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.
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This means a day lasted only 23 and a half hours, according to the new study in AGU's journal Paleoceanography and Paleoclimatology.
The ancient mollusk, from an extinct and wildly diverse group known as rudist clams, grew fast, laying down daily growth rings.
The new study used lasers to sample minute slices of shell and count the growth rings more accurately than human researchers with microscopes.
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.
The new measurement informs models of how the Moon formed and how close to Earth it has been over the 4.5 billion year history of the Earth Moon gravitational dance.
The new study also found corroborating evidence that the mollusks harbored photosynthetic symbionts that may have fueled reef building on the scale of modern day corals.
The high resolution obtained in the new study combined with the fast growth rate of the ancient bivalves revealed unprecedented detail about how the animal lived and the water conditions it grew in, down to a fraction of a day.
"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 on a day 70 million years ago.
"It's pretty amazing," said Niels de Winter, an analytical geochemist at Vrije Universiteit Brussel and the lead author of the new study.
Climate reconstructions of the deep past typically describe long term changes that occur on the scale of tens of thousands of years.
Studies like this one give a glimpse of change on the timescale of living things and have the potential to bridge the gap between climate and weather models.
Chemical analysis of the shell indicates ocean temperatures were warmer in the Late Cretaceous than previously appreciated, reaching 40 degrees Celsius (104 degrees Fahrenheit) in summer and exceeding 30 degrees Celsius (86 degrees Fahrenheit) in winter.
The summer high temperatures likely approached the physiological limits for mollusks, de Winter said.
"The high fidelity of this data set has allowed the authors to draw two particularly interesting inferences that help to sharpen our understanding of both Cretaceous astrochronology and rudist palaeobiology," said Peter Skelton, a retired lecturer of palaeobiology at The Open University and a rudist expert unaffiliated with the new study. ■