This is a really nice bit of work and it also promises to jin up numbers for other epochs. After all this tells us that there is a five day change since the Dinosaurs. We sort of knew as much, but this makes it real
It would be wonderful to establish a real physical time line using these fossils which drove reef building. We do have millions of miles of core from reefs thanks to the oil industry.
It would also be neat to get more information as well, but i actually think that dating would be good enough.
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Ancient shell shows days were half-hour shorter 70 million years ago
March 9, 2020
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. 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
at 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.
Ancient reef-builders
The new study analyzed a single individual that lived for over nine
years in a shallow seabed in the tropics—a location which is now,
70-million-years later, dry land in the mountains of Oman.
Torreites sanchezi mollusks look like tall pint glasses with
lids shaped like bear claw pastries. The ancient mollusks had two
shells, or valves, that met in a hinge, like asymmetrical clams, and
grew in dense reefs, like modern oysters. They thrived in water several
degrees warmer worldwide than modern oceans.
In the late Cretaceous, rudists like T. sanchezi dominated the
reef-building niche in tropical waters around the world, filling the
role held by corals today. They disappeared in the same event that
killed the non-avian dinosaurs 66 million years ago.
"Rudists are quite special bivalves. There's nothing like it living
today," de Winter said. "In the late Cretaceous especially, worldwide
most of the reef builders are these bivalves. So they really took on the
ecosystem building role that the corals have nowadays."
The new method focused a laser on small bits of shell, making holes
10 micrometers in diameter, or about as wide as a red blood cell. Trace
elements in these tiny samples reveal information about the temperature
and chemistry of the water at the time the shell formed. 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.
The new study found 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
"This bivalve had a very strong dependence on this daily cycle, which
suggests that it had photosymbionts," de Winter said. "You have the
day-night rhythm of the light being recorded in the shell."
This result suggests daylight was more important to the lifestyle of
the ancient mollusk than might be expected if it fed itself primarily by
filtering food from the water, like modern day clams and oysters,
according to the authors. De Winter said the mollusks likely had a
relationship with an indwelling 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 traits, and in some cases were demonstrably erroneous.
This paper is the first to provide convincing evidence in favor of the
hypothesis," Skelton said, but cautioned that the new study's conclusion
was specific to Torreites and could not be generalized to other rudists.
Moon retreat
De Winter's careful count of the number of daily layers found 372 for
each yearly interval. This was not a surprise, because scientists know
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.
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.
Because in the history of the Moon, 70 million years is a blink in
time, de Winter and his colleagues hope to apply their new method to
older fossils and catch snapshots of days even deeper in time.
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