70 million years ago days were half-hour shorter

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, which eventually means a day lasted only 23 and a half hours, says new research.

The findings from a new study of fossil mollusk shells from the Late Cretaceous, was published 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 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 data points 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, the 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 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 rudist expert.

Ancient reef-builders

The research 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. 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," de Winter said.

The method focused a laser on small bits of shell, making holes 10 micrometres 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.

Composition of the shell changed more over the course of a day than over seasons, or with the cycles of ocean, tides were found. 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. You have the day-night rhythm of the light being recorded in the shell," de Winter added.

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. 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.