MARTIN LAND / SCIENCE PHOTO LIBRARY MARTIN LAND / SCIENCE PHOTO LIBRARY
Split block of limestone enclosing a single ammonite Arnioceras semicostatum (Young & Bird). The shell along part of the outer whorl has broken away revealing calcite crystals growing into the hollow outer chamber of the shell - that part occupied by the ammonite's body during its lifetime. The ammonites were molluscs of the class Cephalopoda that lived from the Devonian period (400 million years ago) to the end of the Cretaceous (65 million years ago).
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