The Strophomenata, which includes two large orders, the Strophomenida and Productida, is the largest group of Paleozoic brachiopods. Nearly all uncemented strophomenatans possessed an unusual concave brachial valve. Most of these have been considered to have lived epifaunally, but had they rested on the seafloor, not only would they have faced intense predation, but their physical instability would have been fatal. I conclude that nearly all strophomenatans, like similar concavo-convex pectinid bivalves, lived infaunally by ejecting water to create a pit into which they descended, to be protected by sediment covering the concave valve. Strophomenatans have been discovered with this sediment preserved in place. If exhumed and turned upside down, a strophomenatan could have righted itself by squirting water. Many productides had anchoring spines, and some had hinge areas with stabilizing flanges. Small spines on the brachial valves of some productides served to trap disguising sediment. Evolutionary loss of hinge teeth within both the Strophomenida and Productida reduced the friction of valve clapping. Partly because of their slender shape, strophomenides were typically more vulnerable to exhumation than productides. Strophomenides also ejected water less effectively than productides and would have been less adept at righting themselves. The virtual disappearance of the strophomenides during the Devonian can be attributed to their vulnerability to intensifying benthic bulldozing and predation. The success of the productides during the late Paleozoic can be attributed to their relatively deep sequestration in the sediment and ability to right themselves and reburrow effectively when exhumed and overturned.
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Vol. 46 • No. 3