Mallomonas gigantica sp. nov., an Eocene synurophyte possessing the largest known siliceous scales

A new fossil species representing the genus Mallomonas, M. gigantica, is described from the Giraffe Pipe Eocene maar lake locality situated near the Arctic Circle in northern Canada. The new species possesses rolled bristles and two basic types of scales that lack V-ribs, body scales and specialized apical scales. Mallomonas gigantica possesses the largest body scales reported for the genus, including for both fossil and contemporary taxa. Scales reached 12 µm in length and with a surface area near 100 µm2. Body scales are large, square-shaped, with a narrow posterior rim, large and thick anterior submarginal ribs, and a shallow dome with a curved margin that aids in securing the bristle. Base plate pores are small, and closely spaced over the scale surface except under the anterior submarginal ribs and dome. Specialized apical scales are smaller, more circular, asymmetric, and with a forward-projecting spine. The craspedodont bristles, have an open slit running the length of the shaft, an expanded and flat foot, and a row of small teeth lining the apex of the shaft. Although the scale structure is distinctly different, some features of the scales and bristles are most closely related to the fossil species M. schumachii, another taxon with large scales described from the same fossil locality. Given the lack of a V-rib, M. gigantica may represent a stem taxon of section Planae, but similarities with species in the modern section Punctiferae lineage are also discussed. The large, robust and heavy nature of the scales may have posed disadvantageous to the cell by making a slow swimmer more prone to sinking.