JOHN SIBBICK / SCIENCE PHOTO LIBRARY JOHN SIBBICK / SCIENCE PHOTO LIBRARY
Dead ammonites on seabed, illustration. Ammonites, now extinct, were marine invertebrate molluscs with a spiral shell. Most were less than 20 centimetres across. Ammonites first appear in the fossil record around 400 million years ago and became extinct at the end of the Cretaceous Period 65 million years ago. The process of fossilisation begins as sediment covers the dead ammonites on the seabed. If the conditions are right for fossilisation, the structure and anatomy of the ammonites will be preserved for millions of years as the sediments form rocks.
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