When dinosaurs roamed the Earth, our planet rotated faster, 372 times a year compared to the current 365, according to new research on fossil mollusk shells from the Cretaceous Age. During this era, Earth days lasted for about 23 and a half hours, the results of the study were published in Paleoceanography and Paleoclimatology research journal.

Torreites sanchezi and geologic history

An extinct rudist clam, Torreites sanchezi belonging to a group of mollusks once dominated ancient water bodies in the age of dinosaurs about 70 million years ago. These ancient bivalve clams grew by creating rings on its shell which act as a geologic clock. Apart from timekeeping, the shells can reveal details about past climate, ecology, temperature, and chemistry of ancient water bodies where the mollusk lived, along with insights related to the origin of our moon, its proximity and its relationship to Earth. Torreites sanchezi disappeared from the Earth during dinosaur extinction 66 million years ago.

Paleontologists used sophisticated instruments to slice the shells and count growth rings to get the snapshot of what Earth days were like in the Cretaceous period. For the experiments, a fossilized shell of Torreites sanchezi was acquired from Natural History Museum of Maastricht. The specimen was originally collected from a well preserved fossilized rock layer in the Saiwan region of the Sultanate of Oman. The specimen belonged to the Campanian age (72.1 to 83.6 million years ago), for analytical analysis several equipment and techniques were used, including; Micro‐XRF, Stable Isotope Analyses, microscopy, LA‐ICP‐Mass Spectroscopy, MATLAB for modeling, and R for signal processing.

Shorter Earth days in the Cretaceous

Analysis of Torreites sanchezi also showed great seasonal variations, researchers were able to identify different seasons and count the years. They found that years during that time were 372 days long and Earth days were 23 and a half hours long. “It was established by previous research work that Earth days in the past were shorter than today, but this discovery is the most accurate for the late Cretaceous period,” said Dr. Niels De Winters, the leading scientist of the study.

The number of Earth days in a year has not been stable, the length of a year has been constant over time, as Earth’s orbit around the sun doesn’t really change. The length of days has been increasing, as the moon’s gravity creates friction from ocean tides and decreases rotation of the Earth’s. As Earth’s rotation slows down, the pull of tidal forces accelerates the moon, which causes the moon to move farther away each year.

shorter earth days fossil evidence

Image Credit: AGU (press release)
Daily and seasonal layers are visible in a cross-section through the specimen of the rudist clam Torreites sanchezi analyzed in the new study. The red box highlights well-preserved parts of the shell. The inserts show microscopic images of the daily laminae which are bundled in groups likely linked to the 14/28 day tidal cycles.

Ocean and Climate dynamics in the Cretaceous

The researchers used laser techniques to pierce tiny holes in the shell to determine trace element content. “We have about four to five data points per day, and this is something that you almost never get in geological history,” said Dr. Niels de Winter, at Vrije Universiteit Brussel, Belgium. “We can basically look at a day 70 million years ago.”

Chemical analysis revealed information related to ocean temperatures during that era, which showed that the oceans in the Late Cretaceous period were warmer than previously hypothesized. Results showed that ocean temperature in the summer reached 40 degrees Celsius (104 degrees Fahrenheit ) and more than 30 degrees Celsius (86 degrees Fahrenheit) in the winter.

The study also revealed information related to the growth of shells, it was observed that the shell grew much faster during the day than during the night. This suggested that these bivalve shells might have had a symbiotic relationship with another photosynthetic species and might have helped fueled reef-building on the scale of modern-day corals.

Reconstruction and modeling of past climate describe long term changes that occurred on the scale of tens of thousands of years. The present study gives insights regarding changes that occurred over different geologic timescales and has the potential to close the gap between climate and weather models.

Torreites sanchezi and Astrochronology

De Winter’s and his colleagues counted the number of layers in the shell which amounted to 372 for each yearly interval. This did not come as a surprise because scientists already know that Earth days were shorter in the past. The result is, however, the most accurate now available for the late Cretaceous, and has a surprising application to modeling the evolution of the Earth-Moon system.

* Astrochronology is the study of Earth’s history in relation to the solar system (astronomically tuned timescales) by using evidence from sedimentary rock units.

The moon moves farther away from the Earth due to the effect of tidal forces which accelerates the Moon in its orbit while slowing down the Earth’s rotation in the process. It is estimated that the moon is pulling away from us at 3.82 cm (1.5 inches) annually, which is evidenced by precise laser measurements (Earth-Moon distance) left by reflectors from the Apollo mission at the Moon’s surface.

Scientists conclude that the rate of Moon’s receding from Earth has not been constant throughout its history. The current rate of receding if projected linearly back in time would put the Moon inside the Earth only 1.4 billion years ago. While it has been established 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. Which is why, studying ancient fossils helps geoscientists to reconstruct the geologic history of the Earth and to model the origin of the Moon.

Dr. De Winter and his team are hopeful to apply their new methodology to older fossils in order to catch a glimpse of past Earth days even deeper in geologic time.


Ancient shells shows days were half-hour shorter 70 million years ago
AGU press release | March 9, 2020 | American Geophysical Union.


Subdaily‐Scale Chemical Variability in a Torreites Sanchezi Rudist Shell: Implications for Rudist Paleobiology and the Cretaceous Day‐Night Cycle
Paleoceanography and Paleoclimatology Journal

CITE THIS ARTICLE: "Ancient shells reveal Earth days were shorter 70 million years ago," in Geoscience Daily, March 24, 2020, https://geosciencedaily.com/earth/shorter-earth-days-age-of-dinosaurs/.