Analysis: Ireland's fabulous fossils tell a remarkable story of what the country looked like 300 million years ago

By Anthea Lacchia, UCD

Imagine a tropical, shallow sea, its waters glistening in the warm sunshine. Underwater, corals dance in the waves and fish swim gracefully by, as a tiny, tentacle-bearing creature retreats into its spiral shell. Is this your dream holiday destination? Dream no further. This is what Ireland looked like 300 million years ago.

Geologists, the detectives of Earth's 4.6 billion-year-old history, use evidence preserved in rocks to paint a picture of the million- or billion-year-old past of a given area.

In Ireland, the rocks reveal a rich history, punctuated by volcanic eruptions, flooding by ancient oceans, the birth and erosion of mountain ranges, and the appearance of strange and wonderful creatures on our land and in our seas.

These creatures included corals, sea anemones, plants, sea lilies, tentacle-bearing molluscs with hard shells, Giant Irish Deer, amphibians, fish, sharks and trilobites (marine animals that look a lot like present-day woodlice).

A Goniatite Homoceras beyrichianum from Inishcorker, Co Clare. Photo: Gillian Lewarne

Most of the fossils you will see on a typical coastal walk in Ireland are of marine animals without a vertebral column (invertebrates), such as sea-lilies, corals, and bivalves.

What is a fossil?

No, it's not your very, very old relative!

A fossil is the remains of a living organism (animal or plant) preserved in rocks. Any organism, when it dies and falls to the ground or sea floor, can become rapidly buried by sediments. If this happens, it can be preserved as a fossil in the rocks.

Fossils offer a precious snapshot of life on earth at a particular time. Indeed, for many organisms, such as the dinosaurs, fossils are the only evidence we have of their very existence. The study of fossils is called palaeontology and, in Ireland, it is alive and well.

Over half of the rocks that make up Ireland were formed from about 354 to 298 million years ago during the Carboniferous period. And rocks of this age are also prime targets for fossil enthusiasts.

From RTÉ Radio 1's Drivetime, a report on the discovery by Brendan Arrigan of a 340 million year old fossil in a stone wall in Tuam, Co Galway

Life in the Carboniferous

About 350 to 300 million years ago, a warm tropical sea extended northwards over Ireland. The animals that lived in that sea are preserved as fossils in the limestone rock that makes up most of our island. For example, many will be familiar with the limestone terraces of the Burren in Co Clare.

Later on in the Carboniferous, mountain belts started to grow in the south of Ireland and swamps and forests replaced the sea. At this time, rocks like sandstone and shale were deposited in Ireland, as well as coal. What strange creatures lurked in these ancient environments? Let's look at two examples.

Ammonoids

Ammonoids are extinct molluscs that lived as fearsome carnivorous predators in the oceans of the geological past. Related to squid and cuttlefish, these animals were very diverse in terms of size, shape and sculpturing. They had an external shell, divided internally into several chambers. The animal resided in the last formed chamber, which was called the body chamber. Though now extinct, ammonoids were very common in ancient marine waters over Ireland from about 300 million years to 65 million years ago.

From RTÉ Radio 1's Mooney Goes Wild, a report on the 2017 Konservat-Lagerstätten workshop at UCC where leading academics shared their expert knowledge of fossils which could open an invaluable window into our prehistoric past

The best places to find ammonoid fossils in Ireland are in Co Clare and Co Kerry, where the dark shales quickly reveal fossilized ammonoid shell material to the trained eye. The Cliffs of Moher, though inaccessible on the edges, contain ammonoids in the dark, shale-rich layers. The Bridges of Ross, in Loop Head, Co Clare, is an excellent locality to spot some of these fossils preserved in pyrite, or fool's gold. Most of them are less than 1cm in size, so you will have to get very close to the rocks to see them. 

Tetrapods

Tetrapods (four-limbed animals) also roamed around Ireland 325-million years ago, as confirmed by a recent discovery from Doolin, Co Clare. Here, two tiny bones of an ancient tetrapod were found by Dr Eamon Doyle, geologist with the Burren and Cliffs of Moher Burren Geopark, and fossil vertebrate researcher Aodhán Ó Gogáin from Trinity College Dublin. These are the oldest known tetrapod bones in Ireland.

From RTÉ Radio 1's Morning Ireland, geologist Dr Eamon Doyle, Geologist on the discovery of the tiny bones of a 325 million year old amphibian in Co Clare

Another famous tetrapod site is the Tetrapod Trackway in Valentia Island, Co Kerry. Here, the footprints of a salamander-like creature, which was crawling on soft mud, are preserved in the rocks. This fossil, which is 385 million years old, represents the oldest in-situ (preserved in place) tetrapod trackway in the world, and provides some of the earliest evidence of the transition of vertebrates from water to land. A tip: do try to visit the trackway in the morning or evening, when the sun rays make the footprints stand out from the rocks.

What do fossils tell us?

As well as offering key insights into the evolution of life on Earth, including human evolution, fossils provide clues about what ancient environments were like, including climatic conditions. Ultimately, understanding how ancient organisms interacted with their environment can help us understand present-day climate change and how we humans are altering our environment, often for the worst.

But many more mysteries remain to be unearthed from Irish rocks. On your next coastal walk, why not take a closer look at the rocks you are treading on?

Dr Anthea Lacchia is a postdoctoral research fellow at the Irish Centre for Research in Applied Geosciences (iCRAG) at UCD.


The views expressed here are those of the author and do not represent or reflect the views of RTÉ