DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Ammonites are cephalopods with spirally wound, snail-like shells that lived in seas during Jurassic to Cretaceous periods, some 200 to 100 million years ago. Fossil ammonites from these times are found in abundance all over the world and vary in size from a few millimetres to over one meter. The spiral windings of ammonites are divided in small chambers separated by winding partitions that are well displayed in this picture. The picture is of a thin section, taken with polarized, transmitted light, which explains the lively colours taken on by the calcitic crystalline infill of the chambers. Actual diameter is 38 mm.
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