FORM AND FUNCTION OF UNIONOIDEAN SHELL SCULPTURE AND SHAPE (BIVALVIA)

The functions of Recent and fossil unionoidean sculpture are proposed in light of similar theories on the function of marine bivalve sculptu re. Functional models are given for shell morphologies for both soft a nd hard substrata. Most soft substratum taxa have shells of reduced th ickness and dentition, are laterally compressed, and generally are scu lptureless. These characteristics minimize the specific gravity of sof t substratum unionoideans. Sculptured taxa generally are found in hard substrata in large rivers. Shell sculpture is derived from an ancestr al divaricate pattern, and has been modified into the spectrum of unio noidean sculpture found in Recent and fossil species. Shell sculpture in this group is modified for anchoring and anti-scouring functions. B urrowing sculpture, found in many marine bivalves, may not occur in un ionoideans, but was exapted from ancestral burrowing sculpture for oth er roles. Big river taxa have evolved mechanisms for remaining buried, while headwater species have emphasized the ability to to rebury if d islodged. It is proposed that unsculptured big river taxa evolved in h eadwater situations and reinvaded large rivers with alternate methods to facilitate anchoring and reduce scour. These methods form the morph ological facies known as the ''Big River Effect.''