LAWRENCE LAWRY / SCIENCE PHOTO LIBRARY LAWRENCE LAWRY / SCIENCE PHOTO LIBRARY
Ammonite fossil. Ammonites were marine invertebrates. During fossilisation some of the minerals of this ammonite's shell have been replaced by pyrite (iron sulphide, or fool's gold). Ammonites first appeared in the fossil record around 400 million years ago in the late Silurian and early Devonian periods and became extinct at the end of the Cretaceous period, 65 million years ago. Ammonites are index fossils, their presence allows the rock layer in which they are found to be dated to specific geological time periods.
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