Mallomonas discipulorum sp. nov. and M . inaequalis sp. nov. (Synurales, Chrysophyceae) expand our knowledge of species diversity from the early Eocene

Unravelling the fossil record of a clade of organisms is paramount to fully understanding their evolutionary history, and directly aids in aligning a molecular-based phylogeny to geological time. While specimens of most protist lineages are not recorded in the fossil record, many ancient forms representing the Synurales, especially from the speciose genus Mallomonas, have been uncovered over the last two decades. The process of finding, extracting and reconstructing the full array of siliceous components representing each fossil Mallomonas species is especially tedious and time-consuming compared to working with living specimens. Two new fossil species of the genus Mallomonas are described from the Eocene Giraffe Pipe maar lake locality situated near the Arctic Circle in northern Canada. One species, M. discipulorum, represents an extinct member of the large section Mallomonas. This taxon formed a scale type with a unique appendage resembling a paddleboard that is unlike any structure known from the modern flora. The second species, M. inaequalis, is the first fossil species representing the rare section Retrorsae, and is closely related to the modern species, M. retrorsa. Both fossil species were uncovered from mudstones deposited in a shallow, acidic, and humic-stained water body. Together, these new fossil taxa add to the evolutionary history of the genus and will provide additional mileposts for calibrating molecular phylogenies.