ABSTRACT: The families Calciosoleniaceae, Syracosphaeraceae and Rhabdosphaeraceae belong to the order Syracosphaerales and constitute a significant component of extant coccolithophore species, sharing similar ultrastructural bauplans. The literature on Calciosoleniaceae is reviewed and combined light- and scanning electron microscope studies of four extant species including Calciosolenia subtropicus sp. nov. described herein, reveal the unique interpretative V/R structure of the Calciosoleniaceae. Calciosolenia subtropicus sp. nov. is primarily distinguished from previously well documented extant species, Calciosolenia murrayi, Calciosolenia brasiliensis and Calciosolenia corsellii, by the consistent presence of prominent clockwise imbricated wedge shaped R-elements at the inner margin and in distal view of both ordinary and apical scapholiths, distinctly tapering in size towards the major axis of the scapholiths, as observed under the scanning electron microscope. Prompted by the contrasting birefringence displayed by the rim, central area, apical and ordinary coccoliths under crossed-polarized illumination in earlier publications, the Energy Dispersive Spectrometry of Calciosolenia subtropicus sp. nov. reveals the presence of (a) calcite + high silica in the ordinary coccoliths showing dark appearance and (b) high calcite + negligible silica in the apical coccoliths showing bright appearance; besides the c-axis orientation of calcite, the different elemental composition of coccoliths seems to control birefringence. Molecular genetic studies are necessary to provide insight on the relationship between the Calciosoleniaceae and other coccolithophore families including reasons for its successful invasion in contrasting ecological regimes.
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