Neale Monks (UK)
Modern brachiopods are rather obscure animals and even their (supposed) common name, ‘lamp shells’, means little to the average amateur naturalist. However, geologists will be much more familiar with them, because brachiopods are among the commonest fossils in sediments of Palaeozoic age, almost right the way through from the Middle Cambrian to the Late Permian. They were sometimes abundant in the Mesozoic as well, particularly during the Jurassic, and may be quite common in some Cainozoic sediments too. But, on the whole, their post-Permian history was one of decline to a role in marine ecosystems far below that of, say, bivalve molluscs or crustaceans.
Having said that, brachiopods have always fascinated me because they were survivors. Unlike so many of the superstar fossil groups, like trilobites and ammonites, brachiopods declined but they did not die out. They are not particularly diverse today, but the 350 or so modern species is not a bad tally, and they can be found in all the world’s oceans, from Scottish sea-lochs to the coast of California, from Hong Kong harbour to the rocky shores of Patagonia. They may be bit-part players in contemporary marine ecosystems, but, in their own way, they seem to be very good at what they do, more than holding their own in seas and oceans dramatically different to the Cambrian ones, where they first evolved.
While studying for my degree at Aberdeen, I was lucky enough to do a small research project on a modern brachiopod species, Terebratulina retusa, a species found all around the UK coast. My specimens were taken from the Firth of Lorne, where they live attached to horse mussels (Modiolus modiolus), often alongside a second, oyster-like brachiopod called Crania anomala.
What we were interested in was why starfish didn’t eat the brachiopods – a good question, because brachiopods lack the defences sported by the bivalve molluscs, which, at first glance at least, seem to occupy a similar niche. Bivalves tend to burrow into the sediment to avoid being seen by predators, to swim away from predators if they get too close or else to live in an intertidal habitat, where predators do not have good access to them. Clams and cockles are typical examples of burrowing bivalves, scallops and file shells examples of swimming bivalves, and mussels and oysters examples of intertidal bivalves.
By contrast, brachiopods seem to lack all these defences. However, when I offered brachiopods to three different species of starfish, they were completely ignored. Why? To cut a long story short, my hypothesis was that they were not worth the bother. Compared to the amount of shell the predator would have to work through, there just wasn’t much meat, and what meat there was contained spine-like mineral fragments that the predator would need to remove during digestion. In the same way a fisherman would not waste his time landing a small, bony fish with very little usable meat, so too would starfish reject brachiopods in favour of meatier prey.
To be honest, I have no idea if this hypothesis is correct, but my interest in brachiopods has remained with me all my life. Besides being fascinating animals in a low-key sort of way, their fossils have the double advantage of being both common and attractive.
This article will concentrate (but not exclusively) on one fascinating order of brachiopods: the Productida. These first appeared in the Early Devonian, reached their greatest diversity and abundance during the Carboniferous and Permian, but quickly faded away to extinction in the Early Triassic.
Like other brachiopods, productids had soft bodies enclosed by two shells known as valves. One valve – the ventral valve – was deep and bowl-shaped, while the other – the dorsal valve – was flat and lid-like. However, when examined carefully, productids will be seen to be rather more subtly shaped than simple bowls with lids. The valves are actually gently folded into a three-lobed shape, so that when viewed from the front, the commissure (the line between the two valves) dips down, then up and then down again. In modern brachiopods, water flows into the animal from the sides and out through the front, and it is generally assumed the same thing happened here, with the folded commissure being a way of separating the two incoming currents from the one outgoing current.
In addition to this gentle folding of the commissure, the left and right edges of the valves were commonly drawn into wing-like extensions known as alae. These probably served to increase the surface area of the shells, about which more will be said shortly. Unfortunately for collectors, these alae were thinner than the main part of the shell and often break off when the fossil is being removed from the matrix. However, with the bigger species at least (including Gigantoproductus), they are usually sufficiently robust to be preserved to some degree.
As well as these lateral extensions, productid brachiopods were usually ornamented with longitudinal ribs, rather like those seen on modern scallops, though quite a bit finer. Most seem to have had long spines extending from these ribs, although these are rarely well preserved on fossil specimens. There is considerable variation in the strength of the ribs and the length of the spines, and these features are often important in distinguishing different species; Horridonia horrida, for example, is a Late Permian species that gets its name from the Latin word horridus, which means ‘bristly’ or ‘prickly’.
One last and unusual feature worth mentioning is the structure known as the trail. Despite the name, this structure does not have anything to do with movement. As mentioned earlier, the two valves of the brachiopod meet along a line called the commissure. If you think of the valves as lips, then the commissure is the line in between them, that is, the mouth. On most other brachiopods, the commissure lies flush with the front of the animal, but, in productids, it often curved upwards and away from the valves; and like the rest of the shell, it bore ribs and sometimes spines. Fossils of this bit of the brachiopod look like ground that has had a miniature plough dragged over it, hence the name ‘trail’.
The function of this seems to have been analogous to the siphons of modern clams, ensuring that even if the rest of the productid was buried under sediment, its commissure would be above the sediment and so the animal could pump water in and out of its mantle cavity. If it did work this way, it was a neat trick and some palaeontologists believe that many productids did in fact adopt this infaunal habit, making them much more difficult for predators to find than their cousins out in the open.
