What Were Ammonites? The Coiled Shells That Date the Rocks

They were cephalopods, the group that contains octopus, cuttlefish, squid and the living nautilus, and the soft body sat in the outermost chamber of the coil with the head and arms protruding. Almost nothing of that body survives, since it had no hard parts, so the reconstructions draw on their relatives: a ring of arms or tentacles, large eyes, a beak like a parrot's, and jaws, one type of which is found often enough as a fossil to have been mistaken for a separate animal for years. They were predators, taking small fish, crustaceans and floating plankton, and they were themselves eaten, with shells known that carry the tooth marks of a mosasaur in a neat row.

The coil is not solid. It is divided by walls into a series of sealed chambers, and a tube of living tissue called the siphuncle runs through them all, allowing the animal to move gas and fluid in and out and so adjust its buoyancy, in the way that a submarine adjusts ballast. That is the same system the living nautilus uses, and it let the animal hang almost weightless in the water at very little cost. The walls where each chamber meets the outer shell are not flat but crimped into elaborate folds, and the line they trace on the shell surface, the suture, is the single most useful feature the animal has:

• •Simple, gently wavy sutures belong to the earliest forms of the Devonian
• •More folded ones, with a distinctive pattern, mark the Carboniferous and Permian groups
• •The intricate, fern-like sutures of the Jurassic and Cretaceous are diagnostic almost to the species
• •Because the folding strengthens the shell against pressure, the pattern also carries information about the depth an animal lived at

A fossil is useful for dating rock when the species lived over a wide area, evolved quickly into a recognisably different form and is common enough to actually find, and these three conditions are rarely met together. Ammonites meet all of them: they drifted as larvae across whole oceans, so the same species turns up in England, Argentina and Japan; they threw off new species at a remarkable rate; and in some marine strata they are the commonest fossil present. The Jurassic and Cretaceous are consequently divided into ammonite zones, each defined by a particular species or assemblage and lasting a few hundred thousand years, and the scheme worked out from English and German quarries in the nineteenth century still underpins the dating of marine rocks worldwide. When an oil company logs a core, this is often how the age is fixed.

The neat flat spiral is the familiar form, and it is far from the only one. Sizes run from a fingernail to the two-metre Parapuzosia of the German chalk, the largest known. Some, the heteromorphs, abandoned the plane coil altogether: Baculites grew almost straight like a spear, Turrilites spiralled like a snail's shell, Nipponites wound in a series of linked U-shapes that has been described as a knot, and Didymoceras made a corkscrew with a hooked end. These were long read as decadent forms, a sign of a group losing its way before extinction, an interpretation that has not survived. They were abundant, successful and, judged by their distribution, well suited to drifting and feeding on plankton rather than chasing prey, which is a different way of making a living rather than a failure at the old one.

They vanished at the end of the Cretaceous, 66 million years ago, in the same extinction that ended the dinosaurs, and their disappearance is one of the sharpest lines in the fossil record, with shells abundant in the chalk immediately below the boundary clay and absent above it. The likely mechanism is their young. The hatchlings were tiny and drifted in the surface plankton, and the extinction event collapsed the plankton for a period as dust and sulphate cut off the light, which removes both the food and the nursery. The nautilus, whose young hatch large and live near the bottom from the start, came through the same event and is still here, which is the neatest natural experiment the fossil record offers. The name, meanwhile, comes from Pliny, who thought the coils resembled the ram's horns on statues of the Egyptian god Ammon, and in medieval England they were sold as snakestones, coiled serpents turned to rock by a saint, with heads sometimes carved on for the market.

These were shelled cephalopods, relatives of squid and the nautilus, that lived from the Devonian to the end of the Cretaceous and controlled their buoyancy with gas-filled chambers connected by a tube. Their crimped chamber walls leave a suture pattern that identifies them almost to the species, and because they spread worldwide, evolved fast and are common, they divide the Jurassic and Cretaceous into zones a few hundred thousand years long. Coiled, straight and corkscrew forms all existed, and they died out with the dinosaurs while the nautilus, whose young do not drift as plankton, survived.