Last week, we introduced Dinocochlea ingens, a gigantic spiral fossil from the Lower Cretaceous Wadhurst Clay Formation of Hastings, Sussex. Discovered in 1921 during the extension of St Helens Road near Old Roar Glen, this fossil immediately excited local and, indeed, national interest. The specimens were despatched to the British Museum (Natural History) where BB Woodward, a mollusc specialist who had recently retired as chief librarian, formally described the fossil as the new genus and new species – Dinocochlea ingens. The clue to Woodward’s interpretation of the fossil is in the name Dinocochlea, meaning ‘terrible snail’. Woodward (1922) considered Dinocochlea to be the largest snail that had ever lived. By piecing together the fragments found by the workmen building the road, he was able to reconstruct the supposed snail as a monster over 7 feet tall, 14 inches wide and with 23 spiral whorls (Fig. 1).
Not a snail
For a short time, Dinocochlea achieved celebrity status and was exhibited in the public galleries of the BM(NH) between the wars. However, its identity as a colossal snail was soon to be challenged. One of Woodward’s colleagues, the eminent fossil mollusc researcher LR Cox, was the main critic. Cox (1929, 1935) pointed to the variability in the tightness of spiral coiling between specimens of Dinocochlea and the fact that both left and right-handed varieties existed, which would be very unusual for a true snail. Furthermore, the shell itself was not preserved, nor was there was a space between the spiral whorls that would be expected had the shell dissolved, as is so often the case in fossil molluscs. The fatal blow to the snail theory was the structure of the initial whorl. Even in large snails, this begins with a tiny larval shell – the protoconch – a millimetre or less in diameter. In contrast, the final whorl of Dinocochlea is a bulbous structure, measuring 30 to 40mm in diameter, hardly consistent with the tiny free-swimming larva of a snail.
While being convinced that it was not a snail, Cox was unable to reach a firm conclusion about the true identity of Dinocochlea. However, he did entertain two alternative hypotheses. The first was that it is an enormous coprolite, perhaps the fossil excrement of the dinosaur, Iguanodon, the bones and teeth of which had been collected at the nearby Hollington Quarry. Spiral coprolites are known as fossils, mostly excreted by sharks with spiral valves in their lower intestines, but none come remotely close in size to Dinocochlea. Neither does Dinocochlea contain any of the expected remains of partly ingested food.
The second alternative for Dinocochlea was an inorganic origin as a concretion. In favour of this idea was the fact that Dinocochlea was found in association with undoubted concretions, some very difficult to distinguish from the fragments of the fossil itself. The problem was, can an inorganic concretion develop a spiral shape?
A new hypothesis
Prompted by interest from the BBC, who wanted to include Dinocochlea in their behind-the-scenes series about the Natural History Museum – Museum of Life – filmed during 2009/2010 and broadcast in early 2010, it was important to come up with a new and more convincing hypothesis for this striking fossil. The clue came from Cox (1929), who made a telling remark when discussing the concretion hypothesis, noting that, if Dinocochlea was a concretion, it must have grown around some original spiral nucleus long since decayed. Concretions are prone to nucleate around fossil shells – witness the frequency with which concretions in the Lower Jurassic of Dorset and Yorkshire contain an ammonite. Nevertheless, broken whorls of Dinocochlea show no sign of a body fossil within. Instead, cross-sections may reveal concentric bands (Fig. 2) reminiscent of tree rings, implying a centrifugal pattern of growth, as is occasionally seen in concretions (Fig. 3).
In the absence of a body fossil, what else could have formed the template for spiral concretion growth? Concretions can also nucleate around trace fossils. For example, the trace fossil, Thalassinoides, is often preserved in Upper Cretaceous chalks through the growth of flint concretions in and around the branching burrow system made by a crustacean. Fossil burrows, artificially enlarged by concretion growth, have been called ‘mummy envelope concretions’ (for example, Sprechmann et al. 2004).
While branched concretions are easy enough to explain by nucleation around branching burrow systems, can Dinocochlea be accounted for by concretionary growth around a spiral burrow? We believe it can. Several ichnogenera have been erected for spiral burrows, including Daimonelix, Helicodromites, Augerinoichnus and Gyrolithes (Fig. 4). All of these are relatively open, spring-shaped structures. However, concretionary growth around the burrow could conceivably infill the gaps between the turns of the spiral to produce the snail-like appearance of Dinocochlea.
When originally discovered, specimens of Dinocochlea were found horizontally disposed in the Wadhurst Clay. This implies that the original burrow hypothesized to have formed the nucleus for concretion growth was also horizontal. The ichnogenus, Helicodromites, has just such a disposition relative to bedding, but what kind of animal produces the trace fossil Helicodromites? Incipient Helicodromites found at the present day can be made by burrowing threadworms – capitellid polychaetes, such as Notomastus and Heteromastus that inhabit helical burrows in mud. These worms reach lengths of 200mm but have very slender bodies, 1 to 2mm in diameter.
The next question to be answered is why should the burrow made by such a tiny worm cause the growth of a concretion? In fact, concretions are quite common in the Wadhurst Clay where they are known as ‘Tilgate Stone’ and obvious nuclei for their growth are usually lacking. In the case of Dinocochlea, we believe that the decay of organic matter in the vicinity of the burrow of the threadworm locally raised pH levels, triggering precipitation of carbonate cement to form a concretion.
More research remains to be done on Dinocochlea before we can be sure of its origin. Nevertheless, the best hypothesis currently available is that it represents a trace fossil massively enlarged by concretionary growth. It is remarkable to consider that a tiny Cretaceous threadworm, excavating a narrow, corkscrew-shaped burrow, may have been the instigator of the enormous, gastropod-like, Dinocochlea ingens.
Cox, L.R. 1929. A spiral puzzle. Natural History Magazine 2(9), 16-27.
Cox, L.R. 1935. The Hastings giant spirals. The Hastings and East Sussex Naturalist 5, 62-68.
Sprechmann, P., Gaucher, C., Blanco, G. & Montaña, J. 2004. Stromatolitic and trace fossil community of the Cerro Victoria Formation, Arroyo del Soldado Group (lowermost Cambrian, Uruguay). Gondwana Research 7, 753-766.
Taylor, P.D. & Sendino, C. 2011. A new hypothesis for the origin of the supposed giant snail Dinocochlea from the Wealden of Sussex, England. Proceedings of the Geologists’ Association.
Woodward, B.B. 1922. On Dinocochlea ingens, n. gen.et sp., a gigantic gastropod from the Wealden Beds near Hastings. Geological Magazine 59, 242-248.