Oman fossil clam reveals year had 372 days 70 million years ago
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A fossil clam from Oman reveals that 70 million years ago, Earth's year contained 372 days, each about 23.5 hours long. Researchers led by Niels de Winter used laser analysis of daily growth layers in the shell to produce the estimate, with an uncertainty of 8.4 days. The finding reinforces tidal-friction models showing the Moon's gravity gradually slowing Earth's rotation.
The Fossil Calendar
The rudist bivalve Torreites sanchezi lived in a warm Cretaceous sea that is now Oman. With no close living relatives, these clams were primary reef builders of their era. The specimen grew for more than nine years, laying down daily growth layers. Researchers recovered it from Oman’s mountains after the ancient ocean dried up.
Laser Analysis
Led by Niels de Winter, an analytical geochemist, the team used lasers to drill spots 10 micrometres wide inside the shell. They achieved four to five chemical measurements per day, an unprecedented resolution in paleontology. By cross-referencing daily banding with seasonal cycles, they arrived at 372 days per year, with a margin of error of plus or minus 8.4 days.
Earth’s Slowing Spin
The year’s orbital length has remained nearly constant, but Earth’s rotation has decelerated. The Moon’s tidal forces create ocean bulges that brake the spin, transferring angular momentum to the Moon and pushing it away by about 3.8 centimetres per year. Seventy million years ago, days were shorter, packing more into each solar orbit. This clam data fits tidal-evolution models but shows that the deceleration rate has varied over geological time.
What's Next
The findings could refine models of Earth-Moon orbital evolution. The team hopes to extend the method to older fossils, though it remains unclear whether such high-resolution growth layers exist in earlier specimens.
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Oman fossil clam reveals year had 372 days 70 million years ago
