Cyclotella kuetzingiana var planetophora and Stephanocostis chantaicus
These two taxa dominated the upper freshwater diatom assemblage. These diatoms form the planktonic component of the diatom assemblage and their appearance in the fossil record indicates the point at which the transition to freshwater conditions began.
The autecology of these centrics is not well documented. However, both have been recovered from the plankton of northern lakes (Kling and Hakansson, 1988).
Kling, H. and H. Hakansson. 1988. A light and electron microscope study of Cyclotellas species (Bacilaariophyceae) from central and northern Canada. Diatom Research. 3:55-82.