Days were shorter in the late Cretaceous, VUB learns from seashell

Summary

VUB and UGent researchers have established that days used to be 23.5 hours instead of 24, meaning there were also more days in a full year

‘A geologist’s dream’

Just when you thought there weren’t enough hours in a day, you find out days used to be 30 minutes shorter. And that means that a year was a whole week longer. Those are new findings established by researchers at VUB together with colleagues from Ghent University.

According to the study, published in the journal Paleoceanography and Paleoclimatology, a day lasted 23.5 hours and a year 372 days some 70 million years ago, when the dinosaurs were dying out. The finding is based on the study of sea fossils from the late Cretaceous period.

The study analysed a shell of the mollusc Torreites sanchezi. The little creature lived for more than nine years in a shallow tropical seabed, a location that is today dry land in the mountains of Oman.

Ancient molluscs had two shells, or valves, that joined together in a hinge, like mussels. They grew in dense reefs, like oysters do, and thrived in water a few degrees warmer than modern oceans.

Growth rings

In the late Cretaceous period, Torreites sanchezi dominated reefs in tropical waters around the world and played the role that coral plays today. They disappeared like the dinosaurs, 66 million years ago.

Similar to a tree, the shells of the Torreites sanchezi made growth rings, but very rapidly – every day, in fact. The new study used lasers to sample tiny slices of shell and was able to count the rings more accurately than had been possible with microscopes before.

Trace elements in these samples reveal information about the temperature and chemistry of the water at the time the shell was formed. The analysis yielded accurate measurements of the width and number of daily growth rings and seasonal patterns.

We can basically look at a day 70 million years ago

- Niels de Winter

The researchers used seasonal variations in the fossilised shell to identify years. The high resolution obtained in the new study, combined with the rapid growth of the shells, revealed the living conditions of the creature in periods as precise as a few hours in a day.

“We have four to five data points a day, and this is something you hardly ever get in geological history. We can basically look at a day 70 million years ago,” says Niels de Winter of VUB’s Analytical, Environmental and Geo-Chemistry research group. “That’s a resolution you can only dream of as a geologist.”

This type of shell from the late Cretaceous period, he explains, has no equivalent today. “The intricate resolution of the daily layers shows that the shell grew much faster during the day than at night,”  he explains.

Bridging the gaps

Due to the shell’s daily growth, the researchers were able to determine that at that time there were 372 days in a year. The orbital period of the Earth around the sun was no different than today, which means a year lasted as long as it does now, but there were more and shorter days in the year.

“That has to do with the mutual attraction of earth and moon,” says de Winter. “That gravitation caused a gradual slowing down of the Earth’s rotation, combined with a slow distancing of the moon from the Earth.”

Chemical analysis further indicated that days were about 40°C in the summer and 30°C in the winter, roughly the liveable limit for shellfish. De Winter and his colleagues now hope to repeat the research for shellfish that lived at other times in distant geological history.

Climate reconstructions from the past usually describe long-term changes that occur on the scale of tens of thousands of years. What is new about this type of study is that it can tell us something on a time scale of living things. They therefore have the potential to bridge the gap between climate and weather models.

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