Coral Conundrum

Fossil corals are quite common in the limestone of the Burren. They can be easily seen around the Flaggy Shore and the walking trails at Mullaghmore in the Burren National Park. 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. They were living in a shallow tropical sea similar to the sea around the Bahamas today. It would have been great for snorkelling.

These corals belong to two groups; the Rugosa and the Tabulata and they went extinct during one of the largest mass extinctions in geological history at the end of the Permian period about 250 million years ago. Modern corals belong to a different group; the Scleractinia and they appeared after the mass extinction about 240 million years ago. Although they are superficially similar to the Rugosa, they are quite different in how they build the structure of their skeleton, and they use a different form of calcium carbonate to build it. They are best described as ‘distant cousins’.

Modern corals belong to two broad groups, those that live in shallow, warm water (and make such massive structures as the Great Barrier Reef, Australia) and those that live in deep, cold water and do not form reefs (we have important communities of these off the coast of Ireland). However, despite the undoubted importance of these modern corals to our marine environment, we don’t know where they came.

The question is, did modern corals evolve from a few of the rugosa corals that survived the mass extinction event or did they have much older ancestors? Were the scleractinia already around, well before the mass extinction and only came to prominence once all the other corals had been wiped out?

It is generally considered unlikely that a whole new form of complex coral with different internal structure and composition could have evolved so quickly after the mass extinction.

There have been a couple of fossil corals discovered in much older rocks (470 million years old) that have internal structures very similar to modern corals. However, because there is such a long time gap (over 200 million years) until we see the next modern corals after the mass extinction, palaeontologists had thought that these very old forms were an early evolutionary anomaly that only briefly existed.

One way to identify the real ancestors of modern corals would be to find some in rocks that bridge the 200-million-year gap. The rocks of the Burren fall into that age and so when we am out and about looking for fossils, we keep a very close eye out for any slightly different looking fossil corals that may provide an answer to the conundrum of the origin of modern corals.

Dr. Eamon Doyle, geologist for the Burren and Cliffs of Moher UNESCO Global Geopark.