Fossil shells known as Gryphaea are amongst the most familiar of Warwickshire fossils. They are commonly known as ‘Devils’ toenails’, due to their broadly curved shape, which looks a bit like a claw. These fossils are commonly picked up in fields and gardens and people often bring them in to the Warwickshire Museum for identification. They are the shells of extinct oysters, dating back nearly 200 million years ago to the beginning of the Jurassic Period. Most come from the clay and grey limestone bedrock that underlies much of southern and eastern Warwickshire. They prove that these sedimentary rock layers were once seafloor mud.
Gryphaea shells are made up largely of the mineral known as calcite, just like modern oysters. Oyster shells are strong and thick, which is why our Jurassic fossil examples have survived so well, over millions of years. Sometimes the original growth lines can be seen on the surfaces of the fossils.
A complete Gryphaea fossil consists of a larger ‘toenail’-shaped shell and a smaller, flattened ‘lid’. The animal occupied the space between the two shells. The larger, curved shell sat within the sea floor mud andGryphaea oysters must have formed dense colonies on the Jurassic seabed.