EduF127b Pyritised Ammonite

3D Model

Pyritised Ammonite (Quenstedtoceras sp.)

68×52×20mm

Late Jurassic (160-154Ma)

Ammonites are an extinct type of cephalopod, related to squids, octopi and cuttlefish. Unlike many cephalopods except nautiluses, ammonites have a hard, coiled shell which is divided into gas-filled chambers by pieces of shell called septa, with the most of the body tissue in the outermost chamber. By moving water into and out of these chambers using a tube, or siphuncle, the animal can control whether it floats or sinks.

This ammonite has had its shell replaced by the mineral pyrite, or “fool’s gold”, which is iron(II) disulfide. This happens in oxygen-poor environments because the original shell material, aragonite, is relatively unstable and easily dissolves, leaving a space which is filled when sulfides from nearby organic matter decaying, and iron in the seawater react to form pyrite which is more stable in such conditions.

See: EduF466b Nautilus