The Rhynie Fossils

Fossilisation is a process that usually happens after much of a plant or animal has decayed. Soft tissues in particular decay fast, meaning that they are not often fossilised. Insects are rarely fossilised due to their soft, ‘squishy’ nature, and fossilised plants are often imprints of the living plant. Yet at Rhynie, the fossils are so perfectly preserved, that they can be studied at the cellular level. Why? By being trapped in the sinter, they were protected from the attack of oxygen and bacteria, meaning that they did not decay. And as chert is such a hard rock, they were not squashed under geological pressure.

Plants

Today at Yellowstone the plants vary from trees, to shrubs, grasses and mosses. They are not adapted to living right next to, or in, the hot, chemical rich waters – though the mosses can live within a few metres of the hot pools. However, the plants can live closer to the cooler pools, and also in areas which are sometimes flooded by water from the springs. In each case, the plants can still become covered in the silica rich waters and so be trapped in the sinter. These are the fossils of tomorrow.

Here at the cool-water Liberty Pool in Lower Geyser Basin at Yellowstone,
grass can be seen living along the edge of the pool.
This wouldn’t happen next to a hot-water pool.
Photo © Lyall Anderson, National Museums of Scotland.

At the time of the Rhynie hot springs, plants had just evolved the ability to live on land – before then, life only existed in water. Like the Yellowstone plants, the Rhynie plants were not adapted to the hot spring waters, and so lived near the cool springs. In the cherts, we find both plant litter, and semi-upright plants in life position. In other words, we are looking at the ‘ground’ as it was when the plants were living.

They were primitive plants at Rhynie. Some showed similarities to today’s mosses and liverworts, whilst others have no modern descendants. The most common plant found there has been called Rhynia, a plant which, like the other Rhynie plants, grew to a height of about 20cm. Other plants found in the chert include algae, fungi and the earliest known lichen.

Animals

Animal life at Yellowstone today is of course, very common. From bacteria to bears, this national park is home to many species. Like the plants though, they can only survive in certain temperatures. For the water-living animals, that limit is about 45oC, much lower than the temperature of the hot spring waters. So the hot springs may be devoid of animal life, but in and around the cooler springs, animals – especially insects – are abundant. It should be safe to say that the insects of today’s Yellowstone will be the fossils of tomorrow, as they become trapped in sinter, just as it happened at Rhynie.

At Rhynie, there have been fewer fossil insects found than fossil plants. The bulk of the fossil insects are found in the silica crusts that formed from the cool spring algal mats. So the insects clearly preferred to live near the cool water, and were not adapted to living near the hot water. Whilst living, hunting and grazing amongst the plants, sometimes, some of the insects would have drowned in the water and so become trapped in the sinter as it formed.

The perfect preservation of the fossils means that tiny insects such as this
extinct type of spider known as Trigonotarbid,
can be studied at an anatomical detail.
Photo © Hans Kerp University of Munster, Germany.

So although the insects are less common than the plants, more fossils and even new species are being discovered as research continues. Those that have been found to date are very important as they represent the early evolution of insects and life on land. Seven types of arthropods have been found so far, including the earliest known Harvestmen spider (‘daddy long legs’), mite and springtail. Other insects found include other spiders, centipedes and crustaceans. Due to the perfect preservation, the anatomies of the insects can be studied. Mouth parts, eyes, eye lenses, muscle tendons, limb joints, genitalia, ‘lungs’, ‘teeth’, gut contents and moulted exoskeletons have all been discovered, photographed and studied.

Find out more about Scotland’s Devonian history and the Classic Site at Rhynie .

The Rhynie Chert Learning & Teaching Resource -
www.abdn.ac.uk/rhynie/index.htm

The Rhynie Chert and its Flora -
www.biologie.uni-hamburg.de/b-online/kerp/erhynie.html

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Acknowledgements

Thanks go to Dr Lyall Anderson of the National Museums of Scotland, and the Rhynie Chert Research Group, for his knowledge of Rhynie and the use of his photographs.