Pseudoplanktonic and benthic invertebrates in the Middle Jurassic Opalinum Clay, northern Switzerland

A high-resolution study of the Opalinum Clay of northern Switzerland confirmed the hypothesis of Wignall and Simms (1990) that pseudoplankton is exceedingly rare in the fossil record and that even in dark, organic-rich mudstones and black shales it constitutes only a minor proportion of the fauna. In the Opalinum Clay, only a bryozoan, one lepadomorph barnacle, and inarticulate brachiopods and oysters attached to ammonite shells represent the pseudoplankton. The life-style of the posidoniid bivalve Bositra buchi is re-evaluated. Based on the bell-shaped abundance distribution along an oxygenation gradient and nearest-neighbor spatial analysis, it appears that this species was truly benthic and probably byssally attached to the substrate. Previous suggestions of a chemosymbiotic mode of life for this bivalve are dismissed. Rather, this bivalve was an erratic opportunist capable of rapidly colonizing a variety of fine-grained sediments and reached its peak abundances in severely oxygen-depleted palaeo-environments. A benthic mode of life is also proposed for the inoceramid Pseudomytiloides dubius which, however, is also known to occur as (facultative) pseudoplankton. Evidence regarding the mode of life of the small gastropod Coelodiscus sp. is not conclusive, but suggests a benthic life-style.