Dino Days Were Half an Hour Shorter

Life reconstruction of *Dineobellator notohesperus*, Cretaceous period, San Juan Basin, NM (illustration by Sergey Krasovskiy via Wikimedia Commons)

19 December 2020

As we approach the shortest day(*) of the northern year we can take heart that our days are as long as they are. 70 million years ago, in the age of the dinosaurs, the Earth spun faster than it does today. Rather than having 86,400 seconds per day to get things done, dinosaur days were half an hour shorter.

This discovery came to light when paleontologists used lasers to study the growth rings of a rudist bivalve fossil found in the mountains of Oman. During the Cretaceous period rudist bivalves lived in tropical seas and grew throughout their lifetimes, laying down new shell material every day.

Fossil of rudist bivalves (*Vaccinites*) from the Cretaceous of the Omani Mountains, United Arab Emirates (image from Wikimedia Commons)

The lasers were precise enough to identify four to five data points within each day cycle and see that …

The composition of the shell changed more over the course of a day than over seasons, or with the cycles of ocean tides. The fine-scale resolution of the daily layers shows the shell grew much faster during the day than at night.
— AGU.org: Ancient shell shows days were half hour shorter 70 million years ago

Thus “the growth rings allowed the researchers to determine the number of days in a year and more accurately calculate the length of a day 70 million years ago.” They also discovered information on the Moon’s formation and proximity. — agu.org

The Earth, like all of us, is slowing down with age. Every day is infinitesimally longer than the last. (It sure has felt that way during the COVID-19 pandemic!) On the bright side, even when we feel rushed we can be glad we have that daily half hour the dinosaurs missed.

(illustration of dinosaurs from Wikimedia Commons; click on the caption to see the original)

(*) The winter solstice is at 5:02am ET on 21 December 2020.

One thought on “Dino Days Were Half an Hour Shorter”

Interesting! From the link:

The length of a year has been constant over Earth’s history, because Earth’s orbit around the Sun does not change. 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 pull of the tides accelerates the Moon a little in its orbit, so as Earth’s spin slows, the Moon moves farther away. The moon is pulling away from Earth at 3.82 centimeters (1.5 inches) per year. Precise laser measurements of distance to the Moon from Earth have demonstrated this increasing distance since the Apollo program left helpful reflectors on the Moon’s surface.

But scientists conclude the Moon could not have been receding at this rate throughout its history, because projecting its progress linearly back in time would put the Moon inside the Earth only 1.4 billion years ago. Scientists know from other evidence that the Moon has been with us much longer, most likely coalescing in the wake of a massive collision early in Earth’s history, over 4.5 billion years ago. So the Moon’s rate of retreat has changed over time, and information from the past, like a year in the life of an ancient clam, helps researchers reconstruct that history and model of the formation of the moon.

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Dino Days Were Half an Hour Shorter

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