Wednesday, October 17, 2018

Mineralogy Of The Earliest Animal Shells

Carbonate sedimentology and evolution. Two of my favorite subjects converge in this interesting study published in the September 2018 issue of Geology.

Calcium isotope evidence that the earliest metazoan biomineralizers formed aragonite shells-
Sara B. Pruss; Clara L. Bl├Ąttler; Francis A. Macdonald; John A. Higgins

Ediacaran Cloudina and Namacalathus are among the earliest shell-forming organisms. The debated carbonate phase of their skeletons, high-magnesium calcite or aragonite, has been linked to seawater chemistry and pCO2, yet independent constraints on the original mineralogy are lacking. We present a new method to distinguish primary skeletal mineralogy using ╬┤44/40Ca values and trace element compositions of the skeletons and associated cements. Ca isotopes are useful because they are relatively insensitive to diagenetic alteration during burial, and they vary with the primary mineralogical phase of carbonate. We applied this method to microdrilled carbonate and cements associated with both Namacalathus and Cloudina skeletons from the Ediacaran Omkyk Member of the Nama Group in southern Namibia. These data demonstrate that both organisms originally produced aragonitic skeletons, which later underwent diagenetic conversion to calcite. We suggest that calcium isotopes can be used to further constrain unknown skeletal mineralogies through time and to reassess the relationship between seawater chemistry and the mineralogy of biocalcifiers.

Different animals groups acquired the ability to precipitate hard protective shells made up of calcium carbonate at different times between the latest Neoproterozoic (~ 580-541 mya) to early Paleozoic (541 - ~ 450 mya).  What is the larger picture of the evolution of the biomineralization in different animal groups and the mineralogy of the skeleton? Through geologic time the chemistry of sea water has oscillated from that favoring the precipitation of aragonite and high magnesium calcite to that favoring the precipitation of low magnesium calcite. From Late Neoproterozoic to Middle Cambrian sea water chemistry favored the precipitation of aragonite and high Mg Calcite. Animal groups which evolved skeletonization during this time largely adopted aragonite to build their skeletons. Animals which first evolved biomineralization in the Late Cambrian to Ordovician times, during the time of calcite seas, adopted calcite skeletons.

In most animal groups, skeletal mineralogy was conserved even when sea water chemistry changed later in history. There have been only rare instances of animals changing the mineral phase used to build its skeleton. I wrote about one such instance in the Mesozoic when some groups of molluscs switched from  aragonite to calcite. The most prominent example is from the Hippuritoidea (rudist) bivalves. The switch to calcite shells seems to have triggered an evolutionary diversification beginning in the Late Jurassic. By Late Cretaceous times rudist bivalves were so abundant that they displaced corals (which built their skeletons with aragonite) as the chief reef builders in the shallow shelf environments.

Fascinating topic!

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