Tracking The Fossils

Ever since the fuss on the radio about the ancient salamander's footprints in a slate outcrop on Valentia island, Co Kerry, I can't take my eyes off the living-room floor. When it was flagged, some 25 years ago, we shrank from the cost of Liscannor limestone and settled instead for a cheap lorry-load of "demolition" slates: thick, irregular purple sheets salvaged from the roof of one of Westport's earliest town buildings.

In the hey-day of Valentia's slate mine in the 19th century, its slates and flags went everywhere: they roofed the Houses of Parliament at Westminster, made miles of shelving for London's Public Record Office, were used in billiard tables, tombstones, altars and garden seats. Also, perhaps, in Westport, to roof some of the bigger buildings in Lord Sligo's new town.

The slates certainly have fossils - rather as if we had splashed paint around when doing the ceiling and forgotten to clean it off. On my knees, with a magnifying-glass, I can imagine some closer detail in what look like creamy cross-sections through the stems of ancient sea-cucumbers. Alas, not a paw-print in sight.

But the Valencia tracks are scarcely the hot discovery that recent reports made them seem. Take up the new edition of Frank Mitchell's Reading The Irish Landscape (Town House, £19) and there, on page 11, is a picture of the meandering trail in the rock surface, like something a tortoise might have left in wet cement. It was taken by Dr I. Stossel, from Zurich, who made the find three years ago. It is hard to imagine any obsessed antiquarian millionaire wanting a piece of this on his coffee table ("Guess how old that is, then. No, go on, have a guess!"). But at some 375 million years old, and perhaps the earliest track of an emergent amphibian yet recorded in Europe, it might have a certain cachet.

The fear of having this momentous matrix gouged out by international fossil-thieves (as has been happening to dinosaur eggs and bones in other parts of the world) is what has put the Valentia rockslab in the news.

I find it touching that Ireland's very oldest known fossils are also the traces of animal movement. The Oldhamia fossils, found in the middle of the last century in the slates of Bray Head, preserve the radiating burrows, like a series of fans, of one of the earliest creatures of the ocean, living more than 550 million years ago. Ireland at that time was still in two halves, somewhere down around South Africa.

By the time the Valencia salamander was walking about, the two halves had welded together, in the general crunching of continental plates, and Ireland was above sea-level, somewhere around the Equator and drifting slowly north. What the amphibian plodded across was part of the great masses of sediment carried down by rivers. This was cemented into what became Old Red Sandstone, now folded up in our south-western mountains and peninsulas, and here and there, as in Valentia, the sandy mud was hardened into slate.

There are some rather beautiful fossils of ancient plants from this Devonian period, including Archaeopteris hibernica (in the National Museum), found in sandstone layers in a quarry at Kiltorcan in Co Kilkenny. This is the sort of plant that later evolved into the great tree ferns of the swamp-forests, the vegetation of the Carboniferous period that later compacted into coal.

The subsequent erosion of Ireland's substance, up to and during the Ice Age, was massive by any standards. The island lost almost all its coal layer and nearly all the chalk and later sediments above that. We are missing some 250 million years of rocks and their fossils, from the Carboniferous onwards: so whatever sort of Jurassic Park Ireland may have hosted was washed into the sea long ago.

The exceptions lie at the north-east corner of the island, where lava from tectonic upheavals, as the Atlantic opened up again, poured out across some of these rocks and protected them from erosion - hence the chalk that bands the cliffs of Antrim. Here and there in rocks on the shore are ammonites, the up-market spiral-shelled molluscs that people hunt for at Lyme Regis, on England's south coast. And along in Co Down, the red sandstone of Scrabo Hill holds a trail of footprints of one, rather tiny, dinosaur with feet about 7cm long.

But for the rest of the island, the great mass of our fossil heritage is embedded in the central raft of limestone laid down by warm seas about 340 million years ago and exposed when the later rocks were worn away. Two grey chunks I filched from a storm beach on a cliff-top on Inishmore (we are all tempted) now sit among the modern sea-shells on my window-sill. One piece is fluted with columns of coral, the other is studded with shells of small, cockle-like brachiopods: some of the billions of animal skeletons that compose the cliffs of Aran and the Burren hills.

From these marine fossils of the Lower Carboniferous to the fossil bones of Ice Age mammals discovered in Ireland's limestone caves is a huge leap across the ages of our missing geology. Irish fossils of woolly mammoth, musk ox and spotted hyena go back no further than 50,000 years, whereas in unglaciated areas of England there are fossil mammals 1.8 million years old (they include, for your imagining, the woolly rhinoceros).

There was something specially appropriate, amid talk about the Valencia salamander, to be summoned to the little pier at Old Head, beside Louisburgh, to inspect the body of an impressive fellow-reptile, a leatherback turtle almost two metres long. This is the largest of all turtles, and instead of a horny carapace is covered with smooth, black rubbery skin, stretched across seven lengthways ridges like the strakes of a currach.

Leatherbacks regularly arrive in Irish waters in late summer, following the jellyfish in the North Atlantic Drift, and almost as regularly one or two get tangled in the ropes of lobster pots, or nets, and drown. With their wrinkled neck, beaked mouth and hooded eyes they have the look of something primeval, and have, indeed, outlived the dinosaurs by 65 million years. This one had been dead in the sea just long enough to attract scavengers, and the little, bluntheaded, carnivorous crustacea spilling from the turtle's bill were among the oldest creatures of all.