Larval biology of early Cambrian molluscs: The implications of small body size

Current theory suggests that the ancestral larval condition in molluscs was planktic and planktotrophic. Using Recent marine gastropods with similar shell morphologies and Recent molluscan life-history data, we constructed a model that estimates Cambrian univalve fecundities over a range of shell lengths, egg sizes, and body plans. Using the correlation between fecundity and mode of larval development present in Recent marine benthic invertebrates, we infer mode of larval development for Cambrian species. Because of their minute body size, the potential fecundities of most Early Cambrian taxa are substantially below the fecundities correlated with planktic development in Recent gastropods. By the Late Cambrian and Early Ordovician molluscan body size sufficiently increased to produce fecundities similar to those correlated with planktic development in Recent taxa. The life-history patterns present in extant molluscan groups with Cambrian originations also strongly suggest external fertilization, relatively large eggs and lecithotrophic development as the ancestral condition. We propose that the evolution of larval development in Cambrian molluscs proceeded from nonplanktic lecithotrophic development to planktic lecithotrophic development and that planktotrophic larvae are secondarily derived.