Devils’ toenails in Warwickshire

Fossil Gryphaea shells from Fenny Compton, Warwickshire

Fossil shells known as Gryphaea (pronounced ‘gry-fee-a’) or ‘Devils’ toenails’ are commonly found in fields, gardens, and old quarries in southern and eastern Warwickshire.

These distinctive curved fossils are a type of extinct fossil oyster. Most originate within the Jurassic bedrock which underlies much of the region. The fossils show us that a shallow sea covered this area nearly 200 million years ago.

Like modern oysters, Gryphaea shells are made of the mineral known as calcite. The shells are strong and thick, which is why they have survived so well to the present day. Sometimes the original growth lines can be seen on the surfaces of the shells. Occasionally they preserve the remains of worm tubes and other sea animals which grew on their surfaces.

A complete Gryphaea fossil consists of a larger ‘toenail’ shaped shell and a smaller, flattened ‘lid’. The animal occupied the cavity between the shells, just like a modern oyster. The larger, curved shell sat within the sea floor mud. At times Gryphaea oysters must have formed dense colonies, covering many square kilometres of the Jurassic sea bed.

The geology gallery at Market Hall includes a display of local Jurassic fossils including devils’ toenails.

Gryphaea

A fossil Gryphaea shell (‘Devil’s toenail’) from Warwickshire Museum’s collection. Note the thicker, lower ‘toenail’ shell and the flattened ‘lid’. The fossil is about six centimetres long