The bullet shaped fossils are the guards of an extinct order of Mesozoic predators that resembled squid or cuttlefish, and lived from the early Jurassic to the late Cretaceous. In some places they are very useful for establishing the stratigraphy, since, in a similar manner to ammonites, they evolved fairly rapidly. Their changing forms can guide us to which exact period the rocks in question date from. Such identifications are invariably made on the basis of as complete a faunal assemblage as possible, but since many of the organisms used are microscopic, belemnites and other easily identifiable larger fossils serve as an excellent rough and ready field guide to the knowledgeable geologist.
They had 10 equal sized tentacles studded with hooks to ensnare prey, without the two longer ones that characterise modern squid. They also had internal skeletons made of calcite, unlike their modern counterparts, and the most commonly fossilised part pictured here was at the rear of the animal. Rare examples with preserved internal parts indicate that they had ink sacs, hard beaks, large eyes and tailfins similar to modern cephalopods. They are often found in the stomachs of larger predators such as ichthyosaurs.
They have intrigued humans since time immemorial, and were called thunder stones in England (as they were thought to fall from the sky), along with bullet stones, and both the devil’s and St Peter’s fingers depending on religious inclination. The Chinese called them sword stones and the Scandinavians gnome’s candles, testifying to the interest paid to these odd shaed rocks around the globe. Belemnites are also the state fossil of Delaware.
Image credit: cobalt 123