A single fossil clam pulled out of the mountains of Oman has revealed that approximately 70 million years ago, a calendar year consisted of roughly 372 days. According to researchers, this higher count does not mean the year itself was longer; rather, each individual day was significantly shorter than modern days, running about half an hour brief at roughly 23.5 hours per rotation instead of 24 hours.
Unlocking Prehistoric Time From a Fossil Clam
The unusual measurement separates two concepts commonly lumped together as time: the length of a year, which represents one trip around the Sun and has remained essentially unchanged, and the length of a day, which marks a single spin of Earth on its axis.
High-Resolution Laser Analysis of Growth Layers
The remarkable precision came from analyzing a bivalve that lived more than nine years and grew rapidly during the late Cretaceous period, a time when such organisms took on reef-building roles similar to modern corals. The specimen laid down a fresh growth layer each day.
To read the microscopic record, the team used lasers to make spots approximately 10 micrometres wide—roughly the width of a red blood cell—and measured the chemistry inside each one. As de Winter described the exceptional detail: We have about four to five data points per day, and this is something that you almost never get in geological history.
By combining layer counting, spectral analysis, and chemical layer counting across nine growth years, researchers produced a composite estimate of 372 daily laminae per year with a propagated uncertainty of 8.4 days.
Why Earth’s Rotation Has Slowed Over Time
The findings offer a window into planetary mechanics, illustrating how Earth used to spin faster and pack more turns into the same annual orbit. The deceleration of the planet’s rotation is driven primarily by the Moon. Gravitational interaction raises ocean tides that do not line up exactly with the lunar body, a friction that gradually slows Earth’s rotation while pushing the Moon outward.
Data from NASA indicates that the Moon is drifting away by about 4 centimeters a year, with its retreat slowing as it goes.
Clues About the Cretaceous Environment
Chemical analysis of the shell also shed light on the ancient sea where the bivalve lived, indicating extreme warmth. Summer ocean temperatures reached as high as 40 degrees Celsius, while winter temperatures exceeded 30 degrees Celsius.
Furthermore, the chemistry revealed that the organism grew faster in daylight than at night, pointing to a possible reliance on tiny light-loving partners living in its tissues—similar to modern giant clams and some corals—to draw energy from sunlight alongside traditional feeding.
Implications for Future Geological Research
While the study relied on a single shell sampled in exceptional detail rather than a broad survey of the Late Cretaceous, scientists see the technique itself as the breakthrough. de Winter emphasized the potential of the method, stating: We can basically look at a day 70 million years ago. It’s pretty amazing.
Researchers note that if day-by-day chemistry can be read this cleanly from one shell, the approach can be tested across other eras and marine environments to see whether future well-preserved specimens sharpen the 372-day estimate or shift it.
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