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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.

The often tightly wound shells of ammonites may be a familiar sight, but how much do you know about the animals that once lived inside?

What were Ammonites?

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. 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 .

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. While ammonite shells are abundant in the fossil record, it was only recently that scientists have found a very rare fossil of the soft parts of an ammonite. However, fossilised evidence of ammonite arms is yet to be found. Until now, a lot of what we know about ammonites has been inferred based on what we see in living cephalopods.

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