Pleuroceras pyritised ammonite from Buttenheim, Germany. From Jurrasic period approx 200-150mybp. They swam in the Jura ocean that then covered much of Europe. The extraordinary preservation is owed to the fact that organic marine sediments undergo bacterial sulphate reduction producing bisulphide. This can draw in dissolved iron and iron minerals to form pyrites (FeS2) in the fossil cavity. The complex forms of the shell (here seen edge on) can be used for identification - as can the unique pattern of chamber sutures found in many species.
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