The Ordovician was a time of extensive and pervasive low-magnesium calcite (LMC) precipitation on shallow marine sea floors. The evidence comes from field study (extensive hardgrounds and other early cementation fabrics in shallow-water carbonate sequences) and petrography (large volumes of marine calcite cement in grainstones). Contemporaneous sea-floor events, particularly relationships with boring and encrusting organisms and reworking in sequences of intraformational conglomerates, confirm the early timing of such LMC cementation, and also of widespread associated aragonite dissolution. Local evidence points to the dissolved aragonite as a significant source of the calcite cement. This scenario, and the fabrics that provide the evidence for it, are likely to be pointers to other times in the stratigraphic record when LMC was the predominant shallow marine precipitate (Calcite Sea times). The combination of rapid calcite precipitation and aragonite dissolution at a time early in the Phanerozoic when many major invertebrate groups were becoming established may have acted as an influence on the evolution of both their skeletal mineralogy and their ecology.