Our Fossil Roots

In sandstone rocks along the wild Atlantic coast of west Clare we find excellent examples of a fossil called Stigmaria. These are elongate, cylindrical or flattened structures with a surface covered in regularly arranged dimples. First recognized as fossils in the early 1800’s the name Stigmaria was introduced for them in 1822 by the eminent French palaeontologist Alexandre Brogniart.

If you come across a Stigmaria or any other fossil, please remember that these fossils are a non-renewable resource and just take a photo.

At first these fossils were thought to be plant trunks or branches, but the discovery of fine rootlets attached to them made it clear they were roots. In fact, the name Stigmaria refers to the dimples, which are the scars (as in ‘stigmata’) of rootlets that extended from the main root. This is important because roots grow in soil and fossil roots with rootlets still attached means the fossil is in a fossil soil, which indicates land. So, at that moment in time we were above sea-level.

Stigmaria is a general term for the roots of a number of different fossil plants known as ‘Lycopods’ or ‘scale trees’. The most common are called Lepidodendron and Sigillaria. They were very abundant during the later part of Carboniferous times from 320-300 million years ago and were the early equivalent of trees, although they are more closely related to modern clubmosses. Unlike modern clubmosses which are small plants, the early lycopods were tall, sometimes reaching 30m in height and they therefore needed substantial root systems for support. Stigmaria can be long, regularly over a metre long and specimens over 6m long are known from similar aged rocks in the USA.

Stigmaria was a shallow, horizontal, branching root system. Extending from the main trunk the roots branched dichotomously, that is they branched, bifurcating evenly in two. Much finer rootlets extended perpendicularly from the main root. This horizontal root system is thought to be an adaptation for growing in wet, swampy conditions and they are most often found associated with coal horizons. Coal horizons accumulated in swamps where the acidic, boggy conditions encouraged the preservation of the plant material.

These abundant plants were significant because of the impact they had on the climate 300 million years ago. Like now, plants release oxygen and absorb carbon dioxide. It is thought that during the Carboniferous period, forests of these lycopods contributed to global cooling by absorbing CO2 from the atmosphere, storing the carbon in the plant tissues and then having that stored in the swamps and bogs after they died, thus removing the carbon from the system.

300 million years later during the industrial revolution we started burning that preserved plant material (coal) in large quantities and very quickly transferred that carbon dioxide straight back into the atmosphere where it started to trap heat, and so global warming began.

So, these fossils tell us not only the lie of the land of West Clare 300 million years ago but also are a reminder of how our actions in using Earth’s resources have consequences.

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