Page "Exoskeleton" Paragraph 19

With the exception of the mollusks, whose shells often comprise both forms, most lineages use just one form of the mineral.

The form used appears to reflect the seawater chemistry – thus which form was more easily precipitated – at the time that the lineage first evolved a calcified skeleton, and does not change thereafter.

However, the relative abundance of calcite-and aragonite-using lineages does not reflect subsequent seawater chemistry – the magnesium / calcium ratio of the oceans appears to have a negligible impact on organisms ' success, which is instead controlled mainly by how well they recover from mass extinctions.

A recently-discovered modern gastropod that lives near deep-sea hydrothermal vents illustrates the influence of both ancient and modern local chemical environments: its shell is made of aragonite, which is found in some of the earliest fossil mollusks ; but it also has armor plates on the sides of its foot, and these are mineralized with the iron sulfides pyrite and greigite, which had never previously been found in any metazoan but whose ingredients are emitted in large quantities by the vents.