Scale dependent diversity of bryozoan assemblages in the reefs of the Late Ordovician Vasalemma Formation, Estonia

Abstract

The reefs of the Vasalemma Formation, late Sandbian, Late Ordovician, of northern Estonia contain an exceptional rich and abundant bryozoan fauna. They are an example of contemporaneous bryozoan-rich reefs known from around the world, representing the peak diversification interval of this group during the Ordovician. The global Ordovician bryozoan diversification was associated with a decrease in provinciality, a pattern known from other skeletal marine metazoans of this period. The diversification is associated with climatic cooling and increasing atmospheric and sea water oxygenation. However, the mechanisms that led to the bryozoan diversification are poorly known. Here we estimate the bryozoan richness (α and γ diversity) and turnover (β diversity) at the level of samples, reefs, and formations in the Vasalemma Formation and in contemporaneous reef limestone occurrences of the Baltoscandian region. The resulting richness and turnover values differ among the three observational levels and hence are scale dependent. A consistent pattern with lowest between-reef turnover and relatively high between-sample turnover could be detected, reflecting high small-scale (within reef) heterogeneities in lithology and original bryozoan habitat. This is consistent with published work, in which evidence has been presented for small-scale substrate heterogeneity as the most important diversification driver of the Ordovician brachiopod diversification in the Baltoscandian region. The fact that reefs and their local substrate are strongly organism moderated environments sheds light on the potentially important ecosystem engineering role of organisms, such as bryozoans, for the Ordovician diversification.