Shell microstructure and ligament structures have historically been important components of research on phylogeny and taxonomy in the Bivalvia. However, ligament structures, as recently redefined by Waller (1990) and Carter (1990a), remain poorly known for many extinct as well as extant bivalves, and data of shell microstructure have lagged behind other aspects of comparative anatomy because of the persistence of an inadequate descriptive nomenclature, a paucity of data for Paleozoic and Mesozoic families, and the lack of an accurate, comprehensive perspective on evolutionary pathways and convergence. Carter (1990a) recently demonstrated that new data of subclasses Palaeotaxodonta, Pteriomorphia and Isofilibranchia can provide significant new insights into family and higher level evolutionary relationships. The proposed research will extend this database to include the remaining bivalve subclasses Heteroconchia and Anomalodesmata, and it will use the entire database for the Bivalvia in conjunction with published and new data of shell morphology, hinge dentition, shell musculature, gill, palp, pedal and stomach structure, and all other pertinent aspects of hard and soft anatomy to provide the first truly comprehensive, family-cladistic analysis of this entire class. The cladistic analyses will employ two alternative microcomputer programs with efficient routines for finding parsimonious cladograms, as well as a hybrid program, presently under development by Fisher (1991), which will combine some of the salient points of conventional cladistics and stratophenetics. Competing parsimonious cladograms will be evaluated from the perspective of traditional stratophenetics. In addition to providing new perspectives on evolutionary convergence in shell and ligament microstructure and ligament structure, this research is expected to resolve many important but previously intractable questions of bivalve evolution, including the early diversification of shell microstructure in the Bivalvia, the origin of porcelaneous structures in the Veneroida, possible diphyletic origins of the Crassatelloidea, the origin of the myoid superfamilies, and the evolution of porcelaneous microstructures and asymmetrical ligaments in the Anomalodesmata. Additionally, this research will provide a sound phylogenetic and taxonomic framework for future lower-level taxonomic studies, as well as a basis for broader studies of evolutionary radiation and extinction.
