I have become involved in a minor controversy that might be of interest to some of our members.

GRYPHAEA ARCUATA from the lower lias, the familiar ‘toenail’, is a very common fossil near Bath, and is said to be completely extinct, with no modern shellfish of quite the same shape to guide us to an understanding of how it lived, although modern oysters might possibly give a clue about its diet.

Most of the literature on the subject says that it lived on a fairly shallow, muddy, sea floor, though probably not so shallow that it was uncovered at low tide, and that its food, consisting of tiny particles of algae or plankton, was brought to it by currents in the water in the same way as for our sedentary shellfish of today. The shell is said to be sufficiently heavy to keep it on the bottom without further anchorage, and diagrams show the creature lying on the sea floor with its operculum elevated when feeding or closed for defensive purposes.

There’s so much authority for this description that it is probably pointless to make any alternative suggestion, although it might be suspected that authors quote each other back to only one or two originals, without independent thought. However, somehow the diagram does not look very convincing:

1. The heavy ‘knob’ would surely unbalance the creature so that it would tend to point its shell upwards.

2. On firm mud it would be very unstable in the currents it relied on for food. Its weight in water would be only about half its weight in air so that it might be rolled or skidded from place to place in a most uncomfortable manner.

3. Alternatively on very soft mud its aspect would be conducive to its sinking until its food-gathering was terminated by burial. There is no evidence that it had any means of digging itself out.

Now much more sensible the story appears to be if the creature is drawn the other way up:

1. The ‘knob’ would be an effective anchor against movement by the currents that brought its food, its shape and weight assisting this function, helped by the streamlined presentation of the main shell.

2. Its slightly concave underside would resist penetration into the mud, and its operculum could be used to raise it, should it begin to sink below an acceptable amount. It might even have limited ability to bury itself a few millimetres below the surface for defence, or to reduce the effects of strong tides.

3. The ‘knob’ is ideally shaped to allow the main shell to pivot upwards for feeding when the operculum, hinged near the knob, is pressed downwards.

In my opinion these thoughts might be worth further consideration. If the standard explanation is true, it might be expected that the operculum and the surface of the knob are likely to show barnacles or scars where seaweeds became attached, while the main shell is clear because of its contact with the ground, particularly if it lived in an environment of currents so weak that its stability was not impaired. It can hardly have lived in strong currents. In the proposed alternative aspect, any such barnacles or weed attachments are likely to be on the main shell rather than the operculum or knob, although in this case it might have lived in much stronger currents that would have rendered attachment more difficult.

Unfortunately the few samples of Gryphaea in my collection were chosen because of their unmarked appearance, so that they are not very helpful in resolving the matter. One shows some parasite attachment on the main shell, and all the knobs are clear of marking, but it would be unwise to form conclusions from only one example.

If any members have any comment to make, based on their own collections, I would be pleased to hear it. Or if they are sufficiently interested in the idea, perhaps they will join me next summer in a Sunday afternoon expedition to Hook Cliff, where Gryphaea are plentiful, to look at some poor fossils as well as the good ones.

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