I recently read a bit of Why Evolution is True by Jerry Coyne and stumbled upon a fun fact I needed to dig into.
Hundreds of millions of years ago, there were corals, just like today. And, just like today, they grew by depositing a ring of calcium carbonate onto their outer skeleton every day*—similar to the growth patterns in the trunk of a tree. When you look at these growth patterns in living corals, taking into account changes of deposition with seasons, you can see annual growth patterns and, as one might expect, about 365 daily rings per year. When you look at fossil corals from 400 million years ago you see over 400 daily rings per year!
Scientists have long predicted that the rotation of the Earth must be slowing down due to tidal friction—the motion of the tides have been dampening our angular momentum (it’s stolen by the moon!). Not by much, about 1 second gets added to the day every 50,000 years. But, over millions and millions of years, these seconds add up. 600 million years ago, a day was 21 hours long, and over 410 of these days elapsed before the Earth could complete its annual journey around our sun. In 1963 Prof. John W. Wells used a biological calendar—fossilized corals—to corroborate astronomical predictions about our lengthening days.
Anyway, it was the perfect storm of fun facts. The days are getting longer, coral living 400 million years ago experienced over 400 days a year, and we can see this in a biological record.
P.S. Modern corals are still keeping a record. Using “coral chronometers” we have a record of variations in temperature, cloudiness, and even nuclear activity (some bands in corals coinciding with nuclear tests are radioactive). Maybe 400 million years from now somebody (something?) will find corals from today and see the impact of the human era. At least 400 million years from now the postdoc doing this research will have ~27 hours in a day to write up the results.
* I also just read Jurassic Park, hence the Mr. DNA adaptation.