A whole-cell reconstruction of Mallomonas porifera Siver & Wolfe from the Eocene: Implications for the evolution of Chrysophyte cell architecture

Mallomonas porifera was originally described from Middle Eocene lake sediments based on isolated siliceous microfossils and named for the single large pore found centrally on the base plate. Both domeless body and domed scales were described, however it was not known how the scales were positioned on the cell covering nor if the organism possessed bristles. Since the original description, 15 additional samples taken from a long sequence of the sediment core and found to contain M. porifera have been examined, including fractured mudstone sections from each sample. The fractured surfaces were found to contain the remains of whole cells allowing for a detailed description of the siliceous cell covering. Mallomonas porifera contains three types of scales and one bristle morphology arranged in precise positions on the cell. Domed scales are restricted to the anterior of the cell and each is associated with a bristle. The domed scales transition abruptly to domeless body scales that surround the main portion of the cell and are not associated with bristles. The posterior part of the cell covering consists of large triangular-shaped and spine-bearing scales that lack the characteristic large base plate pore found on the other scale types. The relationship of M. porifera to modern taxa within the genus is discussed.