The interesting article on belemnites by Neale Monks (Mollusc World 20:8-11, July 2009) had me searching my garage for a belemnite, collected decades ago and almost forgotten. It came from the Lower Oxford Clay, Jason Zone, at Stewartby about 5 miles SSW of Bedford.
Members of the Department of Palaeontology of the Natural History Museum were there to dig out a pliosaur exposed by a large drag-line excavator. My job was stratigraphy, assigning the pliosaur to a zone in the Oxford Clay, an easy task made even easier by considerable numbers of Kosmoceras ammonites on and around the pliosaur. After logging the section and collecting clay samples, I was free to explore older and abandoned parts of the pit.
The London Brick Company at Stewartby was exploiting a thick bed of bituminous shale which was partly self-firing and, requiring less fuel, produced a cheaper brick. To get this shale bed a considerable amount of overburden clay was removed and deposited in tidy rows of conical heaps, for a hundred yards behind the working face. The furthest and oldest of the conical heaps had weathered to rounded humps and I was curious to see how this raw weathered clay became colonised. I was thinking of plants.
A scatter of ragworts, umbellifers and crucifers came as no surprise, but the number of spiders was unexpected. One, larger than the rest, ran across my path and drew my attention to a nearly complete belemnite Cylindroteuthis puzosiana (illustrated by Neale MW 20, p.10). But this one was not broken and had a surprise hidden in it (figure 1).
figure 1: A near complete belemnite ‘guard’ but without the forward extension of the pro-ostracum. Length 185 mm, diam. 24 mm.
Back home, while washing away the mud, a piece of shell fell off exposing the chambered phragmacone narrowing almost to a point. Impatience and a hair dryer allowed me to focus a hand lens on the point, revealing a shiny hemispherical dome: the protoconch (figure 2). The phragmacone, as Neale pointed out, is made of aragonite, an unstable form of calcium carbonate and therefore usually missing; but not this one. A lucky chance had filled the aragonite chambers with stable calcite, before the aragonite dissolved, leaving a near complete phragmacone with a calcite-filled protoconch right at the tip of the phragmacone.
Belemnite protoconchs, though rarely seen, are not unknown. I have a Belemnopsis from the Mid. Jurassic Fullers Earth Clay of the Fleet, Weymouth, Dorset, which has split along the central axis, through the phragmacone and the protoconch. But it was all preserved and only requires the experience to know what one is looking at. Also, Peter Doyle, top belemnite specialist, told me he had seen protoconchs in Liassic belemnites. So maybe it is not so rare, just knowing where to look and a bit of luck with aragonite to calcite conversion.
figure 2: The protoconch (arrow), lacking growth lines, was laid down as a single cap of shell while still in the egg. Part of the phragmacone, left of the protoconch, is broken off and attached to the inside of the piece of shell which fell off during washing. The aperture of the protoconch is marked by a narrow brown zone just before the first chamber was formed (c.x8). The drawing shows the dimensions of the protoconch made with a 10 mm eyepiece graticule, x100.
In figure 3 I have deliberately avoided the question of the number of arms since we have no evidence. But the ink sac, often present, puts the belemnite in with the coleoid cephalopods, while the internal calcareous shell relates it to the squids and cuttlefish. So the probability is that it had ten arms.
figure 3: A tentative and hypothetical reconstruction of the embryo while still in the egg. The colours are diagrammatic to differentiate between: blue for the shell, red for the animal and yellow for the yolk sac. The enclosing dashed line indicates the egg case. The animal at this stage would have been about 1mm long.