Populations of Epithyris 'subsella' in different Lower Kimmeridgian fossil assemblages show different size-frequency distributions although they exhibit a similar shape of survivorship curves. A population with large-sized adult specimens of E. 'subsella' was characteristic for a community in which most abundant were bivalves: Isognomon subplana, Lopha gregarea and Trichites saussurei. A population with small adult specimens was characteristic for a community with abundant dasyclad alga Goniolina geometrica. Here, among shelled suspension-feeders the most important producers were brachiopods E. 'subsella', Zeilleria humeralis, and Septaliphoria pinguis. Thalli of G. geometrica were the substrate for most of the sedentary organisms in this community. The estimation of biomass and productivity of particular species in fossil communities is discussed.