Gryphaea arcuata

Fossils from the Avon RIGS area.

Gryphaea arcuata

Please read the geologist's code here:-  

  http://www.brerc.org.uk/rigs_site/geologists_code.htm

Name                     Gryphaea arcuata

Phylum                      Mollusca

Class                      Bivalvia

Order                     Pterioidea

Family                    Gryphaeidae

Genus                     Gryphaea

Species                  G. arcuata

Gryphaea arcuata showing 'toenail' and ‘lid’.

Photo credit – Richard Kefford

Other, larger, pictures of this fossil can be found here

Source rock     Lias Group

Age range             Triassic/Jurassic,  Rhaetian 203 Ma – Aalenian 171 Ma

Locations              Several outcrops in area. The one pictured above was found at Hock Cliff in the Sinemurian strata of the Lower Jurassic from about 194 Ma.    SO 722 094

Please note that there are additional risks at this site which include:- unstable cliffs, difficult rocky and muddy foreshore, fast flowing currents and tides which can reach parts of the cliff. 
Check tide times before venturing onto foreshore.
Please see comment below from Andrew Mathieson.

Description of fossil

Gryphaea arcuata is an extinct species of foam oyster, a bivalve mollusc 

in the family Gryphaeidae. Commonly known as Devil’s Toenails.

Their temporal range is from late Triassic to early Cretaceous but their fossils are most commonly found in Jurassic strata.

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 soft parts of 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. These shells are sometimes found in fossil plates along with Turritella, Pentacrinites, Plagiostoma, clams, and sometimes shark’s teeth and fossilized fish scales.

Description of source rock in area – Lias Group.

                   Paleoenvironment

Nearing the end of the Triassic, the landmasses remain united, in the form of the supercontinent Pangea. The Tethys Ocean is actively spreading. The British Isles are at about 30⁰ North.

        During the early Jurassic, Pangea breaks up resulting in Gondwana and Laurasia separating as the South Atlantic starts to rift open. The British Isles continue to drift Northwards to about 40⁰ North. Lithospheric extension and passive rifting occurs to the East, starting to form the North Sea and to the West where the North Atlantic will later open.

        During the early Jurassic, sea levels continue to rise across the British Isles so Triassic desert sediments are replaced by a cyclic succession of fossiliferous dark grey marine mudstones, marls and limestones.

                   Lithology

        Predominantly grey, well bedded, marine calcareous mudstone and silty mudstone; thin tabular or nodular beds of argillaceous limestone, particularly in the lower part; thicker units of siltstone and sandstone, particularly in the upper part, and ironstone, particularly in the middle part. Marginal limestone facies also occur.

Richard Kefford

                  References

             - British geological Survey – Lexicon of rock units