Taphonomic effects of sea-floor exposure on an Ordovician brachiopod assemblage

The taphonomy of the Cincinnatian brachiopod Platystrophia ponderosa reveals a complex history of draft filling and biologic occupation of the shell, commonly followed by disarticulation and preferential destruction of the thinner brachial valve by abrasion and breakage. Polished sections of 50 shells exhibit a repeated filling sequence beginning with mixed and laminated skeletal debris and micrite, deposited from suspension in gaped shells. Pelletal concentrations, produced in situ by a shell-inhabiting organism, truncate the underlying fill. Shell fillings imply prolonged post-mortem exposure of gaped, articulated shells, contrary to arguments that brachiopod shells do not gape following death and rapidly disarticulate. In bulk samples of disarticulated valves, the pedicle:brachial valve ratio increases with abrasion and breakage. This correlation, combined with the greater thickness of the pedicle valve and the lack of brachial valve-dominated assemblages, suggests that skewed valve ratios in Platystrophia ponderosa result from preferential destruction of the thinner brachial valve, rather than from selective transportation of one valve, as observed for pelecypods. Differences in shell preservation are controlled by the duration of post-mortem exposure, not by water energy alone.