A Foot in the Slime
If you had asked me three years ago about open water swimming, I would probably have given you a luke-warm response. Having been persuaded by my partner Rachael to give it a go I can conclusively say that luke-warm is not a suitable response to open water swimming. The North Sea is categorically Baltic in the winter months and swimming at these times comes with a generous cake-slice of insanity, albeit mixed in with a vibrant sense of being alive. One of our favoured spots for swimming is in Whitley Bay from the Panama Swimming Club clubhouse.
The beach here is sandy, but with a variable sized collection of stones strewn across the surface and a reef of beautifully striped sandstone pointing from the Spanish City out towards St Mary’s lighthouse. As an unreconstructed geologist I can’t help scrutinizing the pebbles on the beach as I trepidate towards the freezing water. For the most part these are a collection of water-worn sandstones (some with beautiful stripey patterns) and limestones not infrequently spotted with the remains of the communal coral Siphonodendron. On occasion there are small brown, flat slabs of hard sandy limestone to be found, filled with cream-coloured smiles. These are the cross-section of fossil bivalves preserved in multitudes within these stones. It is one of these stones that features as mystery rock number 23 for the WallCAP Newsletter.
A visit to the beach at Whitley Bay in early March 2021 would not have been the time to go for a swim. A storm the previous weekend had almost completely stripped the beach of its sand, leaving a wasteland of boulders and pebbles. It also exposed more layers of bedrock than the regularly visible reefs of stripy sandstone. Amongst these newly exposed layers was the source of the fossil smiles, a layer no more than a few inches thick just above the stripy sandstones and containing thousands of these fossils.
This layer of rock is the Low Main Mussel Band, which lies just above the Low Main coal seam after which it is named. The mussels, a type of bivalve, are of three species, Carbonicola (which used to be called Anthracosia), Anthrocanaia and Naidites. As usual with fossils, their presence helps unravel what was happening when these layers of rock were being laid down. In addition, this type of shelly layer is found scattered through geography and time in the sequence of late Carboniferous rocks (the Pennine Coal Measures Formation) of Tyneside. So, what are these fossils and what do they tell us?
In the very dim and distant past, I used to belong to the 1st Central scout group which met in a hut adjacent to the underground sidings behind Morden Station in south London. In this pack I rose to the dizzy heights of being a sixer, a role, which brought variable results. At a scout camp competition where we were to show off our camping and woodsman skills, our “six” successfully dug a magnificent latrine. This was discovered inadvertently during the night by one of the judges who fell into it. What we learned was that the middle of a footpath is not a good place for a lat-pit (and we didn’t do well in the competition). On a brighter and better occasion, we camped at a Longridge on the River-Thames near Marlow, a campsite which mercifully had flushing toilets. One of the main purposes of this camp was to learn how to canoe. Learning how to canoe, it seems, involves a great deal of falling out of canoes and a great deal of close up familiarity with the river. One of the things I discovered through multiple visits into the murk of the Thames water was that my feet on the riverbed squelched into layers of silty mud. To my surprise, within this mud lived some magnificent bivalves several inches long and with beautiful glossy green shells. These were swan mussels (Anodonta cygnea). I recollect my surprise at finding bivalves in a river rather than in what I thought of as their natural habitat, which is in the sea. It is the case that the modern landscape of sea-shells is dominated by the phylum of Mollusca either in the form of bivalves (cockles, mussels, clams and the like) or as gastropods (winkles, whelks, limpets and the like). There was, however, a different story to be told of the Carboniferous bivalves, which finally returns us to the question I asked a paragraph back.
