A piece of rock that had been buried for millions of years has become the new clock to understand the rotation of the Earth. According to the analysis of a fossilized bivalve shell from the late Cretaceous era, the days of our planet were only 23 and a half hours (half an hour shorter) 70 million years ago; that is, the Earth rotated faster in the final dinosaur stage than it does today , rotating only 372 times a year, compared to 365 today.
And the thing is that time is not as constant as we imagine (not counting that, as we age, we get the impression that the days go by faster and faster). Although one day means one complete rotation of the Earth on its axis and one year one complete revolution of the Sun, this does not always take the same amount of time.
Earth’s rotation is changing, which affects how many hours there are in a day and how many days in a year. A slower rotation equals a longer day on Earth. Now, thanks to this new work, paleontologists have been able to precisely measure how much time and years passed at the end of the age of the dinosaurs, and all thanks to a small fossil shell.
Calculating the hours
The extinct bivalve is Torreites sanchezi, which comes from a group of bivalves called rudists, similar to a vase (which had two asmetric valves and one of them normally fixed to the substrate), which were annihilated in the Cretaceous-Paleogene extinction event ago 66 million years ago, which wiped out most of the dinosaurs and of which nothing similar currently exists.
What makes it so special? Apart from dominating reef ecosystems, these bivalves had a characteristic identical to modern clams: shells grew at the rate of one layer per day.
Like the rings of a tree stump, scientists from the Environmental and Geochemical Research Group at the University of Brussels have counted the microscopically thin layers of an extinct mollusk to calculate the length of a late Cretaceous day. The scientists were able to study these growth rings to get clues about the mollusk’s environment at each stage.
“We have between four and five data points per day, and this is something that is almost never obtained in geological history,” explained geochemist Niels de Winter, co-author of the work. “Basically, we can see a day 70 million years ago. It’s pretty amazing .”
They used a laser to take small samples of the shell and analyzed the trace elements inside, which gave them information about the temperature and chemistry of the water the bivalve lived in, and how it changed over time. Analysis showed that the composition of the shell changed layer by layer, growing faster during the day than at night. The researchers were able to see groups of layers repeating in clear patterns, indicating changes in the seasons. By carefully counting these layers, the team determined that there were 372 days in each annual cycle.
The effect of gravity on the Moon
The total length of the year has not changed since the Late Cretaceous: a total year consisted of 8,760 hours during the Late Cretaceous, just like a year ago. However, the spin of our planet is becoming progressively slower, thanks to the gravitational effects of our Moon.
Previous studies have shown that about 1.4 billion years ago, a day lasted only 18 hours, basically because the Moon was closer to us, and it made the Earth spin faster. As the Moon is progressively moving away from our planet, at a speed of about 3.8 cm per year, the changing forces of the tides are slowing the rotation of the Earth.
Referencia: Niels J. Winter et al, Subdaily‐Scale Chemical Variability in a Torreites Sanchezi Rudist Shell: Implications for Rudist Paleobiology and the Cretaceous Day‐Night Cycle, Paleoceanography and Paleoclimatology (2020). DOI: 10.1029/2019PA003723
