Molluscs are an extraordinarily successful group of modern organisms. They include slugs and snails, clams, sea-snails and chitons, octopus and squid, and the extinct Mesozoic ammonites that lurk on the beaches of Lyme Regis and many other places. Along modern coasts, they can dominate the ecosystem, and between them have adapted to the most unpleasant and difficult environments, from boulder-strewn breaker zones to brackish lagoons. But they took a long time to get to this stage, and in the Middle Ordovician, they were still a minor group, not very diverse and in most cases decidedly uncommon. The fossils fall into several groups, which I’ll mention individually.
Nautiloids
Perhaps the most abundant were the orthocone nautiloids: predatory squid-like creatures with a long, conical shell that could reach giant proportions, sometimes several metres in length (although those in the Builth area were a mere 30 cm or so, at most). In a world of arthropods and worms, this was certainly among the top predators, and several mangled clusters of graptolites from Llanfawr may be testament to its appetite. The shell of a nautiloid is chambered, with concave septa secreted behind the body as the animal grew, lengthening the conical shell and staying near the opening. The gas content of the sealed chambers could be controlled through a connecting tube, the siphuncle; this would have been used primarily to regulate the animal’s buoyancy; we know this because a similar thing occurs in its modern relative, the Pearly Nautilus of the Pacific.
If they are well preserved, nautiloids have an unexpected use: calculating the water depth when it was deposited! As the shell descends through the water, the external pressure increases, and the chambers implode one by one, each stronger than the last. At least in principle, it is possible to calculate the maximum depth by measuring the thickness and diameter of the last septum that imploded. Since water depth is almost impossible to estimate accurately, this is a very useful little trick. The only problem is, you need lots of specimens, and all uncompressed, with the original shell. This is only likely in limestones. And Builth doesn’t really have any, at least not with nautiloids in. But it’s worth keeping an eye open for.
Bivalves
In addition to nautiloids, there are some familiar groups like the bivalves, known to all by such things as cockles, mussels and scallops. They possessed two shells (valves), one on either side of the body. Compare this with brachiopods, where the shells are above and below the body. For this reason, and because molluscs have a plane of symmetry along their length, the two valves are mirror-images of each other, and individually asymmetric, making a complete shell at least theoretically easily distinguished from that of a brachiopod. At this time, bivalves appear to have been largely restricted to very shallow water, and were rare creatures in deeper water. Only a few specimens have been recovered from Llanfawr, for example, but each is a disctinct species; the diversity could therefore be surprisingly high, despite the very low abundance. Because bivalves mostly have shells made from aragonite rather than calcite, their shells may have dissolved much more often than those of brachiopods. As a result, they may have been far more abundant than we realise. It is striking that the majority of new specimens unearthed are actually undescribed species, suggesting that we are simply not seeing the great majority of their populations.
In a very few places, such as Camnant Ravine, there are very important, diverse early bivalve faunas. In order to be useful to taxonomists, and people studying the evolution of the group, we need to be able to the structures around the hinge – particularly the ‘teeth.’ Today there are several distinctive types of tooth arrangement, particularly in burrowing forms (epifaunal things like oysters often have reduced teeth, because their main purpose is in the burrowing process), including heterodont (cockles) and taxodont (arc shells). In Ordovician faunas, they haven’t yet evolved completely separately, and it is much more difficult to tell them apart, but it makes for an interesting detective story. Sadly, most Ordovician bivalve specimens that turn up have their teeth very poorly preserved, so any good ones are quite important.
Gastropods
Gastropods are the sea-snails: winkles and cone shells, and so on, as well as the majority of fresh-water snails. Land snails, although they are very similar, are actually a separate group (called pulmonates), distinguished by the absence of gills. Although also poorly known from the Ordovician, this is partly because no-one has really studied them – at least, not those from Britain. A fair number have been recovered from the inlier as a whole, with perhaps half a dozen species represented, but there is so little known about them that classifying them is proving very difficult. In general they are small, with a helical, spiral shell; this was the primitive shape of the group, and diversity was as yet limited. Distinguishing gastropod species can be achieved mainly through measurements of the coiling shape, and examination of surface ornament, which in many cases is stunningly beautiful under high magnification. A much under-rated group of fossils, these. One day someone’s going to have to study them properly.
Other molluscs
Several rarer groups of molluscs existed at the time, namely tergomyids (also known as monoplacophorans, ancestral to gastropods), rostroconchs (possibly ancestral to bivalves), polyplacophorans (chitons) and probably the almost unfossilizable aplacophorans. Of these, only the tergomyids have definitely been recorded from Builth, and only from the lowest beds, but the others may well be there somewhere, lurking in the shadows. Tergomyids are very similar to gastropods, but their shells are planispiral rather helical, coiling in one plane only, like an ammonite. Usually, they don’t manage many coils in their life, being essentially a curved, limpet-like cone in extreme examples. They are some of the most shy and retiring of molluscs at the present day, managing to successfully avoid notice by humans, either as fossils or alive, for four hundred million years. Until one was pulled out of the deep sea in 1952 everyone thought that they had been extinct since the Devonian. During the Ordovician they were less shy, although not common, and a few species are known just from the inlier.
Just in case you meet one of the others, this is what they look like. Rostroconchs are effectively peculiar bivalves, but the two shells are fused together with a flexible layer of calcite, leading to a slightly odd appearance. Polyplacophorans have a row of eight imbricated shells, surrounded by tiny organic sclerites that won’t be preserved. They should be familiar from modern seashores to any rock-pool enthusiasts, but isolated shells are not easy to identify, and complete specimens are extremely rare indeed. Aplacophorans are even less likely to turn up, being basically slug-like creatures with a coating of tiny spines or scales. Find one of these, and you’re famous.
Molluscs:
[5]Allumetoceras oneratum Evans 1994. Length up to ~ 15 cm.
[1]Arcodonta regularis Cope 1999. Up to 15 mm.
[5]Bivalve indet. A Approx. 20 mm.
[5]Bivalve indet. B. Approx. 20 mm.
[5]Bivalve indet. C. Approx. 25 mm.
[4]Bivalve indet. D. Approx. 7 mm.
[2]Bivalve indet. E. Few visible details, and no dentition; assumed to be bivalve, but not absolutely certain. Similar morphologies are known from the Upper Ordovician of Wales. At least 30 mm long.
[2]Bivalve indet. F. Little visible detail beyond distinctive outline and low convexity; only specimen 45 mm.
[2]Bivalve indet. G. Only certain specimen (articulated valves) is 60 mm from umbo to distal edge.
[2]Bivalve indet. H. Distinctive break in shell near cardinal area, and only species with moderately clear (palaeotaxodont-like) dentition. Only specimen 10 mm.
[2]Bivalve indet. I. Distinctive outline and clear umbo. The apparent spikes around the edge are probably a taphonomic feature, but given it comes from a strongly silicified bed at a locality known to yield some soft tissue, setae should not be ruled out entirely. Only specimen 30 mm wide.
[2]Bivalve indet. J. Only specimen 27 mm wide.
[2]Bivalve indet. K. Unusual tooth-like ridges on outside of shell. Only specimen 37 mm wide.
[1]Camnantia ampla Cope 1999. Up to ~ 30 mm.
[1]Eritropis peregrinata Cope 1999. Up to 30 mm.
[5]Gastropod indet. sp. A. Diameter up to 15 mm.
[5]Gastropod indet. sp. B. Diameter up to 6 mm.
[1]Gastropod indet. sp. C. Diameter up to 8 mm.
[2]Gastropod indet. sp. D. Diameter up to 4 mm.
[2]Gastropod indet. sp. E. Diameter up to 6 mm.
[4]Gastropod indet. sp. F. Diameter 1 mm; up to 5 mm is known.
[2]Gastropod indet. sp. G. Diameter 8 mm in only specimen.
[1]Glyptarca radnorensis Cope 1999. Up to ~ 30 mm.
[1]Modiolodon ellesae Cope 1999. Up to 60 mm.
[5]Nautiloid indet. sp. A. ~ 40 mm.
[5]Nautiloid indet. sp. B. ~ 50 mm.
[5]Nautiloid indet. sp. C. ~ 70 mm.
[5]Nautiloid indet. sp. D. ~120 mm.
[5]Nautiloid indet. sp. E. ~ 30 mm.
[5]Nautiloid indet. sp. F. ~ 70 mm.
[5]Nautiloid indet. sp. G. ~ 100 mm.
[1]Nautiloid indet. sp. H. This is the specimen that is encrusted by Marcusodictyon? sp., from perhaps the oldest fossiliferous locality yet studied in the Inlier, near Bwlch-y-Cefn. Diameter of preserved section 12 mm.
[4]Nautiloid indet. sp. I. A small species with distinctive sinusoidal ridges on the external surface only. Maximum observed diameter 5 mm.
[5] “Oxydiscus turbinatum”. The name is that given to a specimen in the Sedgwick Museum, Cambridge, but it is currently unclear if it is either valid or accurate. In particular, the genus should probably be Oxydiscoceras, even assuming that it is correctly assigned. Needs redescription. Diameter up to 40 mm.
[1]Praeleda multidentata Cope 1999. Up to 30 mm.
[1]Praeleda subtilis Cope 1999. Up to 20 mm.
[1]Redonia anglica [check author]. Up to 15 mm.
[1]Smilodonta ceryx [check author]. Approx. 20 mm.
[1]Tergomyid indet.
sp. A. Approx 20 mm.
To be drawn:
[1] Lyrodesma cf. secure de Tromelin & Lebesconte 1876.
[2] Indet. bivalve, near-circular outline and highly convex valves.
[2] Indet. modiomorphoid bivalve.
[1] Indet. tergomyid with trilobate section.
[2] indet. gastropod (similar to sp. G.) with longitudinal ridges on whorls.
[1] monoplacophoran indet., reported by Brian Beveridge.
[1] monoplacophoran? reported by Brian Beveridge.
[1] indet bivalve sp. L, Gilwern Hill; reported by Brian Beveridge.
[5] bivalve indet. sp. M, reported by Brian Beveridge.
