Brachiopods have been successfully used for many years to provide local, regional, and worldwide biostratigraphic control. Their value as phylogenetic time-markers is somewhat impaired by the fact that as sessile filter feeders they are particularly susceptible to the impress of varying local environmental conditions. The situation is especially troublesome where precise, finely tuned biostratigraphic zonation is attempted, as it becomes difficult to separate out the effects of time (phylogeny) from the effects of paleoenvironmental variations. Paleoenvironmental interpretations involve many uncertainties, as they are based on indirect evidence, a problem compounded by differences of opinion concerning the data used to provide the basic underpinning.