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Days were half an hour SHORTERduring the Late Cretaceous period 70million years ago before the Moon'sgravity slowed down the Earth'srotation, ancient shell shows

Researchers studied the growth patterns in the shell of an extinct rudist bivalve By counting the daily rings they were able to determine the length of the yearIn the Late Cretaceous period, the Earth span faster, rotating 372 times a yearHowever, interactions between the Moon and tides have since slowed it down

By IAN RANDALL FOR MAILONLINEPUBLISHED: 12:52 GMT, 10 March 2020 | UPDATED: 13:09 GMT, 10 March 2020

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Tuesday, Mar 10th 2020

10/03/2020, 20:13 of 35

Days were half an hour shorter during the Late Cretaceous period 70 million yearsago, an analysis of growth rings in an ancient shell has shown.

Earth at this time would thus have rotated around 372 times every year — rather thanthe current 366.25 rotations that gives us our 365-day year with leap years.

The shell — of an extinct group of mollusc known as the rudist bivalves — had a rapidgrowth rate that allowed it to preserve a detailed record of the past.

Researchers used lasers to sample tiny slices of the shell and count the daily growthrings far more accurately than was previously possible under a microscope.

By looking at the growth patterns over time, the team were then able to determinethe number of days in a year — refining our best guess from astronomical models.

While the length of the year has remained constant across Earth's history, thenumber of days per year has grown as days have gotten longer.

This is because the gravity of the Moon is slowing down the Earth's rotation thanks tothe friction from ocean tides, which also causes the Moon to get further away.

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WHY ARE THE DAYSGETTING SHORTER?

The length of the year has remainedconstant across Earth's history.

However, the number of days per yearhas grown as days have gotten longer.

This is because the gravity of the Moon

The researchers also found the first concrete evidence that rudist bivalves may havecontained photosynthetic symbiotic organisms that helped them to grow.

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Days were half an hour shorter during the Late Cretaceous period 70 million years ago, ananalysis of growth rings in an ancient shell, pictured, has shown

The fast growth rate of the mollusc —which belonged to the species Torreitessanchezi — and the new high-resolutionscanning technique have allowed theresearchers to study the conditions inwhich the shell grew in unprecedenteddetail.

'We have about four to five datapointsper day, and this is something that youalmost never get in geological history,'

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is slowing down the Earth's rotationthanks to the friction from ocean tides.

This also causes the Moon to getfurther away — at around 1.5 inchesevery year, laser measurements reveal.

However, the Moon cannot alwayshave been retreating at this rate — elseit would have been inside the Earthonly 1.4 billion years ago.

In contrast, experts think that theMoon was formed around 4.5 billionyears ago.

Studies like those of this ancient clamwill help scientists work out how theMoon's retreat has changed over time.

almost never get in geological history,'said paper author and geochemist Nielsde Winter of the Vrije Universiteit Brusselin Belgium.

'We can basically look at a day 70 millionyears ago. It's pretty amazing.'

In contrast, normal climatereconstructions are confined to long-term changes that occur on the scale oftens of thousands of years.

By shining lasers on the shell samplesand cutting tiny holes a mere millionth ofa metre across, the team were able toanalyse trace elements in the materialand gain insight into the chemistry andtemperature of the ocean when the shellhas formed.

The specimen — which would have lived in a shallow, tropical seabed in what is nowthe Oman Mountains in the United Arab Emirates — grew its shell for more than nineyears in total.

The researchers found that the composition of the bivalve's shell changed moresignificantly over the course of individual days than it did with the seasons, or theregular cycles of the ocean tides.

'This bivalve had a very strong dependence on this daily cycle, which suggests that ithad photosymbionts,' Dr de Winter explained.

'You have the day-night rhythm of the light being recorded in the shell.'

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10/03/2020, 20:13 of 35

The shell — of an extinct group of mollusc known as the rudist bivalves — had a rapid growthrate that allowed it to preserve a detailed record of the past

Close examination of the individual layers revealed that the shell grew much fasterduring the day than it did at night.

This suggests that rather than just filtering food from the water like modern oysters— a process that is not dependent on the time of the day — Torreites sanchezi likelyharboured symbiotes that fed on sunlight, like modern giant clams.

'Until now, all published arguments for photosymbiosis in rudists have beenessentially speculative,' said palaeobiologist Peter Skelton of the Open University,who was not involved in the present study.

'This paper is the first to provide convincing evidence in favour of the hypothesis.'

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During the Late Cretaceous, Earth would thus have rotated around 372 times every year —rather than the current 366.25 rotations that gives us our 365-day year with leap years.Pictured, as the Earth rotates, the stars in the sky appear to revolve around us

Chemical analysis of the mollusc shells also revealed that ocean temperatures werehotter in the Late Cretaceous period than had been previously thought.

In the summer, water temperatures would have reached around 104°F (40°C)whereas, in winter, the oceans would have exceeded 86°F (30°).

In contrast, modern tropical oceans see summer temperatures of in the order of83°F (28°F) and winter temperatures of 81°F (27°C).

According to Dr de Winter, the high ocean temperatures in the Late Cretaceoussummers likely pushed molluscs to their limits of heat tolerance.

With their initial study complete, the team hope to apply the same method to olderfossils, allowing them to capture snapshots of days further back in time and improveour understanding of the evolution of the Earth–Moon system.

The full findings of the study were published in the journal Paleoceanography andPaleoclimatology.

WHAT IS A RUDIST BIVALVE? Dr de Winter and colleagues studied the ancient shells of an species ofmollusc — Torreites sanchezi — that belonged to an extinct group of molluscsknown as rudist bivalves.

Each individual rudist bivalve had two asymmetrical shells — or valves — thatcame together at a hinge and could be opened and closed by the creature.

Rudists lived in waters that were around several degrees warmer than theaverage ocean temperatures of today, and would have grown together to formdense reefs, much like modern oysters.

'Rudists are quite special bivalves. There's nothing like it living today,' Dr de

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'Rudists are quite special bivalves. There's nothing like it living today,' Dr deWinter explained.

'In the late Cretaceous especially, worldwide most of the reef builders werethese bivalves.'

'So they really took on the ecosystem building role that the corals havenowadays.'

Rudist bivalves died out in the Cretaceous–Paleogene extinction event whichalso saw the end of the non-avian dinosaurs.

Pictured, a fossil of two rudist bivalves from the Cretaceous Period of the OmanMountains, in what is today the United Arab Emirates. Each shell is made up of twovalves — a longer, cone-like one on bottom and a flatter lid at the top (stock image)

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