What are fossils?
Most animals and plants rot and decay when they die, but sometimes they become fossilised. This (usually) happens when they fall into soft mud and are buried.
Over time, the mud and sand compress and turn to rock, along with the creatures inside. People can find fossils by breaking up these rocks to reveal the creatures petrified within.
We generally think of fossils as being millions of years old (like dinosaur remains), but they can be thousands or only hundreds of years old.
Plants and insect fossils
The remains of archaeological plants and insects are preserved in several ways.
They may be found in very wet or very dry environments (where lack of oxygen prevents decay) or because they have been burnt (like charcoal or ash remains from a fire).
Another way is because they have been ‘mineral replaced’. These plants and insects become fossilised because salts replace their organic tissue.
This can happen because they get trapped in sewage or faeces.
What are mineral salts?
Mineral salts are present in bone, tooth enamel, dairy products and plant material. They are also in the food we eat and the faeces we produce.
If there are sufficient salts from, for example, decaying faeces, animal dung or dumped organic waste, and enough liquid, the salts can move into the remains of plants and insects and replace their tissue.
The remains are effectively turned to stone.
Where are plant and insect fossils found?
Typical places where this happens are ancient toilets, cesspits or sewers. Mineralised waste has been found in the sewers of Roman Herculaneum.
The giant fatbergs from the sewers of London are modern examples. They can also be found in massive rubbish heaps like the late Bronze Age midden at Chissenbury on Salisbury Plain.
Various food plant remains can be found in these remains (mainly fruits) and types of flies associated with rotting wet environments.
What can plant and insect fossils tell us?
Fossilised plant foods can tell us what people ate in the past and their living conditions. They can even show us whether (and how) their food was contaminated, for example, with toxic plants or insects.
The above image shows the impression of a seed of corncockle preserved in faecal material.
Corncockle is a poisonous plant that has historically contaminated wheat and was consumed in food such as bread. It would have made the food taste bitter and could have made the person quite ill!
Mineralised Plant and Invertebrate Remains
We have published a new photographic guide to Mineralised Plant, and Invertebrate Remains if you want to learn more.