Time isn’t as constant as we might think. The Earth’s rotation is changing, which affects how many hours are in a day and how many days in a year. Now palaeontologists have managed to precisely measure how long days and years were back in the age of the dinosaurs – and it’s all thanks to a humble mollusk fossil.
Our everyday experience of time sounds simple enough: A day is one full rotation of the Earth on its axis, while a year is one full lap of the Sun. But those things don’t always take the same amount of time – astronomical factors are messing them up.
Although it sometimes feels like days pass faster as we get older and busier, it’s well documented that days are actually getting longer – though not by much. In fact, today will be roughly 1.8 milliseconds longer than this same day 100 years ago. That’s because the Moon is moving away from Earth at a rate of about 3.8 cm (1.5 in) per year, and the changing tidal forces are slowing down the speed of the Earth’s rotation. And a slower rotation, of course, means a longer day.
Previous studies have shown that about 1.4 billion years ago, a day lasted just 18 hours. That’s because the Moon was closer, making the Earth spin faster.
On the new study, the team was able to precisely measure the length of a day much later – about 70 million years ago. This was right at the height of the dinosaurs’ reign, 5 million years before the cataclysm that wiped out roughly three quarters of life on Earth.
Specifically, the team determined that at that time, a day was only 23.5 hours long. And because each day was half an hour shorter, more days could fit into one year – there were 372 days in a year back then, making it one week “longer” than we’re used to.
So how did the scientists figure this out? By studying a fast-growing, ancient mollusk. Torreites sanchezi is an extinct species of bivalve, a clam-like creature that lived in the oceans right up until that history-changing asteroid struck.
These things grew extremely quickly, adding several new calcite layers to their shells per day. Scientists can study these growth rings to get clues about the environment the mollusk at each stage. It’s similar to trees or layers of rock in the ground, but instead of years or millennia, the changes occur over mere hours.
“We have about four to five datapoints per day, and this is something that you almost never get in geological history,” says Niels de Winter, lead author of the study. “We can basically look at a day 70 million years ago. It’s pretty amazing.” The team focused the study on one particular specimen, which appeared to live for about nine years in a shallow tropical seabed. They used a laser to take tiny samples of the shell, and analyzed the trace elements found inside. This told them about the temperature and chemistry of the water that the bivalve lived in, and how that changed over time.
The analysis showed that the composition of the shell changed layer by layer, and it grew faster during daylight hours than at night. The researchers were able to see groups of layers that repeated in clear patterns, which indicated changing seasons. By carefully counting these layers, the team determined that there were 372 days in each yearly cycle.
They also discovered some more details about the kind of world these creatures lived in. For example, the oceans reached temperatures of up to 40 °C (104 °F) in summer and 30 °C (86 °F) in winter. That’s obviously much warmer than they are now, but it’s even warmer than previously thought for the time. The research was published in the journal Paleoceanography and Paleoclimatology.