Days were 30 minutes shorter 70m years ago, according to ancient shells

10 Mar 2020

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A ‘pretty amazing’ discovery has shown that the rotation of the Earth millions of years ago was faster than today, resulting in shorter days.

Palaeontologists have ‘turned back the clock’ with the discovery of a series of fossilised shells that reveal a great deal about the average day on Earth during the late Cretaceous period.

In a paper published to Paleoceanography and Paleoclimatology, researchers from Belgium analysing the 70m-year-old mollusc shells found in the mountains of Oman were able to determine that the Earth spun faster during this time than it does today.

Rotating 372 times per year as opposed to 365, a day during this period would be 30 minutes shorter than what we’re used to.

The ancient shells, from an extinct and wildly diverse group known as rudist clams, grew fast, laying down daily growth rings. These rings allowed researchers to determine the number of days in a year during this time, which could also provide us with more accurate models of how the moon formed and how close to Earth it has been over its 4.5bn-year-old history.

The study also found corroborating evidence that the molluscs harboured symbionts that may have fuelled reef-building on the scale of modern-day corals. High-resolution detail, in combination with the fast growth rate of these ancient creatures, has given unprecedented detail on how they lived.

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

Solving a lunar mystery

By comparison, climate reconstructions of the distant past are typically described on the scale of tens of thousands of years.

The pint-glass-shaped molluscs, Torreites sanchezi, once grew in dense reefs like modern oysters, in tropical waters several degrees warmer than modern oceans. The molluscs disappeared in the same event that killed the dinosaurs 66m years ago.

Until now it has been known days were shorter in the past, but this new discovery provides a much more detailed look at the evolution of the Earth-moon relationship of this time. The length of a day has been growing steadily longer as friction from ocean tides, caused by the moon’s gravity, slows Earth’s rotation.

We know the moon moves 3.82cm away from Earth each year, but scientists came to the conclusion that it could not have been receding at this same rate throughout history as its progress would mean the moon was inside the Earth only 1.4bn years ago.

Creating more accurate models using the data collected from the ancient clams will help scientists see further back in time and answer this lunar mystery, they said.

Colm Gorey was a senior journalist with Silicon Republic