Valve Ultrastructure and Systematic Position of Rhopalodia wetzelii Hustedt (Rhopalodiales, Bacillariophyceae)

Rhopalodia wetzelii Hustedt is a little-known species described as a fossil from the Atacama Desert, Chile. It has subsequently been reported from Argentina and Bolivia, as both fossil and recent. Line drawings accompanied the initial species description in 1927, and light micrographs were published in 1987. To date, only a single scanning electron micrograph with limited information has been documented for this species. Here, we present detailed light and scanning electron microscopy observations of R. wetzelii showing unique features that set it apart from others in the genus. The valve is twisted about its apical axis, the keel is raised substantially off the valve face and externally the striae are comprised of circular areolae. Volate occlusions, seen on the valve exterior in most species of this order of diatoms, are located internally. The systematic position of R. wetzelii is discussed in the context of previous treatments of the genus.