MORE THAN A THIRD OF PALEOZOIC BRACHIOPOD GENERA LIVED INFAUNALLY -- REALLY!

In reconstructions, extinct articulate brachiopods have nearly always been depicted as epifaunal animals. The largely concavo-convex Strophomenata (which include strophomenids and productids) constitute more than a third of Paleozoic articulate brachiopod genera. There has been controversy as to whether members of this group lived concave-up or concave-down on the surface of the substratum, but neither of these positions would have been viable. The concave-up position would have been physically unstable; even weak water movements would have rolled over animals in this position. The concave-down position would have jammed the commissure into the substratum, making feeding and respiration impossible. These problematical postures have only been entertained as possibilities because the animals were assumed to have lived epifaunally: there seemed to be no other option -- but there is one. I conclude that nearly all strophomenatans actually had shallow infaunal modes of life, in which they were both stable and in communication with the water above for feeding and respiration: they lived with the commissure at the sediment surface. Nearly all strophomenatans have a curious concave brachial valve, for which a function has never been found. In the shallow infaunal mode of life I reconstruct for them, the concave brachial valve has an important function. It becomes covered with sand to hide all of the animal but its narrow commissure region. There are excellent analogs here in the modern world. Concavo-convex scallops live in this way. Years ago I studied one of these: Pecten (now Euvola) ziczac. This species attains its shallow infaunal position by clapping its valves to blow away sand. When its commissure reaches the level of the sediment-water interface, it ceases this activity. Because sand on the seafloor seeks its own level, the blown-out area around the animal soon fills in. Some of their blown-out sand ends up on the concave upper valve, and if the valve is not completely covered, sand in traction and suspension soon settles to finish the job. Strophs could easily have evolved this mode of burrowing. The hollow spines of most productids, which would have been too fragile to have survived if their possessors had lived epifaunally, for an infaunal animal would have been protected, while serving to provide physical support.