The Silurian Dicoelosia communities: temporal stability within an Ecologic Evolutionary Unit

Abstract

Dicoelosia communities were a widely distributed, high-diversity type of brachiopod association that inhabited bioturbated muds in outer shelf settings (Benthic Assemblage 4/5). A database from Avalonia, Baltica, and Laurentia for the Early Silurian to Early Devonian shows that brachiopod species diversity in this major community type remained stable for a 30 million years interval during a single Ecologic Evolutionary Unit (EEU). Brachiopod species richness in local communities ranges mainly from 20 to 35, and Shannon indices ( H′) per sample are mainly 1.5–2.5, with a maximum of 2.8; neither value shows any long term trend of stratigraphic increase or decrease. Stability in diversity is accompanied by persistence of the same functional morphotypes of brachiopods. Some genera, particularly among Orthida and Strophomenida, are present in the communities through all or most of their stratigraphic range, while other genera, particularly among smooth biconvex recliners, show a pattern of temporal replacement within a morphotype. Dicoelosia communities appear and disappear in local areas as a consequence of changes in depositional facies, but on the large scale of the three sampled plates, they show no apparent changes related to Silurian eustatic or other global oceanic events. In general, the history of Dicoelosia communities corresponds to the model of ecologic stasis within an EEU. This stasis is shown by species diversity and morphotype content and not by precise generic and specific composition. Data for the Dicoelosia communities do not suggest a ‘locked,’ highly-interactive ecologic structure, and they are more compatible with a model of environmentally-related recruitment in local areas from a widely distributed pool of offshore larvae.