(CNN)When dinosaurs walked the Earth, days on our planet were a little bit shorter than the full 24 hours we know today.
Earth turned more quickly, meaning that a day lasted about 23.5 hours and a year equated to 372 days, according to a new study.
Researchers discovered this fact from a surprising resource: ancient shells, dated to the Late Cretaceous period 70 million years ago.
The fossilized mollusk shell belonged to a group called rudist clams, which grew quickly and recorded their lives in daily growth rings visible in the shells. These specific clams were known as Torreites sanchezi and rudist means that they have two shells, with a hinge connecting them.
Laser sampling produced slices of the shells, allowing the researchers to get an accurate count of the rings. That let them know how many days there were in a year, allowing for the breakdown of how long a day would be.
The study published this week in the journal Paleooceanography and Paleoclimatology, which is published by the America Geophysical Union.
“We have about four to five data points per day, and this is something that you almost never get in geological history. We can basically look at a day 70 million years ago. It’s pretty amazing,” said Niels de Winter, lead study author and analytical geochemist at Vrije Universiteit Brussel.
We’ve long known that an Earth day lasts 24 hours, and that remains constant because Earth’s trip around the sun doesn’t vary.
However, the number of days that make an Earth year have shifted and shortened because days have grown longer. That is thanks to the moon’s gravity, which draws on ocean’s tides and slows Earth’s rate of rotation.
Meanwhile, as the moon tugs on Earth, our natural satellite distances itself about 1.5 inches per year from Earth.
The ancient shell also contained information about the environment the clams lived in. Shell data revealed that oceans during the Late Cretaceous 70 million years ago were much warmer than they are now, reaching 104 degrees Fahrenheit in the summer and above 86 degrees Fahrenheit in the winter.
The maximum temperature would have about reached the limit for mollusks like clams, the researchers said.
But these clams enjoyed temperatures that were warmer than today’s oceans.
The particular clam they studied lived for more than nine years, situated in a shallow tropical seabed. Today, this is dry land in Oman. Rudist clams are unique looking, described in a release by AGU as resembling “tall pint glasses with lids shaped like bear claw pastries.”
Like oysters, the clams thrived in reef environments. And in their day, they acted like coral, building and growing together.
“Rudists are quite special bivalves. There’s nothing like it living today,” de Winter said. “In the Late Cretaceous especially, worldwide most of the reef builders are these bivalves. So they really took on the ecosystem building role that the corals have nowadays.”
And they loved sunlight.
Their shells grew faster during the day in response to sunlight. The researchers believe this means that like modern giant clams, which are covered in algae, these clams were similarly supporting a symbiotic species.
But the clams were wiped out 66 million years ago, just like the dinosaurs.
The data collected from the shell helped the researchers piece together parts of Earth’s past, as well as the evolution of clams. In the future, the researchers hope to study older fossils and learn more about a day in the life of Earth’s distant past.