Scaled chrysophytes in Middle Eocene lake sediments from Northwestern Canada, including descriptions of six new species

Although chrysophyte stomatocysts have a fossil record that likely extends to the lower Cretaceous, to date neither scales nor bristles of the scaled chrysophyte genera older than Holocene have been reported. This leaves the origin of this important group of freshwater algae a mystery. We discovered abundant and diverse scaled chrysophyte microfossils in ca. 47 Ma sediments (Middle Eocene, Lutetian) deposited as the post-eruptive infill of a crater formed during intrusion of the Giraffe Pipe kimberlite, in the Northwest Territories of Canada. The microfossils are exceptionally well preserved and represent at least eleven different scaled chrysophyte taxa, including 7 species of Mallomonas, 1 of Synura,2 of Spiniferomonas and 1 of Chrysosphaerella. Six of these are herein described as new species. Although some taxa do not appear to have modern analogs, others can be readily traced to, or are morphologically identical with, extant species. Our findings prove definitively that the most common genera of scaled chrysophytes found in freshwater habitats worldwide today were well established by the middle Eocene, and that these organisms are most likely much older than 47 Ma. Simple as well as highly complex scale morphologies are equally represented among Eocene chrysophytes. Furthermore, our observations lead us to hypothesize that bristles and domes associated with Mallomonas evolved well after the genus had become an established component of lake phytoplankton. NB: 47 Ma = 47 million years