A fossil coral can preserve more than the shape of an animal that lived in an ancient sea. Fine lines on its skeleton can also record repeated cycles of growth. Count those lines within a yearly band, and the fossil becomes evidence about how quickly Earth was turning.

For corals from the Devonian, roughly 400 million years ago, the famous result is about 400 days per year. That does not mean Earth took substantially longer to travel around the Sun. It means more, shorter days fitted inside approximately the same annual journey.

The calendar on a coral skeleton

Cornell paleontologist John W. Wells published “Coral Growth and Geochronometry” in Nature in 1963. His question linked a familiar paleontological object to a problem in astronomy: could fossil growth patterns test the expectation that Earth’s rotation had slowed?

As the Paleontological Research Institution explains, Wells distinguished fine growth lines from thicker annual bands, or annulations. Modern corals provided a basis for interpreting those different rhythms. In well-preserved Devonian specimens from central New York, he found an average of roughly 400 fine lines within an annual band.

The comparison depended on recognizing two clocks in the same skeleton. The larger seasonal pattern supplied the year; the finer increments supplied the proposed daily count. Neither the fossil’s size nor the total number of visible ridges alone could answer the question.

The crucial measurement was the number of days inside a year.

Why 400 days means about 22 hours

The arithmetic is straightforward once the units are kept separate. A modern year contains approximately 365.24 days of 24 hours each, or about 8,766 hours. Dividing that duration among 400 ancient days gives approximately 21.9 modern hours per day.

That calculation explains the headline’s rounded figure of 22 hours. It is not a claim that a fossil provides a clock reading accurate to the minute. Both the geological age and the number of daily increments are approximate, and the calculation assumes the annual orbital period was close enough to today’s for this comparison.

“More days” can sound like “more time,” but here the counting unit itself was smaller. Imagine dividing a fixed length of ribbon into 400 pieces instead of 365. The greater number of pieces does not require a longer ribbon; each piece is shorter.

The distinction also prevents a misleading picture of extra seasons. A year could contain more sunrises while retaining its annual seasonal cycle. What changed was how often the planet turned during its trip around the Sun, rather than the number of times it completed that trip.

The Moon helps explain the slowdown

NASA’s explanation of lunar tidal acceleration describes the physical connection. The Moon raises tides on Earth, and Earth’s rotation carries the tidal bulge ahead of the Moon. The gravitational interaction transfers angular momentum from Earth’s spin into the Moon’s orbit.

Earth’s rotation slows while the Moon moves outward. Lunar laser ranging, using reflectors left on the surface, measures a present recession rate of about 3.8 centimeters per year. This is a direct modern measurement of a process whose accumulated effects become substantial over geological time.

The present rate is not a ruler that can simply be extended backward unchanged through hundreds of millions of years. As AGU notes in its discussion of fossil timekeeping, the Moon’s recession rate has varied. Ancient observations therefore provide useful constraints on the history, rather than merely illustrating a calculation based on today’s motion.

A younger shell kept a different count

The broader approach did not end with Wells’s corals. A 2020 study reported by the American Geophysical Union examined a fossil rudist bivalve, Torreites sanchezi, from what is now Oman. It lived about 70 million years ago, much later than the Devonian corals.

Researchers used laser sampling and the shell’s growth record to investigate daily and seasonal patterns. They identified approximately 372 daily layers per year, implying a day about 23.5 hours long. The individual had lived for more than nine years, providing repeated annual intervals to examine.

This was a different animal, a different geological age and a different analytical technique. Its result should not be folded into the coral count as though both measured the same moment. Instead, it provides another point in the reconstruction: a younger fossil recorded a day closer to the modern length.

Reading the record with care

The Digital Atlas of Ancient Life gives Wells’s Devonian counts as 385 to 410 daily lines per annual cycle, with 400 typical. That range is a useful reminder that these are biological structures interpreted as records of periodic growth, not manufactured instruments.

A successful reading requires identifying which bands represent which intervals and preserving enough detail to count them. Assigning an age to the fossil is another step. The inference becomes meaningful when the growth interpretation, geological context and physical explanation agree.

A coral did not measure Earth’s rotation intentionally. It built a skeleton as it lived, and that ordinary biological activity left a pattern that could survive in stone. The remarkable connection is that a small feature of an ancient animal can help answer a question about the motion of an entire planet.