Article: Constructional morphology of bivalves: evolutionary pathways in primary versus secondary soft-bottom dwellers [twenty-fourth annual address, delivered 10 March 1983]

In contrast to the minor within-habitat improvements in shell shape and sculpture of primary soft-bottom dwellers, the transition of fixosessile rock dwellers back to soft substrates has resulted in fast and drastic morphological changes. They were facilitated by the available ecologic stepping stones that caused morphogenetic programmes—first to 'derail' (rock habitats), then to be shortened (to fit the size of dead shell habitats), and finally to extend again (in order to increase mechanical stability on soft bottoms). The third step allows only a limited number of adaptational strategies (flat, outriggered, and edgewise recliners; mud stickers; 'pickabacks') that led to convergent shell forms in different groups of bivalves. Within groups, however, phylogenetic and morphogenetic constraints, as well as the adaptational landscape, channel evolution to such a degree that it becomes difficult, at least in the fossil record, to resolve the multitude of parallel and iterative lineages.