`Molar-tooth microspar': a chemical explanation for its disappearance ∼ 750 Ma

Molar-tooth structures are intricately crumpled, microsparry calcite fissure fills that formed during the Precambrian. Strontium isotope stratigraphy constrains the last occurrence of volumetrically significant molar-tooth structure (MT) in the geological record to ～ 750 Ma. Although the disappearance of MT is commonly ascribed to the influence of metazoans on sediment cohesion, this now seems less plausible because there is no evidence for significant sediment disruption by metazoans before ～ 550 Ma. It is proposed here that the most likely alternative explanation for MT disappearance is a change in ocean chemistry. A decrease in CaCO₃ saturation and/or an increase in the concentration of precipitation inhibitors in mid-Neoproterozoic seawater may have contributed to MT disappearance, and might also help to explain the approximately contemporaneous decline in stromatolite diversity.