The Ancient Earth Turned Half-Hour Faster 70 Million Years Ago

Ancient Earth spun faster 70 million years ago, with half-hour shorter days.
Brad Bergan

The ancient Earth turned faster when the dinosaurs roamed the planet, rotating 372 times per year compared to the current 365, reports a new study of fossil mollusk shells from the late Cretaceous period.

In other words, ancient days were only 23.5 hours long, reports a new study published in AGU's journal Paleoceanography and Paleoclimatology.


Ancient Earth had shorter days

An ancient mollusk from an extinct and wildly diverse group called the rudist clams grew incredibly fast, laying down growth rings every day. The recent study used lasers to sample small slices of shell, counting growth rings with more accuracy than microscopes.

Observing growth rings allowed researchers to discern the precise number of days in a year and the length of a day 70 million years ago. This new measurement also tells us how the Moon formed, and its changing distance to Earth during the 4.5-billion-year Earth-Moon gravitational dance.

The recent study also found evidence that supports the notion that mollusks harbored photosynthetic symbionts capable of fueling reef-building on a scale matching modern corals.

Extreme precision on the deep past

The research team also found high-res information about how the animal lived and thrived in its water conditions, with measurements so precise they account for a fraction of a day.

"We have about four to five datapoints per day, and this is something that you almost never get in geological history. We can basically look at a day 70 million years ago. It's pretty amazing," said Niels de Winter, an analytical geochemist at Vrije Universiteit Brussel and lead author of the study.

Reconstructions of what ancient Earth's climate was like in the deep past usually show long term changes that happen on a scale of tens of thousands of years. This study and others like it also show how change works on the timescale of living creatures, which could one day bridge a scientific gap between climate and weather models.

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