The geometry of conchiferan shell evolution: origins of coiling and bivalved morphologies.

The article focuses on a geometric model explaining fundamental features of conchiferan shell morphology, including the origin of coiling, lateral compression, and the evolution of bivalved forms within the Diasoma (the clade comprising Scaphopoda and Bivalvia). The model posits that these shell features arise from genetically controlled asymmetries in mantle mineralization rates along the shell margin, expressed as growth stimulus gradients influencing shell shape during ontogeny. Three hypotheses for the origin of bivalved shells are tested: derivation from laterally compressed univalved ancestors, bifurcation of growth gradients producing pseudobivalved Rostroconchia, and bifurcation of the shell field itself yielding true bivalved Bivalvia. The study suggests that multiple independent and convergent evolutionary events in Cambrian soft-sediment environments contributed to the diversity of laterally compressed and bivalved morphologies, with the model providing a mechanistic framework linking developmental processes to shell form evolution.