Did ancient ammonites sink or swim? This X-ray image of a fossil is bringing their shells back to life to help tell us if they could move freely through water.
We think that ammonites, which lived up to 65 million years ago, used their shells as a flotation device, pumping out water through gaps in the structure and filling them up with buoyant gas. The photo below shows what the actual holes look like.
To work out whether the structure would have allowed for swimming, René Hoffmann of Ruhr University in Bochum, Germany, used X-rays to produce a 3D model of an ammonite. Using the reconstructions, he was able to calculate the ammonite's weight and volume, and thus determine whether the chambers really could help it float. To check his results, he also created models of the nautilus, a present-day relative of ammonites that are able to float. Since he could access living specimens, he was able to compare their body weight with calculations based on the volume and density of the simulated shell.
His results suggest that young ammonite shells had 11 chambers, enough to keep hatchlings afloat. Although he has yet to confirm the buoyancy of adults, ammonites developed more shell chambers as they aged and became heavier, which would have helped support them in the water.
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