While most productids shared the same basic shape, they varied dramatically in size. The biggest ones, like Gigantoproductus giganteus, can measure as much as 30cm in width, while the smallest ones, such as Muhuarina haeretica, barely reach 1mm. Notwithstanding such ‘micro-productids’, the average productid was actually quite large by the standards of brachiopods, and even casual collectors will have specimens that would have measured a good 5 to 10cm in width when alive. They also had quite thick shells, which is one reason why productid fossils are so commonly and easily collected.
As mentioned earlier, the lateral extensions of the shell are believed to have increased the surface area of the animal. That was important because, unlike other brachiopods, they rested directly on the seafloor. Most other brachiopods attach themselves to firm surfaces, such as rocks, using a strong, adhesive stalk known as a pedicle (which, incidentally, secretes one of the strongest glues known in nature). However, productids were adapted to life on sandy, muddy or gravelly substrates, and they needed a wide shell to stop themselves from sinking into the sediment. In effect, their shells were flattened out like snowshoes to spread out their weight as efficiently as possible.
Besides the alae, it is often assumed that their long spines helped the brachiopod in the same sort of way, partly by spreading out its weight and partly by keeping the brachiopod in a stable orientation. This second function is important, because productid brachiopods could not move themselves, so if the current rolled them over in such a way, they could not open their shells and they would quickly suffocate or starve to death.
However, some alternative theories have been put forward by palaeontologists who have noticed some very odd things about the spines on at least some productid brachiopods. For a start, the spines are hollow, which seems an unnecessary modification for structures used only for stability. One explanation for this is that lengths of sensitive tissue ran from the body of the animal through the tube-like spines, turning the spines into touch or taste receptors. Should a predator like a starfish come too close, it would be detected by the spines and the brachiopod would know to close its shell before the predator got too close.
An alternative explanation is that the tissue running through the spines was able to secrete some sort of cement or adhesive the brachiopod used to fix itself in place. While fossil productid brachiopods are often found in seafloor deposits, it does not necessarily follow that that is where they lived, and the fossils of at least some species frequently have other organisms cemented to them, including bryozoans, crinoids and other brachiopods. As many as 85% of the specimens of the productid brachiopod Heteralosia slocomi collected at one locality in Nevada in the USA have been reported as being attached to bryozoans. Was it the case that, in life, these productids were attached to other organisms using their spines and it was only when they died that their shells ended up scattered about on the seafloor?
Most brachiopods are fairly small animals. The biggest living species is the subantarctic species Magellania venosa, a species that grows to a shell length of about 10cm. Going back a couple of million years to the Pliocene and you can find Terebratula species that grew to a similar sort of size. British geologists usually call their big Pliocene brachiopods Terebratula maxima, and it can be found in both the Red Crag and Coralline Crag at places like Orford and Walton-on-the-Naze. However, there were several other species that lived at about the same time, superficially at least, all very similar in size and shape.
While the bigger Magellania and Terebratula can be big enough to fill a man’s hand, they are small fry compared to the biggest productids like Gigantoproductus giganteus. At up to 30cm in width (including the alae), these were the biggest brachiopods to have ever lived, and that’s what makes them such fun to collect. In the world of brachiopods, they’re veritable giants.
However, while I’ve visited many Carboniferous Limestone exposures and quarries over the years, I never managed to find any Gigantoproductus. That was, until my partner and I went to North Wales to help celebrate her brother’s birthday …
My Gigantoproductus specimens came from an exposure of the Lower Carboniferous (Clwyd Limestone Group) at the cliffs near Caim, a small village on the eastern edge of Anglesey.
Once you have driven to Caim, getting to the exposure is not difficult. There is a dead-end lane that runs more or less due east from the village and, at the end, is a cottage called Pentir. Walk through the gate and then turn about 45o towards the north, so you’re walking diagonally across the field towards the two obvious headlands in front of you. The Carboniferous limestone can be seen outcropping all over the place, but as ever, some bits are more fossiliferous than others. However, Gigantoproductus are too big to overlook, and it didn’t take long to collect a dozen or so decent specimens, as well as lots of other Carboniferous bits and pieces.
On the occasion when I visited, the tide was in, so for the first hour I had to content myself with looking over the rocks at the top of the cliffs. However, as the tide receded, more and more limestone became exposed and, at low tide, I was also able to collect fossils from the foreshore. While a good place for experienced collectors, it has to be said that Caim is not an easy locality to explore, and it is not recommended for families or those with limited mobility.
Another place famed for its Gigantoproductus is Trearne Quarry near Glasgow, but this is a working quarry so not at all family-friendly. Fortunately, there are some more accessible localities where Gigantoproductus can be found, including Burley Hill in north-eastern Wales. The UKGE website (ukfossils.co.uk) has information on the Caim, Trearne and Burley Hill sites, including maps, directions and all the relevant geological details.
Brachiopods are often overlooked by collectors, who find themselves attracted by the more charismatic fossil invertebrates like trilobites and ammonites. That’s a shame, because, in their own way, brachiopods are attractive and interesting animals. I hope that my hunt for Gigantoproductus stimulates you to give these fascinating fossils a bit more attention next time you’re out collecting.