Fossil Shells

22°C, 5.50 pm.
THESE PIECES of sandstone at the corner of the pond provide access to the water for birds and perhaps some cover for newts and frogs. They also help disguise the edge of the black plastic pond-liner. The first pond that we dug when we moved here was lined with builder’s damp-proofing plastic – blue polythene sheeting – which wasn’t resistant to ultra-violet light. I covered the edges with turves and spread garden netting across the bottom of the pond and pressed sub-soil from the hole we’d dug into it. This produced a more natural-looking pond than our present one but it had a tendency to wick water away and the netting wasn’t a good idea; on one occasion I saw a frog that had been drowned when it had gone down into the mud and got caught in it.

Brachiopods

I noticed these impressions of fossils shells in the rock on the far right (top). Superficially they look like cockle-shells but these fossil creatures weren’t bivalve molluscs; the Brachiopods (the name is from the Greek meaning ‘arm-foot’) appeared in the Cambrian explosion of life on Earth and have been around for 570 million years. Three thousand fossil species have been described but today only about 100 living species remain. They have suffered from competition with bivalves such as cockles, oysters and mussels.

Modelling clay cast of brachiopod. I can't decide whether this is a complete shell or whether these are two valves from separate individuals which just happen to have fossilised together.

These fossils are negative impressions of the shells so I pushed a piece of modelling clay into the hole and made this positive  cast of the original shell. When this shell was last visible on the surface of the earth it was on the seabed at a time when our part of the Earth’s crust lay close to the equator, some 300 million years ago in the Upper Carboniferous period. As the fossil is in sandstone, I guess these brachiopods must have been living near the mouth of a river.

The ribs on the shell bear this out as they indicate that this species of brachopod was adapted to live in shallow water in a strong current. A smooth shells would indicate a species that lives in deeper, calmer waters. That zig-zag line which marks the opening of the shell is known as the commisure (below right); brachiopods developed a folded commisure to increase feeding area while preventing sand particles entering.

Brachiopods stood on a pedicle stalk anchored on the seabed and opened and closed the two valves of their shells to feed. The hole in the shell where the stalk protruded led to them being given the name ‘lamp-shells’, as the larger valve resembles a Roman oil-lamp, with a hole for the wick.

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