Using critical path method to analyse the radiation of rudist bivalves

It has been suggested that certain families of rudist bivalves (superfamily Hippuritoidea) were ‘preadapted’ to inhabit different environments and radiated to fill them. This scenario was based on a hypothesis of a geometric constraint of shell growth, a differential increase in the number of rudist genera possessing uncoiled shells relative to those possessing spirogyrate (i.e. coiled) shells, and a speculative phylogeny. In this paper, these ideas are reformulated and tested by comparing a postulated sequence of structural changes, expressed according to critical path method of management theory, and phylogeny, inferred from cladistic analysis. The hypothesis of constraint is unfalsified, and the cladogram is consistent with a proliferation of rudists possessing uncoiled shells. The proliferation may be attributed to the origin of an invaginated ligament, which should be considered as a direct adaptation.