PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY
Ammonite Sphenodiscus sp. fossil. 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. This specimen measures 30 centimetres in diameter and was found in Mexico.
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