The Evolution of Shell Composition in Brachiopods  

The ability to secrete mineralized hard parts is crucial to many animal phyla, as it enables the formation of structural components that commonly define the overall shape of the organism.  Because of this fundamental role, biomineralizing systems evolve relatively conservatively, and only rarely show a drastic change in composition.  Brachiopods are an example where this has happened: while the subphylum Linguliformea exclusively uses apatite as its mineral component, the other two subphyla Rhynchonelliformea and Craniiformea secrete calcite shells.  As the oldest known brachiopod faunas from the Tommotian already contain both phosphatic and calcitic taxa, the split between these two fundamental biomineral systems must have occurred at the very base of the phylum. 

Recent work has shown that the shell structures and compositions of early Cambrian brachiopods are more diverse than generally assumed.  They transcend the traditional pigeonholes of phosphatic and calcitic shells, and include shells which may have been entirely organic, and shells that are microstructurally and compositionally intermediate between the typical organophosphatic and calcitic types.  In order to reconstruct the sequence of shell mineral acquisition, we need an unambiguous stem group that allows us to polarize shell composition. 

Currently, the most compelling scenario roots brachiopods within tommotiids.  This is supported by the shared shell-penetrating setal tubes of tannuolinids and various problematic brachiopods, and by the recent discoveries of articulated scleritomes of the apparently sessile tommotiids Eccentrotheca and Paterimitra which suggest that the bivalved brachiopod body plan evolved through the successive shortening and simplifying of a multi-element tube.  New microstructural data now provide additional independent evidence suggesting that organophosphatic skeletal secretion in tommotiids is homologous with paterinids, the oldest known organophosphatic brachiopods.  Most studied tommotiid taxa and several paterinid taxa exhibit prominent polygonal structures that permeate the entire sclerite. 

The transition from tommotiids to paterinids included a gradual shift from a comparably dense lamination with micron-sized laminae to increasingly larger laminae with a decreasing phosphate/organic ratio.  The observed microstructural changes probably reflect an adaptation to a more fluctuating phosphorous availability, resulting from the switch of the presumed vagrant lifestyle of basal tommotiids to the sessile lifestyle of more derived tommotiids.  The origin of calcitic shells from such organophosphatic ancestors was probably also driven by the need to permanently maintain sufficient phosphorous levels for general metabolism.  The amounts of phosphorous needed for shell secretion would have made it particularly difficult to thrive in nutrient-poor regions such as many tropical reef environments, in which calcitic brachiopods prospered.