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A new discovery reveals Earth has not always had a 24-hour day

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If you have ever wanted extra hours in the day, then take heart: A prehistoric mollusk reveals how dinosaurs lived 70 million years ago — 23.5 hours a day, 372 days a year — and how much longer days might become over the course of the next several million years.

The revelation that Earth has not always had a 24 hour day and orbited the Sun over the course of 365 days every year comes from the discovery of an ancient clam dating back to the Cretaceous period, when dinosaurs ruled the Earth.

The discovery is described in a new study published this week in the journal Paleoceanography and Paleoclimatology.

“We now have a new, more accurate method for determining the number of days in a year.”

“Our finding is exciting because climate reconstructions this far back in time are normally limited to long trends, thousands or millions of years,” Niels de Winter, an analytical geochemist from Vrije Universiteit Brussel and author of the study, tells Inverse. Researchers made the discovery after they analyzed the shell using lasers to zero in on how the clam grew over time.

This unprecedented window into prehistory allows researchers to paint a detailed picture of the minutiae of climate conditions during the dinosaur days — including the temperatures dinosaurs dwelt in.

“Our study shows how shells of bivalves can be used to create "snapshots" of the local climate, millions of years ago,” de Winter says.

Snapshot of the ancient clam

Niels de Winter

The study also marks the first time researchers have managed to pinpoint such precise details about the 4.5-billion-year-long love-and-hate relationship between the Moon and the Earth. Ultimately, the results could inform how we construct models to predict the future workings of the universe — and the Earth's journey around the Sun.

“We now have a new, more accurate method for determining the number of days in a year,” de Winter says.

Peering into a clam

The 70 million-year-old shell is a fossil of a clam called Torreites sanchezi from the Natural History Museum of Maastricht.

Before it went extinct some 66 million years ago in the same asteroid catastrophe that killed the T.Rex and most of the other dinosaurs, the creature liked to nestle in the shallow, tropical seabeds in the warm ocean that once covered now dry mountain ranges in Oman. Though it is technically a clam, the mollusk was somewhat like a coral: it dominated the tropical oceans by building extensive reefs.

“Rudists are quite special bivalves. There’s nothing like it living today,” de Winter said.

The clam had a life span of over nine years, and it grew quickly, laying a new growth ring every day.

“We can basically look at a day 70 million years ago. It’s pretty amazing.”

In the past, scientists had to count these layers by eye through a microscope to get an idea of how each clam might have lived. But new laser technology enabled de Winter and his team to make 10 red blood-cell sized holes in one of the fossils to study shell formation more closely than ever before.

“With our new high-resolution laser technique, we can detect the chemical changes in the shell composition that happen over these daily layers and let the computer count the layers for us based on the data, which is free of human error and therefore more accurate,” de Winter says.

“We did this for four different chemical records through nine growth years in the shell, yielding a total of 36 estimates which makes us pretty confident about the number of days in a year around the time our specimen grew." In this clam's case: 372 days per year.

“We can basically look at a day 70 million years ago. It’s pretty amazing," he said.

A window in time

By counting the growth rings, analyzing trace elements in the microscopic samples, and observing seasonal patterns, the researchers were able to obtain unprecedented details about how the creature lived, where it lived, how long for, and more.

From these measurements, the researchers draw a very detailed picture of everything around the clam, too.

Chart showing growth over time, according to daily circles: Microscopic observations of shell laminae and bundling.

Niels de Winter

Here are three things researchers could observe from looking at this shell:

3. Strange growth patterns

By looking at the composition of the shell and how it is layered, researchers reveal the clam changed a lot during the course of a day — and less so over the course of the seasons.

During the day, the clam grew faster than during the night. These findings suggest that the ancient mollusk fed on sunlight and filtered nutrients through the water, much like a modern clam.

“It is likely that this symbiosis with photosynthesizing algae is what allowed them to grow so fast and be so sensitive to the day-night cycle,” De Winter says.

“We don't know if all rudists did this, so it would be interesting to try and find out whether they show the same changes in their shell.”

2. Climate change in the dinosaur era

By looking at the stable carbon and oxygen isotopes in the shell, de Winter and his team could tell that ocean temperatures when this shell thrived were around 86 degrees Fahrenheit during the winter, and 104 degrees Farhenheit in summer.

