Days on Earth When Dinosaurs Lived Were Half an Hour Shorter Than They Are Now, Ancient Fossil Reveals

A fossilized shell from the late Cretaceous has provided evidence that it takes the Earth a half-hour longer to spin on its axis today compared to 70 million years ago. This supports the predictions made by astronomical models.

Scientists writing in Paleoceanography and Paleoclimatology worked this out by analyzing daily growth rings in ancient mollusks. Using various measurements and dating techniques they were able to count the number of days in a year and even measure light variance across a single day.

The results suggest a dinosaur's year would have been 372 days long—a week longer than ours—but their days would have been almost half-an-hour shorter.

"We have about four to five datapoints per day, and this is something that you almost never get in geological history," lead author Niels de Winter, an analytical geochemist at Vrije Universiteit, said in a statement. "We can basically look at a day 70 million years ago. It's pretty amazing."

Fossil rudist bivalves (Vaccinites)
Fossil rudist bivalves (Vaccinites) from the Al-Hajar Mountains, United Arab Emirates. Wikipedia, Wilson44691 – Own work, Public Domain

The basis of the study is a 70-million-year-old Torreites sanchezi mollusk, which spent more than nine years living in a shallow seabed in what is today entirely dry land in Oman's mountains.

Lasers were used to drill holes in the shell just 10 micrometers in diameter—a size a little larger than the width of a red blood cell. The study's authors say this technique enabled them to count daily growth rings with more accuracy than methods involving microscopes.

The method yields very detailed information about changes in the environment at a scale of just days, de Winter told Newsweek.

"The 'snapshots' of climate we can obtain in this way teach us how warmer climates, such as those in the Late Cretaceous, 70 million years ago, affect seasonal differences in temperature and extreme weather events," he said.

The study was also the first to provide "convincing evidence" that mollusks like T. sanchezi had a symbiotic relationship with photosynthesizing species, said de Winter. Patterns in the rings suggests the shells grew faster in the day than at night and was driven by solar (rather than lunar) cycles. The researchers believe T. sanchezi co-existed with a second species that lived inside the mollusk and fed on sunlight—like algae found in giant clams today.

T. sanchezi contained two uneven shells attached on a hinge—a little like a twenty-first century asymmetrical clam. It lived in reefs, submerged tropical waters, and would have served a similar function in their ecosystem as modern-day corals. That is, before the creatures died out during the Cretaceous–Paleogene extinction event that killed non-avian dinosaurs 66 million years ago.

"There's nothing like it living today," said de Winter.

The researchers say that while it has been predicted by astronomical models that days 70 million years ago were shorter than they are now, this is the most accurate calculation of how long a year would have been during the late Cretaceous.

As Newsweek previously reported, days gradually get longer as Earth's gravitational grip on the Moon weakens—a phenomenon that currently takes place at a rate of 1.5 inches (3.82 centimeters) per year. As the Moon retreats, the Earth's rotation on its axis longer to complete as the friction from tidal forces wanes.

Information about the rotation of the Earth is important for those wanting to understand how the moon formed and how the relationship between the Earth and moon has changed over time, de Winter explained. Data like that described in the paper could be used to help test astronomical models measuring how the length of a single day has changed over the course of the planet's history.

"Deciphering fingerprints of length of day in a rudist (the T. sanchezi) is a unique tool to reconstruct the evolution of the Earth-Moon system," Mingsong Li, Assistant Research Professor in the Department of Geosciences at Pennsylvania State University, told Newsweek.

The study of the length of a day is a cornerstone to understanding past climate change at a much higher temporal resolution (daily) in comparison to the millennial‐ to million-year-scale currently used to monitor paleoclimate changes, said Li.

This article has been updated to include additional comments from Dr Niels de Winter and Prof Mingsong Li.