DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Ammonite fossil. Section through a fossil of the ammonite Sphenodiscus lenticularis. Ammonites are extinct cephalopod molluscs that lived in the oceans during the Jurassic and Cretaceous periods. A fossil is created when an animal is quickly buried in sand, mud or volcanic ash after death, and the normal decay processes do not occur. The minerals of the hard parts of the animal, such as shell or bone, are gradually replaced with rock. The detailed septa (partitions) between the chambers of the shell's interior are particularly well seen. This specimen measures 6 centimetres across. S. lenticularis fossils are scarce except in North America.
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