Fossil species of Mallomonas from an Eocene Maar Lake with recessed dome structures: Early attempts at securing bristles to the cell covering?

The genus Mallomonas, the largest within the Class Synurophyceae, consists of motile, unicellular, freshwater heterokonts that are covered with an outer layer of overlapping siliceous scales. The scales are flat, circular to oval structures with taxonomically distinct designs. In addition to scales, the cell coverings of most Mallomonas species contain a second siliceous component known as a bristle. Bristles are composed of a long thin shaft and a flattened proximal foot. The foot of the bristle is fastened under the distal end of a scale in such a way as to allow the shaft to radiate out from the cell and rotate on its axis. For many species, the distal scale margin is distinctly raised, forming a hollow space that secures the bristle foot, and an inverted U-shaped opening from which the shaft emerges. This structure is called the dome. We describe three fossil species of Mallomonas from an Arctic Eocene maar lake that contain domes that are recessed to varying degrees back from the distal scale margin. This design is not known in the modern flora and we hypothesize that it represents an early attempt at securing bristles to the scale coat.