ammonites, Chapman's Pool, Dorset

Ammonites are the distinctive fossils of the warm, Mediterranean-style seas of the Jurassic Period. The dark oil shales have been compressed down to about 10% of their original thickness as they formed into rock; accordingly, these two shells have got a bit squashed. In life, ammonites were fearsome predators, zooming around in the warm waters in an upright position, propelled with jets of water. There are six different ways that fossils can form and be preserved, but here the white aragonite mineral is, quite simply, the creature's original shell. The large one is about 20cm wide.

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