Coral kept the time of ancient days
Growth bands in Devonian corals show about 400 days in a year. The year had more days because each day was shorter.
Strong2 min read
John Wells noticed that living corals lay down a daily ridge and a yearly modulation. Devonian specimens gave him about 400 ridges to the year. If the length of the year in absolute time was similar, the day was about 22 hours. The moon was closer. The tides were a larger machine.
Later fossil clocks — bivalves, stromatolites — broadly agree that the day has been lengthening. The method is geology leaning on biology. It matches the physics of tidal braking, which we now measure directly with lasers.
Caveats
- Band counts are noisy and not every coral is a perfect diary. The direction and the rough size of the change are the robust result.
Sources
- 1Wells J. W.. Coral Growth and Geochronometry. Nature 197, 1963.