Field and petrographic data from Lower Ordovician rocks in New York suggest the intertidal development of undisturbed cryptalgal structures during periods of elevated paleosalinities, whereas mottled cryptalgalaminites represent times of near-normal paleosalinities. The extent to which algal-laminated deposits are preserved is related to salinity controls on the peritidal distribution of grazing and burrowing invertebrates. The invertebrates, if abundant because salinity was near-normal, will destroy the laminated nature of the algal mats in an environment in which these invertebrates thrive. In contrast, algal-laminated deposits are well preserved in environments of elevated salinities because of the paucity or absence of algae-scavenging invertebrates. These conclusions lend support to the concept of an antipathetic relationship between blue-green algae and predatory herbivorous or burrowing invertebrates. This relationship is a second-order process operating on a blue-green algal community that was already displaced to sea-marginal environments since at least the Middle Ordovician.