Abstract
Pedicle valves greatly outnumber brachial valves in disarticulated samples of the Upper Ordovician brachiopod Platystrophia ponderosa. Some articulated specimens are compressed (shortened), probably by burial and compaction. The shortening is accommodated by fracturing, which is largely confined to the brachial valve. This preferential fracturing of the brachial valve can account for the observed paucity of whole brachial valves of P. ponderosa in the fossil record, and suggests that mechanisms other than differential current transport contribute to valve disproportionation. Discriminating between autochthonous and transported assemblages on the basis of valve ratios may lead to erroneous paleoecological interpretations, because brachial valves could be destroyed preferentially, without significant transport to the burial site.
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