No satisfactory genetic interpretation of rhythmic marl-limestone sequences has yet been published. Previous studies of the problem have relied mainly on sedimentological data, and, as far as the author is aware, there has been no investigation of the faunal assemblages contained in such sequences. In this present paper an attempt is made to combine sedimentological and palaeoecological data. The case treated is theOoser Plattenkalk(Upper Devonian I of the Eifel area, Rheinisches Schiefergebirge; see Fig. 1). This facies unit consists of a rhythmic sequence of calcareous shales and argillaceous limestones with distinct carbonate-clay ratios.