Planothidium scrobiculatum sp. nov. (Bacillariophyta), a new monoraphid diatom from freshwater Pleistocene deposits of South America

A new monoraphid diatom species appeared successively in paleolimnological records from a Quaternary sediment core (1.5±0.1 Ma) retrieved from Colonia basin (Brazilian Coastal Plain, Atlantic Forest, Sao Paulo Metropolitan Region), and is formally described as Planothidium scrobiculatum sp. nov. We studied the new species through light and scanning electron microscopy and discussed its similarity to other members of the genus. The new species has a unique combination of morphological features that allow its separation from all other species in the genus: (1) lanceolate valve outline with cuneate apices, (2) multiseriate striae composed of two to four rows of irregular sized areolae on the rapheless sternum valve (SV), not interrupted at the valve mantle junction, and (3) arrangement of areolae on the valve mantle, grouped in 3 or 4 rows, each composed of 2 to 3 areolae on the SV. Nevertheless, the new species main diagnostic feature is the occurrence of numerous, unusual, small shallow depressions externally located on the SV along the axial area, cavum, and virgae. As a result, P. scrobiculatum is different from similar species under LM and SEM by some evident characters or a combination of characters. This study contributes to the knowledge of the so far understudied biodiversity of microorganisms of tropical regions. In addition, the above-mentioned diversity patterns and the wealth of new diatom taxa inside and around the Colonia basin (many yet to be described) suggest that further samplings from other localities may provide an insight into its habitat preferences and range, since this area is an exceptional biological entity, worthy of further study.