The bellerophont controversy revisited

An old controversy reestablished itself in the late 1970s and early 1980s t hat focused on the systematic placement of the enigmatic Bellerophontoidca (informally, "bellerophonts"). a group of planispirally coiled, wholly foss il molluscs. The controversy embraced three fundamental concepts that ale b ased on different philosophical interpretations of shell form, muscle scar patterns, and other preserved shell features. 1) all belleropbonts were mon oplacophorans; ?) all bellerophonts were gastropods; and 3) some belleropho nts were monoplocophorans and some were gastropods. A review of the main is sues appearing in the literature since the early 1980s indicates that these three philosophical divisions still exist and, indeed, have become entrenc hed. An examination of the relevant anatomical and shell features of recent gastropods and monoplacophorans, and comparison with preserved features in enigmatic fossil forms, convinces us that the bellerophontoideans and the coiled and high-domed "monoplacophorans" (Cyclomya) were gastropods. Only t he flattened, spoon- and cap-shaped monoplacophorans (Tergomya) were true m onoplacophorans. We present a hypothetical scheme for the morpho logical di versification of gastropods from early monoplacophorans that could account for Cyclomya, Bellerophontoidea, Patellogastropoda. and Prosobranchia.