Ammonite (Skirrocera sp.) fossils. Ammonites were marine invertebrates. They first appeared in the fossil record around 400 million years ago in the l

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Ammonite (Skirrocera sp.) fossils. Ammonites were marine invertebrates. They 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. These specimens date from the Bajocian stage (between 171 and 167 million years ago).

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