Nature

Ancient Shell Has Revealed Exactly How Much Shorter Days Were 70 Million Years Ago


A chunk of rock that had been buried in the ground for millions of years has become a new clock for understanding Earth’s rotation. Analysis of a fossilised Cretaceous-era bivalve shell has revealed that our planet’s days were half an hour shorter 70 million years ago.

 

In turn, knowing this can now help scientists to more accurately piece together the rate at which the Moon is slowly moving away from our planet.

Understanding how Earth’s rotation has changed over time is a pretty interesting challenge. We can’t exactly go back and experience it or record it; instead, we have to rely on how our planet has recorded those changes over time.

For instance, by studying how changes in solar radiation recorded in ancient rock matched up with the Sun’s cycles over tens of thousands of years, scientists were recently able to determine that Earth’s days were just 18 hours long around 1.4 billion years ago.

But obtaining information on a more granular scale has proven somewhat challenging. This is where an extinct bivalve called Torreites sanchezi comes into play. T. sanchezi comes from a group of bivalves called rudists that were wiped out in the Cretaceous-Palaeogene extinction event 66 million years ago, and nothing like them exists today.

They were sort-of shaped like a vase, with a lid at the wider end; these bivalves dominated reef ecosystems. But they did have a few things in common with modern clams – one of which is that their shells grew at the rate of a layer per day.

 

You can probably guess where this is going. Just as tree rings contain information about the year in which they grew, those shell rings can be analysed, too. Specifically, they can reveal water conditions, such as temperature and chemistry, on sub-daily timescales, showing us how these animals lived. 

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

The team obtained a single T. sanchezi fossil and subjected it to a variety of analysis techniques, including mass spectrometry, microscopy, stable isotope analysis, and micro X-ray fluorescence.

Chemical analysis of the shell revealed the oceans were much, much warmer 70 million years ago. The bivalve thrived in waters that reached temperatures of 40 degrees Celsius (104 degrees Fahrenheit) in summer and more than 30 degrees Celsius (86 degrees Fahrenheit) in winter.

Shell rings also display seasonal variability; for example, in modern clams, layers grown in winter will be darker. Such seasonal variability allows scientists to identify yearly timescales within the line patterns seen in the shell, as seasonal rings match each other.

 

The team used this aspect to calculate the length of day when the bivalve lived. They determined that their fossilised T. sanchezi had lived for nine years. Then, they counted the rings in each year, both visually and chemically. If you did that today, you’d get 365 rings per year – but instead, they got 372.

We know the length of a year has remained more or less the same, since Earth’s orbit hasn’t changed. So that means the length of a day – determined by the speed of Earth’s rotation – must have changed, lengthening since then from 23.5 to 24 hours.

That Earth’s rotation is slowing down is actually pretty well established, and it’s been linked pretty conclusively to the Moon, because the deceleration of Earth’s rotation is caused by friction from Earth’s tides. This effect is called, funnily enough, tidal friction.

Those tides are caused by the gravitational pull of the Moon, which causes them to bulge. However, Earth’s rotation skews the bulge slightly ahead of the Moon’s position in orbit around the planet. This creates a rotational force between the two bodies that accelerates the Moon, causing it to gradually move farther away from Earth.

Currently, the Moon is orbiting away from Earth at a rate of about 3.82 centimetres (1.5 inches) per year, as determined by precise measurements that use lasers bounced off markers set there by astronauts during the Apollo missions.

If we used this rate to extrapolate the Moon’s initial position 4.5 billion years ago (when we think it formed), something doesn’t add up: the satellite would have been so close to our planet, it would have been torn apart by tidal forces.

Which leads scientists to the conclusion that the rate at which the Moon moves away has probably changed – accelerated – over time. But precisely how fast it was moving away at any given point in time is hard to determine.

Finding more geological records that let us calculate the length of days at different points in Earth’s history would help us to plot the Moon’s acceleration more precisely; in turn, then we could find out when our Moon formed. And finding those data points is exactly what the team hopes to do, with even older mollusc shell fossils.

But that’s not even all. The study also revealed that the shell rings grew more quickly during the day. This, the researchers said, suggests that T. sanchezi formed a symbiotic relationship with photosynthetic organisms – similar to today’s giant clams, which have a symbiotic relationship with algae.

“Until now, all published arguments for photosymbiosis in rudists have been essentially speculative, based on merely suggestive morphological traits, and in some cases were demonstrably erroneous,” said palaeobiologist Peter Skelton of The Open University, who was not involved in the research.

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

Pretty wild that you can tell all that just by looking at an old shell, huh?

The research has been published in Paleoceanography and Paleoclimatology.

 



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