The daily growth layers arranged into seasonal and tidal patterns, present in calcified structures of many modern as well as fossil organisms, provide evidence on the length of lunar month and year in the geological past.
The data presented are mainly from molluscan shells and indicate that the number of days per lunar month and per year has decreased significantly since Ordovician time. The change possibly has not taken place at a uniform rate.
Banding and periodical patterns in stromatolites reflect cycles of daily, tidal, and seasonal nature, but rarely, if ever, represent a complete growth record. Such lack of completeness is a function of many parameters like water depth, energy of the environment, biological and physical disruptions, which are not always determinable in fossils. Phanerozoic stromatolites show a higher degree of incompleteness than precambrian ones. Figures obtained from stromatolites counts cannot be compared to those from molluscs. Preliminary data indicate that the length of the synodic month during Gunflint time (≥1750 m.y. ago) was between 36 and 39 days. The oldest known stromatolites (≃ 2800 m.y.) show patterns that are interpreted as due to tidal influence and have a periodicity of 10, 20 and 40 laminae. The obvious conclusions are that the Moon has been associated with Earth since at least Early Precambrian times, that all theories implying a late capture of the Moon should be revised and that the calculated secular changes in the Earth's rotation rate cannot be accepted as representative for the all geological history.