How Historic Human Poo Can Preserve Plant Fossils

What can plant and insect fossils tell us about the way people ate? Archaeologist Dr Ruth Pelling explains all.

What are fossils?

When they die, most animals and plants rot and decay, but sometimes they become fossilised. This (usually) happens when they fall into soft mud and are buried.

Ammonite fossils from Lyme Bay, Devon and Dorset © Ruth Pelling
Ammonite fossils from Lyme Bay, Devon and Dorset © Ruth Pelling

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 insects

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).

Photograph of fossilised bracken
Bracken can sometimes be found in cesspits where plants were used as toilet wipes © Historic England

Another way is because they have been ‘mineral replaced’. These plants and insects become fossilised because their organic tissue is replaced by salts.

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.

Photograph of fossilised grapes
Grapes have been found in urban medieval and post-medieval cesspits © Historic England

If there are sufficient salts from, for example, decaying faeces, animal dung or dumped organic waste, as well as enough liquid, then the salts can move into the remains of plants and insects and replace their tissue.

The remains are effectively turned to stone.

Where are these 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 interior of one of the sewers at Herculaneum, prior to excavation © Erica Rowan 2016
The interior of one of the sewers at Herculaneum, prior to excavation © Erica Rowan 2016

The giant fatbergs from the sewers of London are modern examples. They can also be found in giant rubbish heaps like the late Bronze Age midden at Chissenbury on Salisbury Plain.

A variety of food plant remains can be found in these remains (particularly fruits) as well as types of fly which are associated with rotting wet environments.

Photograph of cesspit fly
Cesspit fly are widely found in urban faecal deposits, particularly Roman and medieval cesspits © Historic England

What can these fossils tell us?

Fossilised plant foods can tell us about what people ate in the past, and about their living conditions. They can even show us whether (and how) their food was contaminated, for example with toxic plants or insects.

Photograph of Corncockle
Corncockle, poisonous in high concentrations, has been found in Anglo-Saxon and medieval cesspits © Historic England

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!

If you’d like to learn more, we have published a new photographic guide to Mineralised Plant and Invertebrate Remains.

Further reading:

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