All corals that have a calcified outer wall display minute growth lines on their outer surfaces. The carbonate is secreted by symbiotic algae (zooxanthellae) which, responding to day and night, create a series of diurnal growth increments. Studies on Devonian isolate corals indicate a 400-day year and, therefore, a 22-hour day. Post-Devonian data confirm a near-linear deceleration of the Earth's rotational velocity towards the present 24-hour day. The number of daily growth lines can therefore be used in a crude way to calibrate the geologic record.
Subjects: Ecology and Conservation — Earth Sciences and Geography.