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Abstract:

Rare but exquisite phosphatization of intercrystalline soft tissue in some shells of the latest Ordovician orthide brachiopod Plaesiomys anticostiensis from Anticosti Island, eastern Canada, is interpreted to have been microbially induced while the brachiopod was still alive, or immediately after death, before intensive decay of soft issue could have taken place. This is indicated by the ubiquitous preservation of filamentous microbes only in areas of the shell with exceptional phosphatization of soft tissue, down to subcellular detail, but absent in areas lacking phosphatization. Nonpenetrating epipunctae and aditicules are shown to have housed small and relatively large setae, which were in turn connected by organic membranes located between calcitic shell layers. In phosphatized areas, the setae display excellent subcellular-level setal structures (e.g. microvillous canals and their membranous walls with details to the 100-nm scale). In nonphosphatized areas, each seta was replaced by a single calcite rod, resembling the core of strophomenide pseudopunctae, that previously have been puzzling to interpret because of the lack of modern analogues. The permineralized microvillous canals of Plaesiomys left clearly marked longitudinal striations on the inner surface of epipunctae. This study suggests that the strophomenide pseudopunctae may have been soft tissue permineralized or replaced by calcite and confirms the setigerous nature of tubular microstructures with striated inner walls in other stem-group brachiopods.

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GeoRef, Copyright 2018, American Geological Institute.<br/>2011-019563<br/>Plaesiomys anticostiensis<br/>Velleda Member