Molluscan Phylogeny

Abstract

Mollusks are likely descendants of turbellarian- like flatworms and have probably never been segmented. The first mollusks were probably spiculose and lacked a shell; thus, it seems reasonable to regard the Aplacophora as direct descendants of these first mollusks and not as a degenerate group that has lost the shell. A shell probably evolved twice in mollusks, once in the multivalved polyplacophorans and once in the univalved monoplacophorans. The bivalved pelecypods are likely descendants of the univalves. The class Monoplacophora is probably ancestral to the Gastropoda, Cephalopoda, and Rostroconchia. In turn, rostroconchs probably gave rise to the Scaphopoda and Pelecypoda. Possible fossil ancestors of the cephalopods, rostraconchs, pelecypods, and scaphopods are documented and figured.