“Molar-tooth” structures are microcrystalline calcite-filled features of long-debated origin that are common in Mesoproterozoic and early Neoproterozoic fine-grained, shallow-water carbonates. We have constrained the environment of their formation from the standpoint of depositional and diagenetic conditions inferred on the basis of petrographic and geochemical data derived from examples in the Helena Formation, a calcareous marine interval within the Mesoproterozoic Belt Supergroup. The collective data set supports an early-diagenetic origin for molar-tooth structures and implies environmental factors that promoted rapid precipitation of calcite into CO2-generated voids in cohesive muds. We propose that the temporally restricted distribution of molar-tooth structures in the geologic record reflects a unique combination of the environmental parameters that control their formation. Of these factors, the most fundamental include CaCO3 saturation and redox conditions in shallow marine waters that promote prolific carbonate production, tectonic quiescence and the availability of extensive epicratonic sites for carbonate accumulation, and the absence of bioturbators.