Lemer et al. (2016) regard the simple prismato-nacreous shell structure of pterioids and pinnoids as having limited their life habit repertoire compared with other pteriomorphs that had evolved the foliated shell microstructure. Nevertheless, we suggest that the combination of the two microstructures has produced effective armour. The development of the asymmetry described here in the pterioids also enabled morphological novelty, and a broader range of life habits, including becoming pleurothetic and lying on one valve.