Ireland’s rocks remarkably rich in evidence of ancient sea life

Brittle-star belonged to deep-water life of a vanishing ocean 435 million years ago

The  brittle star, a  Silurian species new to science,  has been named by international experts for an Irish geologist, Dr Eamon Doyle

The brittle star, a Silurian species new to science, has been named by international experts for an Irish geologist, Dr Eamon Doyle

 

The western hills can look quite glorious in winter, sculpted by a raking sun. Bogs cloaked in dying moorgrass can shine as golden as a lion’s mane. The little blue lakes can gleam like sapphires.

That, I’m getting afraid, was before climate change. Now the west must be shrouded in endless rain and hammered with storms of wind and hail. The hills have been laced with white water, slicing the peat, baring the bedrock, ponding the twisting roads.

But news of a tiny fragment of a mountain has drawn me out of their gloom. A gritty fossil the size of a thumbnail has led me to engage with the giant turmoils of the planet and infinite stretches of time.

Crepidosoma doyleii, a sea-bed brittle-star, belonged to the deep-water life of a vanishing ocean 435 million years ago. A Silurian species new to science, it has been named by international experts for an Irish geologist, Dr Eamon Doyle, a rare honour for an Irish scientist. He made the discovery in the 1990s, on his field research for a PhD from NUI Galway.

Five of the minute fossils, their body-discs and arms remarkably intact, ended up high in the scarp of Muintir Eoghan in the Maamturks of Connemara. Drive south from Leenane and over the rise into the Maam Valley; it’s up on the right, above the clear-felled forestry.

The fossils’ story predates anything in the modern western landscape and was set long before Jurassic dinosaurs, the popular image of an ancient Earth.

In the planet’s Silurian period, 440 to 410 million years ago (and named, if it matters, for a tribe of Celts on the Welsh borders), Earth was recovering from an Ordovician ice age. First land plants were growing in moist regions near the equator and early insects were living among them.

First fish

The first fish with jaws appeared in the oceans and coral reefs expanded. So were the crinoids, the living “sea lilies”, and seabed animals including brittle-stars, spidery relatives of starfish and sea urchins.

Ireland at that time was still in two halves, each a nameless little zone in a crustal plate somewhere south of the Equator. What is now the northwestern portion of Ireland was in the shores of a big and barren Laurentia continent and the southern bit in the shore of the smaller Avalonia. The plates were separated by the Iapetus Ocean, narrowing slowly as they inched towards each other.

Their eventual collision was titanic, folding and refolding the crustal rock into mountains on the scale of the Himalayas. Today’s hills of Connemara and south Mayo are mere relict stubs, worn down by fierce erosion and furrowed by great glaciations of ice.

The last, chaotic throes of the disappearing Iapetus produced volcanic islands, their ash and magma flowing into basins of stormy sea and smothering much of their life. The brittle-stars of Dr Doyle’s discovery were living in deep water, perhaps 50m down. Preserved as moulded forms in compressed sedimentary rock, they ended within the one massive supercontinent of Pangaea.

In the Jurassic period, starting some 200 million years ago, Pangaea split apart. The northern half, Laurentia, then also fragmented, opening basins of the later Atlantic Ocean.

In the hills of modern Connacht, marine rocks from the Silurian stretch from east to west at the borders of Galway and Mayo in what geologists call the Kilbride Formation. It runs from rocks above Lough Mask in Joyce Country almost to the coast south of Killary Harbour, where crowded beds of today’s brittle-stars wave with their long, whiplike arms. They can shed one or two of them readily and grow new ones. Brittle, indeed.

The Kilbride rocks hold changing seabed communities, gathered in beds at the fringe of stormy Silurian seas. They offer fossil bivalves, corals, sea lilies, trilobites. But in the western reaches of Kilbride, Crepidosoma doyleii emerged as Ireland’s first recorded Silurian brittle-star.

This is described, with a map and fossil photographs, in the Irish Journal of Earth Sciences, published by the Royal Irish Academy (online at http://iti.ms/2F68jTV). Its authors include Dr David Harper, who supervised Eamon Doyle’s PhD, and scientists in America and the Netherlands.

Twenty years, it seems, is not inordinately long for international recognition of a new species with a name honouring the discoverer. Dr Doyle is now the geologist for the Burren and Moher cliffs geopark, developing fossil trails in the famous limestone hills.

Ireland’s rocks are remarkably rich in their evidence of ancient sealife and of Earth’s ancient dramas and upheavals. Bringing their stories alive is the challenge of Ireland’s new Unesco geoparks. Dr Doyle’s is the third for the island, after Marble Arch caves in Fermanagh and the “Copper Coast” of Co Waterford with its early mining history

Crepidosoma doylei, soon for display in the National Museum, may justify a fourth geopark in Maam Valley and the hills of Joyce Country.

Michael Viney’s Reflections on Another Life, a selection of columns from the past four decades, is available from easons.com; viney@anu.ie

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