A case for biotic morphogenesis of coniform stromatolites

Mathematical models have recently been used to cast doubt on the biotic origin of stromatolites. Here by contrast we propose a biotic model for stromatolite morphogenesis which considers the relationship between upward growth of a phototropic or phototactic biofilm (v) and mineral accretion normal to the surface (λ). These processes are sufficient to account for the growth and form of many ancient stromatolities. Domical stromatolites form when v is less than or comparable to λ. Coniform structures with thickened apical zones, typical of Conophyton, form when v≫λ. More angular coniform structures, similar to the stromatolites claimed as the oldest macroscopic evidence of life, form when v⋙λ.