"What is a tetrapod?" we wondered when we looked at our tour itinerary to Valentia Island. Located off Southwest Ireland, in Co. Kerry, Valentia Island is famous for its tetrapod trackway.
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| Tetrapod footprints in rock, County Kerry |
| Photo © Barb & Ron Kroll |
Tetrapod footprints are the oldest evidence in the world of four-legged vertebrates (animals with backbones) walking on land. Eventually, tetrapods (Acanthostega) evolved into mammals and ultimately into humans.
How old are tetrapod tracks?
The fossil trackways were formed 385 million years ago, during the Middle Devonian period, when Ireland was part of the southern hemisphere continent of Euramerica.
No mammals existed then. Neither did dinosaurs, which first appeared in the Jurassic period, 155 million years later. Radioactive dating of nearby volcanic ash deposits determined the age of the tetrapod trackway.
In 1993, Iwan Stossel, a Swiss geology student, discovered the tetrapod trackway location in the northeast region of Valentia Island. It is now owned by the Ireland Government.
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| Tetrapod trackway sign |
| Photo © Barb & Ron Kroll |
Where is the tetrapod trackway?
Finding the tetrapod footprints is not easy, so we were happy that our knowledgeable tour guide, Frank Walsh, knew how to get there.
He drove us along a road lined with orange montbretia flowers, pink fuchsia and purple heather, to The Lighthouse Cafe, where he parked the van. Walking past a thatch-roofed white stucco house with flowers painted on the walls, we crossed a cattle grid.
Ignoring a "Beware of the Bull" sign, we continued to the Dingle Bay coast. A Tetrapod Trackway sign directed us to a cliff. Climbing down the rocks, we found the stone footprints behind a fence.
What do tetrapod footprints look like?
Two parallel rows of 150 basset hound-sized tracks form the curved track. Frank Walsh pointed out faint marks where the Valentia Island tetrapod dragged its tail.
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| Valentia Island tetrapod tracks |
| Photo © Barb & Ron Kroll |
How was the fossilized trackway formed? After the Devonian tetrapod walked over soft silt flood deposits, another sediment layer buried the footprints before water could wash them away. In time, they dried and were compacted into rocks.
The Valentia Island tetrapod footprints are the most extensive of the four Devonian trackways in the world. (The others are in Tarbat Ness, Scotland, Genoa River, NSW Australia and Glen Isla in Victoria, Australia.)
Ripple marks
We also saw fossilized ripple marks, which formed in sheets of silt deposited after rivers flooded. New silt deposits covered the ripple marks after the pools dried out.
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| Water ripple marks in stone |
| Photo © Barb & Ron Kroll |
Eventually, high pressures turned the silt to stone. Coastal erosion allowed us to see these ripple marks, made 385 million years ago.
What did tetrapods look like?
The first water-dwelling creatures, which crawled out of the water and became the first animals to walk on land, looked like large lizards. Tetrapods (animals with four feet) were about one meter (3.3 feet) long.
Are tetrapods missing links? Before tetrapods walked on land, all known vertebrates were fish. Gradually, their fins evolved into primitive legs and amphibians developed the ability to breathe air outside of water.
English and Irish/Gaelic signs on Geokaun Mountain describe the Valentia Tetrapod Trackway, how the tetrapod footprints were preserved in the wet mud and what the Devonian World was like when tetrapods made their tracks.
TRAVEL INFORMATION
Tourism Ireland: www.discoverireland.com
More things to see and do in Ireland:
Irish Breakfasts at Ireland B&Bs
Gap of Dunloe Tours - Hike, Bike or Jaunting Car?
Killarney Ireland Hiking Trail - Co. Kerry
