Riddle of the shells

This is an extract from the newsletter of The ‘Black Country Geological Society‘, No. 201 June 2010. This was part of the regular section entitled ‘The Dudley Bug’ written by members Alison Roberts and Chris Broughton.

Counting the brachiopods

During April 2010, as part of a KITTS graduate training placement at Dudley Museum we visited the Wrens Nest to see what are the most common fossils found in two of the main formations found at the national nature reserve. The two formations which were chosen was the 430 million years old  Much Wenlock Limestone Formation, Nodular Member and the older Coalbrookdale Formation. After a short period of collecting we returned to the museum where we laid the fossils out in rows of the same species. What we found was rather interesting.

Atrypa reticularis brachiopods

Firstly, Atrypa reticularis was the most common brachiopod at both locations, suggesting that this was the most successful species throughout the Wenlock Epoch. There were significant differences between the Atrypa brachiopods from the older Coalbrookdale Formation and the younger, shallow marine Much Wenlock Limestone Formation. The Coalbrookdale Formation is associated with deep marine conditions as the sediment is a lime mud, deposited in low energy conditions. In this environment the brachiopods were larger and had wings on their shells, possibly to spread their weight on the surface of the soft sediments, whilst in the Much Wenlock Limestone Formation, the Atrypa brachiopods were much smaller in size due to living in the shallow reef mound conditions. The photo opposite displays the size and shape variations between the Atrypa species, possibly due to environmental pressures (Coalbrookdale Formation on the left). The ages between these brachiopods could be around 3 to 5 million years, so could this be evidence of evolution within this species?

Eospirifer brachiopods

This pattern is also apparent when the Eospirifer sp.brachiopods are studied. The Coalbrookdale specimens are considerably larger than the Nodular Member specimens. This can be seen in the opposite photo with the Coalbrookdale Formation specimen on the right. Further differences noted the lack of tabulate (colonial) corals in the deeper Coalbrookdale Formation, which are so abundant in the shallower reef mound environment.

Chris Broughton
Geologist
Wolverhampton Art Gallery

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