Large brachiopods are quite common in the limestone of the Burren. They can be seen in walls and loose rocks and in the rock of the limestone pavement that we so enjoy walking over. They were alive and thriving about 330 million years ago when the area that has become the Burren was over 5,000 km away, near the equator. The large brachiopods that are found in the Burren are called Gigantoproductus, not because they are gigantic but because they are relatively large compared to most other brachiopods, living or fossil. There are other fossil brachiopods in the limestone of the Burren, but the big brachiopod Gigantoproductus is the one you are most likely to see.
Brachiopods look like bivalves such as scallops or cockles (which are types of mollusc) because they have two opposing shells that they can open and close, however internally they are very different because brachiopods have a feeding structure known as a lophophore, which is different to the gill system used by bivalve molluscs. Brachiopods evolved at the same time as bivalve molluscs around 530 million years ago when many creatures first started making protective calcareous shells for themselves and thousands of species have evolved since then.
Brachiopods are alive and well and living in all the worlds oceans today, however you are unlikely to have come across one unless you scuba dive, as they are small and live in deep seas with little light. Today they are hugely outnumbered by their bivalve mollusc cousins which are easy to find as they are common in shallow water and can be found along any coast. That wasn’t always so, in the past, brachiopods dominated the seas; during the Carboniferous Period, 330 million years ago they would have been much more abundant than bivalves. However, they were decimated by the Permian mass extinction 250 million years ago. Their slower metabolism likely hindered their recovery and bivalve molluscs took the opportunity to take over ecological niches left vacant by the extinct brachiopods.
It is interesting to think that when the limestone in the Burren is dissolved by rain, the fossil brachiopods in the limestone are also dissolved and washed into the sea. The calcium and bicarbonate ions then become available for living brachiopods (and other shelled creatures) to extract from the water to make their shells today. Some of these new brachiopod shells will in turn be buried and preserved as fossils. This is an example of the Rock Cycle.
However, a warning is required; if global warming continues and ocean acidification continues the chemistry of the oceans will change so that it will be impossible for the shelled creatures in the sea to make their shells and we will have created another mass extinction.
Dr. Eamon Doyle, geologist for the Burren and Cliffs of Moher UNESCO Global Geopark.