Abstract

Brachiopods were one of the most diverse groups of reef-dwellers during the Paleozoic, and their degree of specialization for reef habitats provides an important way of assessing the ecologic complexity in reef communities. Silurian (Wenlockian) reef brachiopods in Gotland and Wisconsin are compared to level-bottom brachiopods in Gotland, Wisconsin, and the Welsh Borderland. The reef—level-bottom comparisons are made at the level of single bulk samples, as combined groups of samples from single reefs and from single level-bottom communities, and as combined groups of samples from several reefs that occupy the same stratigraphic horizon. Species diversity of reef brachiopods is higher than that of level-bottom brachiopods, but the amount of this difference decreases from the scale of entire reefs to the scale of single samples, where Shannon indices (H′) and rarefaction curves of reef and level-bottom samples commonly overlap. In terms of species richness, maximum reef values reach 44 species (n = 3452 individuals), while maximum level-bottom values reach 32 species (n = 7732 individuals). Cluster analysis separates associations of reef and level-bottom brachiopods, but there is also some compositional overlap of reef and level-bottom samples. Although the reefs contain more pedunculate brachiopods and fewer strophic, free-lying brachiopods than level bottoms, the reef brachiopods include no morphotypes or genera that are not also known from the level bottom. Wenlockian reef brachiopods in Gotland and Wisconsin were open surface dwellers with very close taxonomic, functional, and ecologic ties to level-bottom communities. As such, they indicate very similar levels of ecologic complexity between reef and high-diversity level-bottom communities. Cryptic reef brachiopods are known from the Silurian, but appear to have been rare. Brachiopods did not include common reef specialists until the Late Paleozoic. The slow pace of brachiopod specialization for reefs and the very close resemblance of Silurian reef and level-bottom brachiopods reflect the ecologic simplicity and long-term diversity plateau of the Paleozoic evolutionary fauna.

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