Modern crinoid in the Pacific near Japan

Breakup of crinoid stem.

A: piece of crinoid stem in life, held together by ligaments (pink) and skin (dashed line)

B: after death, the ligaments and skin decay, and the pieces of the stem start to separate

C: the pieces of the skeleton are scattered by currents

The fossil crinoids can appear differently, depending on the way the block of limestone has broken and the orientation of the crinoid pieces within it. Where the crinoid pieces are cut in cross section, they appear as scattered white circular structures, about 2-10 mm wide. Where they are cut along their length, they appear as two parallel white lines with a jagged edge where they face each other.

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A B C

Crinoids in the Burren limestones

Crinoids in the Burren limestones

A: Colonial corals in limestone. B: Goniatite fossil in shale. C: Trace Fossils in siltstone. D: Trace Fossils in siltstone. E: Crinoid stems in limestone

A: Crinoid fossils in limestone, Doolin Point. B: Brachiopod fossils in limestone kerbside, Ennistymon. C: Goniatite fossils in the shale, Fisherstreet, Doolin. D: Fossil corals in limestone, Burren National Park. E. Colonial coral fossil in limestone, Flaggy shore. F: Brachiopod & coral fossils in steps, Cliffs of Moher.

A: Brachiopod fossils in limestone, Allidie. B: Brachiopod fossils with geopetal structure, Cliffs of Moher. C: Fasciculate colonial corals, Black Head. D: Vertical and horizontal trace fossils in fine sandstones, Cliffs of Moher. E: Pyritised goniatites, Fisherstreet Bay.

A

A

B

B

C

C

C

D

D

E

A E

D

B F

E