Mallomonas aperturae sp. nov. (Synurophyceae) reveals that the complex cell architecture observed on modern synurophytes was well established by the middle Eocene

摘要

Mallomonas, the largest genus within the class Synurophyceae, consists of unicellular, flagellated, heterokont organisms each with an external cell covering comprised of species-specific overlapping siliceous scales. Most species also possess bristles, which are elongated needle-like siliceous structures that radiate out from the scale coat. A new fossil species, Mallomonas aperturae, is described from the middle Eocene Giraffe Pipe locality situated near the Arctic Circle in northern Canada. The new species possesses uniquely sculptured tripartite scales, each possessing a well-developed V-rib, dome and anterior wing-like extensions. The most unique feature is a large and elongated opening along the posterior side of the dome, a structure known only on scales of the modern species, Mallomonas paludosa (Synurophyceae). It is proposed that the opening on the dome may provide a means for securing bristles to the scales and aiding in bristle rotation. Remarkably, remains of cells with scales still in position were recovered from Giraffe Pipe mudstones, allowing for examination of the alignment and overlapping nature of scales making up the cell covering. The V-rib and anterior wing structures on the scale surface were used to precisely space and orient the scales, indicating that the ability to produce a highly structured cell covering, a hallmark of synurophyte algae, was well established by the middle Eocene. This evidence further supports the hypothesis that fossil species bearing scales with well-developed V-ribs can be used as a marker for the ability of the organism to build a highly organised cell covering.