Abstract

A uniquely preserved occurrence of the plectambonitoid brachiopod Sowerbyella rugosa from the Upper Ordovician of Northern Kentucky shows several individuals preserved in direct association with burrow-like structures and oriented with the commissural plane vertical and the hinge line down in the upper laminated siltstone of a tempestite bed. Two types of structures are present: (1) structures that document translational motion of convex-up brachiopods to the surface, then rotation to a vertical position with the commissure near the sediment-water interface; and (2) structures that show slightly longer-term occupation and sediment clearing around the brachiopods before they expired in the post-storm accumulation of fine mud. The abilities demonstrated by these individuals require a rethinking of the helpless-strophomenid paradigm. The peculiar plectambonitoid traits, mantle threads (as evidenced by interarea canals) and external mantle flaps (as evidenced by comae) were unlikely to have contributed to the ability of these individuals to burrow upward. Valve clapping is the only apparent means by which Sowerbyella rugosa could have moved through the sediment. If plectambonitoids could have moved by clapping their valves, then the same may have been true of strophomenoids, such as Rafinesquina alternata. The ability to move sediment and adjust position explains how strophomenids could live in a convex-up position or even, if partially buried, in more vertical positions, as documented by previous epibiont studies. These abilities suggest that geniculation may have served to anchor shells of active individuals in an inclined or vertical position in higher-energy environments of shifting substrates rather than to lift commissures of passive individuals in a convex-down position in lower-energy environments of rapid mud accumulation.

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