Days were a half hour shorter when dinosaurs roamed the Earth 70 million years ago

Earth turned faster at the end of the time of the dinosaurs than it does today, rotating 372 times a year, compared to the current 365, according to a new study of fossil mollusk shells from the late Cretaceous.

  • A day lasted only about 23-and-a-half hours.
  • The Earth turned faster than it does today.
  • The new study used lasers to sample tiny slices of a mollusk’s shell and count the growth rings.

When dinosaurs roamed the earth, days were about a half-hour shorter than they are today, a new study said.

Why is this? 70 million years ago, the Earth turned faster than it does today, rotating 372 times a year, compared to the current 365, according to an analysis of an ancient fossil mollusk shell from the late Cretaceous period. 

This means a day lasted only about 23-and-a-half hours.

The study findings back up what astronomers had theorized about the length of days millions of years ago.

Researchers analyzed a single mollusk that lived for over nine years in a shallow seabed in the tropics – a location that’s now dry land in the mountains of Oman.

The mollusk grew fast, laying down daily growth rings. The new study used lasers to sample tiny slices of the mollusk’s shell and count the growth rings.

“We have about four to five datapoints 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 study lead author Niels de Winter, an analytical geochemist at Vrije Universiteit in Brussels, Belgium.

The new method focused a laser on small bits of the mollusk’s shell, making holes 5 micrometers in diameter, or about as wide as a red blood cell.

The analysis provided accurate measurements of the width and number of daily growth rings as well as seasonal patterns. The researchers used seasonal variations in the fossilized shell to identify years.

Trace elements in these tiny samples also revealed information about the temperature and chemistry of the water. 

Chemical analysis of the shell showed that ocean temperatures were warmer in the Late Cretaceous than had previously been thought, reaching a toasty 104 degrees Fahrenheit in summer and over 86 degrees during winter. The summer high temperatures probably approached the physiological limits for mollusks, de Winter said.

The species of mollusk that was studied is now extinct, disappearing in the same extinction event that killed the non-avian dinosaurs 66 million years ago.

The length of a year has stayed steady throughout Earth’s history, because Earth’s orbit around the sun stays the same. But the number of days within a year has been shortening over time because days have been growing longer.

The length of a day has been growing steadily longer as friction from ocean tides, caused by the moon’s gravity, slows Earth’s rotation.

The study appeared in the journal Paleoceanography and Paleoclimatology, a publication of the American Geophysical Union.

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