Gryphaea

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Gryphaea

Kingdom:	Animalia
Phylum:	Mollusca
Class:	Bivalvia
Order:	Ostreoida
Family:	Gryphaeidae
Genus:	Gryphaea

Gryphaea (Lamarck, 1801), also known as Devil's toenails, are a genus of extinct oysters, marine bivalve mollusks in the family Gryphaeidae. They often exist as fossils from the Jurassic or Cretaceous periods. Gryphaea fossils are common in many parts of Britain.

Fossil bivalve Gryphaea arcuata from the Jurassic of England

These oysters lived on the sea bed in shallow waters, possibly in large colonies. The complete fossils consist of two articulated valves: a larger gnarly-shaped shell (the "toenail") and a smaller, flattened shell, the "lid". The animal occupied the cavity between the two shells, just like modern oysters. The shells also feature prominent growth bands. The larger, curved shell sat within the mud on the sea floor.

A classic location to find these fossils is Redcar on the North-East coast of England. There used to be a common folk belief that carrying one of these fossils could prevent rheumatism.

Note: the name "Devil's toenail" is also used for some fossil species of the genus Exogyra, which is in the same family (Gryphaeidae) as Gryphaea.

The Pycnodonte newberryi (Stanton), is another Gryphaea species from the Cretaceous period, 90 to 130 million years old. Pycnodonte newberryi fossils generally exist in large layers or beds. In the state of Utah, in the USA, in the Capital Reef National Park area this layer of fossils ranges up to ten feet thick. The layer is present in the lower Tununk section of the Mancos Shale Fraction, just above the Dakota Sandstone and below the Blue Gate Shale layer. In the Hanksville, Utah area the Pycnodonte newberryi oyster bed is exposed over a large area. Studies are currently being conducted to determine if there are more than one species in these beds.