This estimate contradicts previous models of ocean conditions during the time, suggesting water temperature was warmer than researchers had thought, the study suggests.

“These findings change how we can study past greenhouse climates in high detail."

Taken together, the findings provide the foundation for a reconstruction of the climate during the time of the dinosaurs.

“This allows us in the future to look at changes in the environment of these bivalves on a much finer scale,” de Winter says. “More or less like we are looking at weather, rather than climate."

One of the main areas the findings will inform is the study of past greenhouse gas levels, de Winter says. That could in turn inform our predictions for our own climate future, he says.

“We are interested in these climates because they can teach us something about how the Earth will change in the ongoing human-caused global climate emergency.”

“Researchers and climate modelers are very interested in how warming on Earth influences the occurrence of extreme seasons and weather events, and now we potentially have a tool to study this in the past with greater precision," he says.

1. Star-crossed lovers

Counting the daily layers of this tiny mollusk enabled de Winter and his team to pinpoint the number of days in a year during the Late Cretacean era with unprecedented precision.

Knowing that there were 372 days in a year informs scientists’ understanding of how Earth’s rotation has slowed down over time. It can also enable us to predict how long days and years will last in the far future.

The Earth’s orbit around the Sun has stayed constant over the course of the planet's history, but the number of days in year is decreasing over time. The reason why is that the length of a day is determined by Earth's rotation, which is slowing steadily under the influence of the Moon.

The Earth experiences friction from ocean tides, as the Moon gravity pulls and pushes the waters of the planet to a rhythm. Over time, this rhythmic gravitational dance pushes the Moon farther away from the Earth at 1.5 inches per year, slowly widening the Moon’s orbit — and, in turn, slowing down the Earth’s rotation.

This delicate dance is something researchers can observe using data collected by space agencies like NASA, for example, but these data can't offer an accurate representation of what has always happened.

If the Moon had always moved 1.5 inches away from the Earth, then the Moon would have to have been located inside the Earth 1.4 billion years ago to be where it is now.

Will Earth days keep getting longer?

Tracing how the Moon’s breakup with the Earth has changed over time can help us predict the future dynamics of our planet, de Winter says.

“This is important information for astronomers if they want to investigate the history of our planet and the formation of the Moon,” he says.

“Our own human history is too short to notice the effect of increasing the length of day, but on longer geological timescales, this effect is noticeable.”

“The recession of the Moon away from Earth will continue in the future, and our days keep slowly getting longer, resulting in less days per year in the far future,” he says.

“You can imagine that these processes play at very long timescales.”

“All I know is that ultimately, we're talking billions of years," he says. Eventually, he notes "we will actually lose our Moon, because if it is far away it will escape Earth's gravity."

De Winter isn't ready to put a date on when that will happen, however, but we likely won't be around to see it.

"I'm not sure how long this would take, and if other events, such as the death of our Sun, will precede it, so it may never occur, but it would be quite a spectacular event indeed.”

Abstract: This study presents subdaily resolved chemical records through fossil mollusk shell calcite. Trace element profiles resolve periodic variability across ~40‐μm‐thin daily growth laminae in a Campanian Torreites sanchezi rudist bivalve. These high‐resolution records are combined with seasonally resolved stable isotope and trace element records that allow shell‐chemical variability to be discussed on both seasonal and daily scale. A combination of layer counting, spectral analysis of chemical cyclicity and chemical layer counting shows that the rudist precipitated 372 daily laminae per year, demonstrating that length of day has increased since the Late Cretaceous, as predicted by astronomical models. This new approach to determine the length of a solar day in geologic history through multiproxy chemical records at subdaily resolution yields considerably more control on the uncertainty of this estimate. Daily chemical variability exceeds seasonal variability in our records, and cannot be explained by diurnal temperature changes. Instead, we postulate that rudist shell chemistry is driven on a daily scale by changes in light intensity. These results together with those of stable isotope analyses provide strong evidence that Torreites rudists had photosymbionts. Bivalve shell calcite generally preserves well. Therefore, this study paves the way for daily‐scale reconstructions of paleoenvironment and sunlight intensity on geologic time scales from bivalve shells, potentially allowing researchers to bridge the gap between climate and weather reconstructions. Such reconstructions improve shell chronologies, document environmental change in warm ecosystems, and widen our understanding of the magnitude of short‐term changes during greenhouse climates.

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