From the fossil shell of a single bivalve mollusk from the end of the Cretaceous period, a group of researchers was able to conclude that 70 million years ago, a year lasted 372 days and the days only lasted 23 and a half hours – 30 minutes less than our current 24 hours.
According to the study, published in the academic journal Paleoceanography and Paleoclimatology, the 10 cm mollusk is part of an extinct group of animals known as rudist mussels, which grew rapidly and, consequently, established daily growth rings.
To see these rings accurately, scientists used lasers to cut tiny slices from the shell of the bivalve. Using only microscopes, the researchers would not get as much information.
A look at a day 70 million years ago
The high resolution of the images obtained combined with the rapid growth rate of the mollusk was enough to reveal unprecedented details about the life of the animal and the environment that surrounded it. “This is something you rarely get in geological history. Basically, we can look at a day 70 million years ago. It’s incredible,” said Niels de Winter, an analytical geochemist at the Vrije Universiteit Brussel and lead author of the research.
The shorter days may have been the result of the distance between the Earth and the Moon at the time. It is known that the translation of our planet around the Sun never changes, however, as the Moon moves away from Earth (3.82 centimeters per year), the friction of the ocean tides decreases and the rotation of the planet too, increasing the length of days over millions of years.
The environment 70 million years ago
According to the chemical analysis of the bivalve, the ocean temperatures at the end of the Cretaceous period were much warmer than imagined: the water naturally reached 40°C in the summer and even exceeded 30°C in the winter.
“The high fidelity of this data set has allowed the authors to draw two particularly interesting inferences that help to improve our understanding of Cretaceous astrochronology and rudist paleobiology,” said Peter Skelton, a retired professor of paleobiology at Open University in the United Kingdom.
The studied mollusk lived 70 million years ago, at the bottom of a shallow sea that today corresponds to the dry lands of the mountains of Oman. Scientifically called Torreites sanchezi, the bivalve looked like glasses with lids and was only able to thrive due to the high temperature of the waters at the time.
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They also found that the shell grew much faster during the day than at night, which indicates that the mollusk was a photosymbiont – that is, it had a vital association with some species that fed on sunlight.
These bivalves went extinct in the same event that killed all dinosaurs 66 million years ago.