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Pyxilla? dubia  was originally described from Jutland, but subsequently transferred to  Pseudopyxilla . However, the discovery, over eighty years apart, of two enigmatic specimens with tubular heterovalvate frustules featuring a  Pseudopyxilla dubia  valve at one end and a rhizosolenioid valve at the other end resulted in a new combination,  Rhizosolenia dubia . Herein, we document in detail the valve morphology of specimens observed in early Eocene samples from Mors (Jutland) using light and scanning electron microscopy. Our results reveal the presence of both valve types (but never as heterovalvate frustules). The rhizosolenioid valves possess a proboscis with three spinulae at the tip (the middle one being very extensive), a longitudinal slit on the dorsal side, and claspers on the basal part of the domed valve on the ventral side. Our results suggest that the  Pseudopyxilla  frustule is the resting spore of a yet, unknown diatom, while the ‘rhizosolenioid’ valve represents the vegetative phase of a new species,  Proboscia pinocchioi  sp. nov. From an evolutionary standpoint,  P. pinocchioi  lacks the longitudinal ridges of the  cretacea  lineage, but bears the characteristic polar spinulae and limited pore distribution of the  barboi  lineage. However,  P. pinocchioi  surprisingly possesses claspers (normally observed only in the  alata  lineage) – representing the oldest evidence of claspers in rhizosolenioid diatoms.

diatom research

On the enigmatic diatom  Rhizosolenia dubia , its relationship to  Pseudopyxilla  and  Proboscia , and description of a new species,  Proboscia pinocchioi  sp. nov.