OPEN WIDE! HOW MUCH IS KNOWN ABOUT THE GAPE ANGLE IN STROPHOMENID BRACHIOPODS?

The maximum gape between the valves of a strophomenid brachiopod is estimated from the muscles and lever system. If the valves continued to open until the diductor muscles pulled directly down the cardinal process, then a gape of about 40º is inferred for specimens from five genera. But could a strophomenid open this widely, or did other morphological features block the gape? Silicified specimens of Rafinesquina are capable of opening to about 40º. Two dimensional models based on closely-spaced serial sections show that neither the teeth—socket arrangement nor the pseudodeltidium—chilidium acted to block the shell opening. The mechanical efficiency of the cardinal process drops rapidly as the gape widens, but once the inertia was overcome and water sucked into the shell, the force required to open the next 30º may have been quite low. No morphological or mechanical features were found in the specimens examined that preclude a wide gape.

What are the implications of a wide gape for the life style of strophomenid brachiopods? The function of geniculation in preventing sediment entry is not affected by the degree of opening. In a concave-up position, a trail on the convex valve would prevent sediment entry regardless of the gape. Rotational models show that features such as the diaphragm and submarginal rim, suggested by previous authors to limit the gape and act as sediment strainers, were not effective in narrowing the shell opening to prevent sediment entry.

A wide opening between the valves of strophomenid brachiopods may imply a different feeding system from the inhalant-exhalent model used for biconvex brachiopods with complex spiral lophophores. Unlike biconvex brachiopods, the concavo-convex shape of strophomenids limited the shell volume and lophophore size, but maximized the mantle area. If the strophomenids opened widely, exposing the mantle on the large flat valves to currents, perhaps the mantle was ciliated and carried food particles to the small lophophore.

There are alternative models to the free lying, concave-up lifestyle. If moat-making were an habitual activity for strophomenids that lived convex-up, then a wide gape (powered by the extremely large diductor muscles that characterize most strophomenids) may have been a way to displace sediments. These alternate models need to be considered.