What Is an Ammonite? A Shell That Was a Buoyancy Device

The animal lived only in the outermost chamber and built new chambers as it grew, sealing off the ones behind with walls called septa. A thin tube of living tissue, the siphuncle, ran back through every chamber and allowed the animal to move fluid in and out of them, which changed the overall density of the shell and therefore its buoyancy, in the same way the modern nautilus works. That system lets an animal hover at a chosen depth without swimming, which is a substantial energy saving for a predator that hunts by waiting and darting. The junction between each septum and the shell wall is not simple but folded, and the visible line of that junction, the suture, becomes progressively more elaborate through the group's history, from simple curves in early forms to extraordinarily convoluted patterns in later ones. The function of that complexity is debated, with strengthening against pressure the traditional explanation and others including increased attachment area and control of fluid movement proposed.

The animal belongs firmly within a group whose living members are familiar:

• •Cephalopods, the group containing squid, octopus, cuttlefish and nautilus, so the animal had tentacles, good eyes and jet propulsion
• •More closely related to squid and octopus than to the nautilus, despite the superficial shell resemblance, which is a case of similar appearance from a shared ancestral feature rather than close kinship
• •Predators, with jaws preserved in some specimens and with stomach contents including small crustaceans and other shelled animals
• •Sexually dimorphic in many species, with two size classes found together, the larger conventionally interpreted as female
• •Enormously variable in shape, including tightly coiled, loosely coiled, straight, hooked, helically spiralled and forms resembling a paperclip, particularly late in the group's history
• •Present for over three hundred million years, from the Devonian until the end of the Cretaceous, when they disappeared entirely while the nautilus survived

Ammonites are among the most important index fossils for Mesozoic marine rocks and meet every requirement. They evolved rapidly, with species lasting on the order of a million years or less, which gives fine resolution. They were free-swimming and widely distributed, so the same species correlate rocks across continents. They are abundant in many marine sequences. And they are distinctive and identifiable, since shell shape, ribbing, ornament and suture pattern together give a reliable signature. The Jurassic in particular is divided into ammonite zones that remain the standard framework, with each zone named for a characteristic species and representing a period short enough to be genuinely useful. That scheme was developed in the nineteenth century and underpins geological mapping and petroleum exploration, which is a case of a fossil group having direct commercial value more than a century after the work establishing it.

Late in the Cretaceous a number of lineages abandoned tight coiling for shapes that look chaotic, and these heteromorphs were once interpreted as evidence of degeneration preceding extinction, which was a teleological reading now rejected. Analysis of their hydrodynamics and likely life positions indicates they were functional designs suited to particular feeding strategies, including slow drifting and vertical migration, and several were abundant and widespread, which is not what a failing group looks like. Their extinction at the end of the Cretaceous was complete and is conspicuous because the nautilus, superficially similar, survived. The favoured explanation concerns early life history, since ammonites hatched from very small eggs producing tiny planktonic young dependent on the plankton community, which collapsed severely after the asteroid impact, while nautiloids lay a few large eggs producing well-developed young that live on the sea floor and did not depend on that food web. That difference in reproduction rather than in adult ecology appears to be what separated survival from extinction.

The animal occupied only the outer chamber and used a tube running back through the sealed ones to move fluid and control buoyancy, so it could hover without swimming. It was a tentacled predator closer to squid than to the nautilus. Rapid evolution and wide distribution make the group the standard tool for dating Jurassic marine rock. Tiny planktonic young appear to explain why they died out where nautiloids survived.