The Carboniferous bivalves to be found in the Low Main Mussel Band are, like the River Thames’ swan mussels an indicator of fresh or brackish water. This marks a significant change from the swamp-land conditions in which the coal of the Main Coal seam was forming and may relate to a rise in sea-level. These mussel bands, of which there are many in the Pennine Coal Measures Formation, are surprisingly continuous over many kilometres. This makes them a useful signpost, not only of the geological conditions in which they were laid down, but also of where you are in the rock sequence and in time. A coal miner coming across the Low Main Mussel Band would know that they were immediately above the Low Main Coal seam. Each of the other mussel bands scattered through the Pennine Coal Measures Formation has different types of bivalves (and other fossils) in them. This was not just down to the mussel beds forming in different environments (which favoured one or other type of creature) but also a result of evolutionary change. In the time interval between the formation of different mussel bands, some creatures had become extinct and new creatures had evolved.
So what happens if we make the comparison between a river or estuarine environment and a marine environment in the Carboniferous as we did for the modern day? It reveals that the dominant species of sea-shell in the Carboniferous were not molluscs of any stripe, but another group of creatures with two shells, the brachiopods. The Brachiopoda form a phylum in their own right completely separate from the Mollusca. The name brachiopod comes from the ancient Greek and means arm-foot. The “foot” is the most obvious part of a live brachiopod, with a muscular column called a pedicle, which extends from the bottom (ventral or pedicle) valve of the animal. Brachiopods use this foot to secure themselves to the sea floor. The “arm” is found on the inside of the shell as part of the upper (ventral or brachial) valve. It is not an arm in any mammalian sense, rather a support structure for a part of the animals feeding apparatus called a lophophore, a whiskery, horseshoe shaped structure. These brachial structures, which support the lophophore vary in their complexity. For example, in the brachiopod family of Spirifers (common in the Carboniferous) they form elegant spiral structures which give the Spirifers their name.
Brachiopods are also known as lamp shells. This comes from the similarity in shape between the brachiopod family of Terebratulids and Roman oil lamps. The name Terebratulid comes from the Latin for hole borer (terebra) – the reason for the derivation is not clear, but maybe because the small circular hole in the shell, through which the pedicle would have emerged and
looks like it has been drilled, is so clearly seen in these animals. The Terebratulids are also one of a handful of brachiopod orders that survived the largest mass extinction event, known as the Great Dying, at the end of the Permian period 252 million years ago. This event also marked the fulcrum around which the balance of ecological power
between the molluscan bivalves and the brachiopods hinged. The brachiopods are remarkable in their persistence through geological time. There is one species, Lingula, which may be found in brackish estuarine sediments (hostile to many organisms) and there is a remarkably similar (although evolutionary distinct) burrowing brachiopods which is preserved in rocks of Cambrian age (circa 550Ma). However, the rich list of brachiopod species and the range of ecological niches they filled during the Palaeozoic era (including the Carboniferous period), has now been usurped and almost entirely filled by molluscan bivalves. Next time I visit Whitley Bay for a bracing dip, it will be the limpets and cockles (molluscs all) I will share the swim with, whilst I contemplate the brackish waters of the Low Main Mussel Band and their ancient cousins.
Attributions
Swan Mussel: Anodonta cygnea. Jakob Bergengren, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Mussels: Derrick Mercer, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
Cockles: Cardium indicum Lamarck, 1819 – hians cockle. James St. John, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
Spirifer: Two specimens of Spirifer striatus (named as Spirifera striata in the original). From Plate XXXI of Monograph of British Fossil Brachiopoda Volume 4 Part 3.
Roman lamp: Ancient Roman oil lamp in D. Diogo de Sousa Museum, Braga, Portugal.. Joseolgon, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
Terebratulid: Terebratulid brachiopod from the Campanian (Upper Cretaceous) of southwestern France. wilson44691, CC0, via Wikimedia Commons
Lingula: Lingula anatina shell found in the Mediterranean Sea, in a laboratory of practices of the Faculty of Sciences of the University of Corunna. I, Drow male, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Lingulella: Lingulella lingulaeformis Mickwitz, Leptembolon lingulaeformis (Mickwitz, 1896). Estonian Museum of Natural History, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons