Giant file clams: A common fossil as an exciting document of Earth history

Jens Lehmann (Germany)

Plagiostoma – a record of about 200 million years

Are there any boring fossils out there in the ground? I do not think so and to demonstrate this, an “ordinary” fossil find is focussed on here. We are talking about “just” a mussel, but one that belongs to an extinct group and even this alone makes it exciting. Plagiostoma is a member of the file clam family. This genus is only known from the fossil record and appeared probably in the Permian and certainly by the Mid-Triassic period, about 240 million years ago. This genus became extinct at the Cretaceous-Palaeogene boundary – according to the most recent timescale dated 66 million years ago. At least two dozen of species have been described so far.

The shells of Plagiostoma are moderately inflated and can reach the size of a hand, with an oblique oval outline. It is a common fossil, for example, in the Muschelkalk (Middle Triassic) in the Germanic Basin and quite a few continental countries. It is also well known from the Blue Lias of Southern England and stratigraphic equivalents, and can be frequently found at Monmouth Beach in Lyme Regis, a world-famous spot for fossil hunting.

The giant Plagiostoma

The species frequently encountered at Lyme Regis is Plagiostoma gigantea and the name already suggests that it is a giant amongst the representatives of the genus. The specimens figured here (Fig. 1) are of moderate size, with a total length of each about 8cm, but there are shells can reach almost 12cm in total length.

The shells of this species are moderately to strongly inflated. They always show this typical oblique oval outline and are smooth – except for faint radial ridges and growth lines (Fig. 2). This species already appears in the Rhaetian of Southern England and occurs throughout the Lower Lias in this area (for details, see Palmer 2010).

Preparation and pieces

Walking along the seashore at Lyme Regis, you can see many Plagiostoma sticking out of the rock – although it might be quite challenging to recognise them in cross-section. Please keep in mind that you are not allowed to collect them in situ at this World Heritage site. However, it is unnecessary since you can collect many specimens from loose boulders at the beach, like the ones figured here. The mussel on the right side of the rock slab in Fig. 1 was sticking out of the matrix and the second specimen was discovered while smashing the boulder into pieces.

The bivalve on the left broke into two pieces during fieldwork and thus a time-consuming preparation took place before the photo was taken. Although both valves show many cracks (Fig. 3), those have not been caused by gluing the smashed rock pieces together again.

In other words, it was not recent but rather ancient stress that was affecting the rock. After the shells were embedded in the limy mud on the seafloor, the rock became lithified. However, the sediment stack that was accumulating on the seafloor was increasing and becoming heavy; and, before lithification took place, the overload cracked the calcareous shells that were rigid by nature. However, this does not reduce the scientific value of the fossil – it still tells a rather nice story. Furthermore, the pattern looks aesthetic and, after preparation, the fossils were moderately painted with a rock care product except for the cracks. This led to an even higher contrast with the aim of emphasising the cracks (Fig. 3).

Lifestyle of the giant

The preparation even revealed details like the wings of the shell. These are small in Plagiostoma gigantea, as they are in allfile clams (Fig. 2). The wings can be found left of the umbo – the part of the shell where the growth started – and this indicates that the figured specimens are both right valved. Despite the fact that both valves are of the same size they definitely did not belong to the same specimen – roughly 200 million years ago. What else do we know about these bivalves and their ecology? Plagiostoma was lying on the seafloor, only attached with an organic anchor – called a byssus – to the ground; and they were feeding on very small food particles that were floating in the water – called suspension feeding. After their death the valves have been transported along the seafloor by currents and thus usually both valves became isolated eventually. At Lyme Regis the energy of the palaeo-currents was strong enough to bring the single valves into a stable position, with the convex side up. This can be well seen at many of the worn in situ rock surfaces on the foreshore.

Speech for a boring fossil

At the very end, I need to insist that common fossils tell exciting stories – it is just important to listen carefully. Each fossil is worth looking at seriously. As a specialist on ammonites myself, I suggest that collectors, who collect ammonites but throw away bivalves since they are less interesting, really should have a new look at these and other “accessory” fossils.

About the author

Professor Jens Lehmann of the Faculty of Geosciences at the University of Bremen (Germany) is Head of the Geosciences Collection (https://www.geosammlung.uni-bremen.de/en/geosciences-collection-of-the-university-of-bremen/). His research interests are evolution, diversity and palaeoenvironmental reconstruction. His particular interest in these topics is on the Cretaceous and ammonites. His research trips have taken him to places all around the world and, currently, his research focusses on the Triassic of Nevada/USA.

References

Palmer, P.C., 2010. Mollusca – Bivalves, in: Lord, A.R., Davis, P.G. (Eds.), Fossils from the Lower Lias of the Dorset Coast. Field Guides to Fossils. University Printing House, Oxford, London, pp. 124-146.