Days were half an hour shorter during the Late Cretaceous period 70 million years ago, 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 than the 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 rapid growth 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 growth rings far more accurately than was previously possible under a microscope.
By looking at the growth patterns over time, the team were then able to determine the 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, the number 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 to the friction from ocean tides, which also causes the Moon to get further away.
The researchers also found the first concrete evidence that rudist bivalves may have contained 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, an analysis of growth rings in an ancient shell, pictured, has shown
WHY ARE THE DAYS GETTING SHORTER?
The length of the year has remained constant across Earth’s history.
However, the number 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 to the friction from ocean tides.
This also causes the Moon to get further away — at around 1.5 inches every year, laser measurements reveal.
However, the Moon cannot always have been retreating at this rate — else it would have been inside the Earth only 1.4 billion years ago.
In contrast, experts think that the Moon was formed around 4.5 billion years ago.
Studies like those of this ancient clam will help scientists work out how the Moon’s retreat has changed over time.
The fast growth rate of the mollusc — which belonged to the species Torreites sanchezi — and the new high-resolution scanning technique have allowed the researchers to study the conditions in which the shell grew in unprecedented detail.
‘We have about four to five datapoints per day, and this is something that you almost never get in geological history,’ said paper author and geochemist Niels de Winter of the Vrije Universiteit Brussel in Belgium.
‘We can basically look at a day 70 million years ago. It’s pretty amazing.’
In contrast, normal climate reconstructions are confined to long-term changes that occur on the scale of tens of thousands of years.
By shining lasers on the shell samples and cutting tiny holes a mere millionth of a metre across, the team were able to analyse trace elements in the material and gain insight into the chemistry and temperature of the ocean when the shell has formed.
The specimen — which would have lived in a shallow, tropical seabed in what is now the Oman Mountains in the United Arab Emirates — grew its shell for more than nine years in total.
The researchers found that the composition of the bivalve’s shell changed more significantly over the course of individual days than it did with the seasons, or the regular cycles of the ocean tides.
‘This bivalve had a very strong dependence on this daily cycle, which suggests that it had photosymbionts,’ Dr de Winter explained.
‘You have the day-night rhythm of the light being recorded in the shell.’
The shell — of an extinct group of mollusc known as the rudist bivalves — had a rapid growth rate that allowed it to preserve a detailed record of the past
Close examination of the individual layers revealed that the shell grew much faster during 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 likely harboured symbiotes that fed on sunlight, like modern giant clams.
‘Until now, all published arguments for photosymbiosis in rudists have been essentially 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.’
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 were hotter 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 of 83°F (28°F) and winter temperatures of 81°F (27°C).
According to Dr de Winter, the high ocean temperatures in the Late Cretaceous summers likely pushed molluscs to their limits of heat tolerance.
With their initial study complete, the team hope to apply the same method to older fossils, allowing them to capture snapshots of days further back in time and improve our understanding of the evolution of the Earth–Moon system.
The full findings of the study were published in the journal Paleoceanography and Paleoclimatology.
WHAT IS A RUDIST BIVALVE?
Dr de Winter and colleagues studied the ancient shells of an species of mollusc — Torreites sanchezi — that belonged to an extinct group of molluscs known as rudist bivalves.
Each individual rudist bivalve had two asymmetrical shells — or valves — that came together at a hinge and could be opened and closed by the creature.
Rudists lived in waters that were around several degrees warmer than the average ocean temperatures of today, and would have grown together to form dense reefs, much like modern oysters.
‘Rudists are quite special bivalves. There’s nothing like it living today,’ Dr de Winter explained.
‘In the late Cretaceous especially, worldwide most of the reef builders were these bivalves.’
‘So they really took on the ecosystem building role that the corals have nowadays.’
Rudist bivalves died out in the Cretaceous–Paleogene extinction event which also saw the end of the non-avian dinosaurs.
Pictured, a fossil of two rudist bivalves from the Cretaceous Period of the Oman Mountains, in what is today the United Arab Emirates. Each shell is made up of two valves — a longer, cone-like one on bottom and a flatter lid at the top (stock image)