The Earth was spinning faster at the end of the dinosaur age than it is today, rotating 372 times a year instead of 365 today, according to a new study of fossil mollusk shells from the late Cretaceous published in the AGU journal Paleoceanography. and Paleoclimatology. This means that a day lasted only 23 and a half hours.
The mollusks, from an extinct group known as rudist clams, grew rapidly, creating daily growth rings. The new study used lasers to sample tiny bits of shell and count growth rings more precisely than researchers could using microscopes.
Growth rings allowed researchers to determine the number of days in a year and more accurately calculate the length of a day 70 million years ago. The measurement also informs models of the Moon’s formation and how close it has been to Earth during the 4.5 billion-year history of the Earth-Moon gravitational dance.
The study also found corroborating evidence that molluscs harbored photosynthetic symbionts that may have fueled reef building on the scale of today’s corals.
The high resolution obtained in the study, combined with the rapid growth rate of ancient bivalves, revealed unprecedented details about how the animal lived and the water conditions in which it grew, down to a fraction of a day.
We have about four or five data points a day, and this is something that is almost never achieved in geological history. We can basically look at a day 70 million years ago. it’s quite amazingsaid Niels de Winter, an analytical geochemist at the Vrije Universiteit Brussel and lead author of the 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 take a look at the change in the time scale of living things and have the potential to bridge the gap between climate and weather models.
Chemical analysis of the shells indicates that ocean temperatures were warmer in the Late Cretaceous than previously appreciated, reaching 40 degrees Celsius in summer and exceeding 30 degrees Celsius in winter. High summer temperatures probably approached the physiological limits of molluscsdeWinter said.
The high fidelity of this data set has allowed the authors to draw two particularly interesting inferences that help sharpen our understanding of both Cretaceous astrochronology and rudist paleobiology.said Peter Skelton, an Open University professor of paleobiology and a rudist expert unaffiliated with the study.
The study looked at a single individual that lived for more than nine years on a shallow seabed in the tropics, a place that is now, 70 million years later, dry land in the mountains of Oman.
At the end of the Cretaceous, rudist molluscs such as T.sanchezi they dominated reef building in tropical waters around the world, fulfilling the role that corals play today. They disappeared in the same event that killed off the non-avian dinosaurs 66 million years ago.
Rudists are very special bivalves. There’s nothing like it living todayDeWinter said. Especially in the late Cretaceous, around the world the majority of reef builders are these bivalves. So they really took on the role of ecosystem builders that corals have today..
The study found that the composition of the shell changed more over the course of a day than over the seasons, or with ocean tidal cycles. Fine-scale resolution of the daily layers shows that the shell grew much faster during the day than at night.
This bivalve had a very strong dependence on this daily cycledeWinter said. We can see the day-night rhythm of light being recorded on the shell.
This result suggests that daylight was more important to the ancient mollusk’s lifestyle than might be expected if it fed primarily by filtering food from the water, like today’s clams and oysters, according to the authors. De Winter said the mollusks likely had a relationship with a resident symbiotic species that fed on sunlight, similar to living giant clams, which harbor symbiotic algae.
Until now, all published arguments for photosymbiosis in rudists have been essentially speculative, based on merely suggestive morphological features, and in some cases demonstrably wrong. This work is the first to provide convincing evidence in favor of the hypothesisSkelton said, but cautioned that the new study’s conclusion was specific to turrets and could not be generalized to other rudists.
De Winter’s careful counting of the number of daily layers found 372 for each annual interval. This was not a surprise, because scientists know that days were shorter in the past. The result is, however, the most accurate now available for the late Cretaceous, and has surprising application for modeling the evolution of the Earth-Moon system.
The length of a year has been constant throughout Earth’s history, because the Earth’s orbit around the Sun does not change. But the number of days within a year has been getting shorter over time because the days have been getting longer. The length of a day has been steadily growing as ocean tidal friction, caused by the Moon’s gravity, slows Earth’s rotation.
Tidal pull accelerates the Moon a bit in its orbit, so as Earth’s spin slows, the Moon moves farther away. The Moon is moving away from Earth at 3.82 centimeters per year. Precise laser measurements of the Moon’s distance from Earth have shown this increasing distance since the Apollo program left useful reflectors on the Moon’s surface.
But scientists conclude that the Moon could not have receded at this rate throughout its history, because projecting its progress linearly back in time would put the Moon within Earth’s orbit 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, more than 4.5 billion years ago. So the rate of the Moon’s retreat has changed over time, and information from the past, such as a year in the life of an ancient clam, helps researchers piece together that history and the model of the Moon’s formation.
Because 70 million years in the Moon’s history is just the blink of an eye in time, de Winter and his colleagues hope to apply their new method to older fossils and capture snapshots of days even deeper in time.
Sources
Agu 100 / de, Winter, NJ, Goderis, S., Van Malderen, SJ, Sinnesael, M., Vansteenberge, S., Snoeck, C., Belza, J., Vanhaecke, F. and Claeys, P. (2020 ), Subdaily‐Scale Chemical Variability in a Torreites Sanchezi Rudist Shell: Implications for Rudist Paleobiology and the Cretaceous Day‐Night Cycle. Paleoceanography and Paleoclimatology, 35: e2019PA003723. doi:10.1029/2019PA003723