Bimineralic bryozoan skeletons: A comparison of three modern genera

A combination of light microscopy, field-emission scanning electron microscopy (FE-SEM), electron microprobe, and Raman spectroscopic analysis is used here to study three cheilostome bryozoan taxa with bimineralic skeletons: Odontionella cyclops, Margaretta cereoides, and Pentapora spp. Typically for bimineralic bryozoans, all skeletal walls forming the box-like zooids are constructed primarily of calcite with only the outer layer of the frontal shield consisting of aragonite. The two minerals have characteristic lamellar (calcite) and fibrous (aragonite) ultrastructures visible in FE-SEM images. Similarities in skeletal ultrastructure between the three taxa are evident in the basal and vertical walls, which have almost the same thicknesses and fabrics. These calcitic walls consist of an inner layer with crystallites oriented parallel to the wall and an outer layer with crystallites perpendicular to the wall surface. The bimineralic frontal shields of Odontionella cyclops and Pentapora spp. are ca. 150 μm thick and Margaretta cereoides ca. 40-100 μm. Interfaces between calcite and aragonite layers in these walls are variable: in Odontionella cyclops the interface between the layers is relatively planar, while in the other two species it is undulose. In these bimineralic taxa frontal shields in the innermost parts of the calcite layer comprise low Mg-calcite with 3-4 mol% Mg, which increases to over 5 mol% Mg (intermediate Mg-calcite) near to the boundary with the overlying aragonite layer; Mg content drops to a minimal value of 0.2-0.3 mol% Mg in the aragonite. These findings are significant for our understanding of both biomineralization processes in bryozoans and the use of Mg skeletal chemistry as a temperature proxy.