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Strength of pedicle attachment in articulate brachiopods: ecologic and paleoecologic significance

It has been claimed that articulate brachiopods are excluded from turbulent environments by the weakness of their pedicle attachment. The evidence does not support this conclusion. Experiments on the force required to remove brachiopods from their substrate show that removal force often equals that of bivalve mussels from turbulent environments having the same valve area. Intertidal brachiopods probably occupy cryptic habitats to avoid desiccation, not to avoid waves. Subtidal brachiopods are abundant in tidal currents of 4 knots (2 m/sec) where they are usually attached in exposed positions. Although paleoecologists often assume that the size of the pedicle foramen is directly proportional to attachment strength, the relationship is not precise. Experimental detachment of brachiopods resulted from failure of the substrate, pedicle-substrate attachment, pedicle, or pedicle adjustor muscles, in approximately equal frequencies. At the same attachment strength, the foramen in Terebratalia is four times the size of the foramen in Hemithiris. Although subapical foramina are supposedly unable to enlarge as the brachiopod grows, the subapical foramen of Hemithiris appears to do so.