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

'Lithostrotion'

Fossils of Avon RIGS region 

'Lithostrotion'

Please read the Geologist's code here:-

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

Name: Siphonodendron (formerly Lithostrotion) martini

Phylum: Cnidaria

Order: Rugosa

Horizon: Carboniferous

Photo credit - T. Bolton

Source rock: Clifton Down Limestone Formation

Age: Tournasian stage of the Lower Carboniferous Period (345 - 359Ma)

Locations: Avon George, Broadfield Down, Mendip Hills. Tickenham Ridge

Note: The underlined area is the type location for this Formation

Description of fossil

Siphonodendron martini corals were colonial, cylindrical and showed growth increment bands. Increment bands arise as a result of daily changes in light (which would affect the algae living in the outer cells of the organism) or temperature as well as monthly changes, associated with the lunar cycle. In the cool season or at night the corals secrete less calcite and so monthly and daily bands are visible on the surface of the corallites. Devonian corals studied at a different location show 400 daily bands in a year while these in the Lower Carboniferous have 391. This would indicate that the earth is slowing down on its rotational axis with tidal friction thought to be a major causal factor. The decrease in speed of the Earth's rotation is shown to accelerate in the Lower Carboniferous due to widespread occurrence of shallow shelf-seas.

Siphonodendron martini has a solid rod columella, major and minor septa (28 septa in each order) and dissepiments. In some areas many corallites did not reach maturity, only growing to 3mm. in diameter. Three ecotypes of the fossil are identified probably representing growth stages in the coral. The third ecotype is characterised by small radius, comprising only twenty septa and a single row of dissephnents. Some samples showed rejuvenescence i.e. the corallites reached adult dimensions then stopped growing only to recommence with smaller radius and more juvenile features. Measured growth was reduced from 4 mm a month in the limestone down to 2 mm a month in the calcareous shales where stunted growth is also observed to affect the diameter of the corallites. In the shale, rejuvenescence took place and there was little evidence of asexual breeding or monthly bands as the increase in nutrients from high levels of sedimentation is likely to have restricted breeding. Alternatively, a decrease in light levels  light may have affected the algae living in the outer cells of the organism, have reduced breeding and thus eliminated the monthly banding.

Photo credit - T. Bolton

Description of source rock ( Clifton Down Limestone )

Paleoenvironment

During the Lower Carboniferous the British Isles were just south of the equator and under water. As the Laurasian continent drifted north a large delta carrying eroded sediments from the north deposited its load in the sea, eventually killing the coral reefs and starting the Millstone Grit beds – forming the Quartzitic Sandstone Formation in the Avon RIGS area.

Lithology        
The lithology is dominated by calcite mudstones with a locally abundant but low diversity fossil assemblage. At Burrington Combe the formation is about 170 m thick, and three subdivisions can generally be recognised across the Mendip area. The lowest unit comprises a mixture of calcite mudstones, white oolitic limestones and dark splintery limestones. This interval is relatively expanded in the Cheddar area, where a 38 m thick dark limestone ('Cheddar Limestone Member') is overlain by a 58 m thick white oolitic limestone ('Cheddar Oolite Member'). The middle part of the succession is dominated by fine-grained, grey-black limestone with nodules and bands of chert and abundant remains of the coral Siphonodendron['Lithostrotion'] martini ('Lithostrotion Limestone'). Porcellaneous calcitic mudstones dominate the highest part of the formation, including locally developed algal mudstones and stromatolites, indicating deposition in a very shallow-water, near-shore or lagoonal environment.

Richard Kefford

References

- British geological Survey – Lexicon of rock units

- Natural History Museum – British Natural History
- The Black Country Geological Society – Newsletter No. 129.  June 1998.
- BGS. 1:50.000 series. Bristol 264 (S & D)
- BGS. 1:25,000 series Clevedon and Portishead. Geological Sheet ST 47.
- Post by T. Bolton on www.ukfossils.co.uk/ forum - photo credits