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Ammonites were shelled cephalopods that died out about 66 million years ago. Fossils of them are found all around the world, sometimes in very large concentrations. 

Before we understood what they were, one of the explanations for ammonites was that they were coiled-up snakes that had been turned to stone, earning them the nickname 'snakestones'. But ammonites weren't reptiles: they were ocean-dwelling molluscs, specifically cephalopods.

An ammonite fossil with a carved snake's head SC: Natural History Museum
An ammonite fossil with a carved snake's head SC: Natural History Museum

Zoë Hughes, Curator of Fossil Invertebrates at the Museum, explains, 'Ammonites are extinct shelled cephalopods. All of them had a chambered shell that they used for buoyancy.'

The group Cephalopoda is divided into three subgroups: coleoids (including squids, octopuses and cuttlefishes), nautiloids (the nautiluses) and ammonites.

Ammonites' shells make the animals look most like nautiluses, but they are actually thought to be more closely related to coleoids.

'Some of their morphology was closer to that of the coleoid group,' says Zoë. 'We think it’s more likely that ammonites would have had eight arms rather than lots of tentacles like a nautilus, though the shell is more similar to that of a nautilus.'

Ammonites were born with tiny shells and, as they grew, they built new chambers onto it. They would move their entire body into a new chamber and seal off their old and now too-small living quarters with walls known as septa.

Ammonites looked a bit like nautiluses but are thought to be more closely related to coleoids, a group that includes octopuses and cuttlefish © Esteban De Armas/Shutterstock

Zoë adds, 'The ammonite would have lived in one chamber, but we don't know how often they built a new one.

'Previously it has been suggested this could have been a monthly occurrence, but there is no evidence for that. Some studies looking at the chemical composition of the shells - a field called sclerochronology - are starting to gain some insight of how long ammonites might have lived.'

Ammonites' growing shells typically formed into a flat spiral, known as a planispiral, although a variety of shapes did evolve over time. Shells could be a loose spiral or tightly curled with whorls touching. They could be flat or helical. Some species would begin growing their shell in a tight spiral but straighten it out through later growth phases. There were also some more unusual shapes - the species Nipponites mirabilis, which is found in Japan, is exceptionally rare and looks a bit like a knot.

All of these Madagascan fossils at 12 Stones and Lavender, come from the Mahajanga River Basin, near the village of Ambatolafia, in the Boeny region of northwest Madagascar, located in the former Mahajanga Province.  (Province de Majunga in French.)  They date from the Early Cretaceous period, Albian Age (~100 to 113 million years ago).

The ammonites and nautilus from Mahajanga are typically preserved by yellow to brown calcite in glauconitic sandstone.  The same locality produces a more limited number of white and black fossils, colored by manganese.

(* = Some scientists believe that these Madagascan ammonite species now belong under the Aioloceras genera instead.)