The skeletal make-up of marine calcareous organisms such as corals and molluscs throughout geologic time is controlled by recovery from mass extinctions, rather than global changes in ocean chemistry. Previous work had suggested that changes in the magnesium content of the oceans controlled what mineral the animals used to create skeletons.
Marine calcifying organisms secrete shells of either the aragonite or calcite forms of calcium carbonate, depending on the organisms’ physiology. In the modern ocean, which favours aragonite, less than 20% of organisms secrete calcite.
Online in Nature Geoscience this week, Wolfgang Kiessling and colleagues used large palaeontology databases to examine this relationship over the past 500 million years. They find that the percentage of calcite secretors has a stronger relationship with mass extinctions and recovery periods than with ocean chemistry. The authors suggest that evolutionary fate of groups of marine calcifyers strongly depends on selective recovery from mass extinctions, rather than periodic changes in ocean chemistry.