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Paper No. 5
Presentation Time: 2:30 PM


CARLSON, Sandra J., Department of Geology, University of California, Davis, One Shields Ave, Davis, CA 95616, ROOPNARINE, Peter D., Invertebrate Zoology & Geology, California Academy of Sciences, 55 Concourse Dr, Golden Gate Park, San Francisco, CA 94118, ANDERSON, Laurie C., Geology and Geological Engineering, South Dakota School of Mines and Technology, 501 East St. Joseph St, Rapid City, SD 57702 and GOODWIN, David H., Department of Geosciences, Denison University, 100 Sunset Hill Drive, Granville, OH 43023,

Are bivalved shells the expression of common, underlying historical, genetic, and developmental mechanisms? The bivalved condition has evolved multiple times in Lophotrochozoa, most notably in bivalved mollusks and brachiopods. We hypothesize that mollusks and brachiopods share, by common ancestry, genetic mechanisms of conchological growth and development, reflected in patterns of modularity and integration within and between the two valves. In a bivalved mollusk or brachiopod shell the valves grow by commissural accretion. The mantles that mineralize them are in continual contact at the growing edge during ontogeny, maintaining an enclosed space between valves. The mechanism joining the two valves and allowing them to move relative to one another, commonly about a hinge, is powered by muscles that originate on one valve and insert on the other.

Certain bivalves exhibit patterns of modularity among landmarks of a single valve, and surprisingly different degrees of integration and modularity between left and right valves: left valves are highly integrated with two distinct modules; right valves are poorly integrated and no modules are recognized. This asymmetry in integration suggests that one valve drives the growth and morphology of the whole bivalved skeleton, carrying the morphology of the second valve along more or less passively. To test the generality of this prediction among bivalved lophotrochozoans, we are investigating patterns of modularity and integration in bivalved mollusks and articulate brachiopods. Considering the differences in orientation and symmetry between bivalved mollusks and brachiopods, and differences in valve mechanics, some differences will manifest themselves. We predict, however, that the brachiopod ventral valve is more highly integrated than the dorsal, and that the left and right sides of each valve form two modules. We will also investigate the covariance of integration with size, ontogeny and stratigraphic age in each clade.

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