Dinosaurs saw shorter days and longer years, says fossil shell study

Dinosaurs saw shorter days and...
A fossil bivalve from the Late Cretaceous period
A fossil bivalve from the Late Cretaceous period
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A cross-section of the bivalve shell, highlighting the layers
A cross-section of the bivalve shell, highlighting the layers
A fossil bivalve from the Late Cretaceous period
A fossil bivalve from the Late Cretaceous period

Time isn’t as constant as we might think. The Earth’s rotation is changing, which affects how many hours are in a day and how many days in a year. Now palaeontologists have managed to precisely measure how long days and years were back in the age of the dinosaurs – and it’s all thanks to a humble mollusk fossil.

Our everyday experience of time sounds simple enough: A day is one full rotation of the Earth on its axis, while a year is one full lap of the Sun. But those things don’t always take the same amount of time – astronomical factors are messing them up.

Although it sometimes feels like days pass faster as we get older and busier, it’s well documented that days are actually getting longer – though not by much. In fact, today will be roughly 1.8 milliseconds longer than this same day 100 years ago. That’s because the Moon is moving away from Earth at a rate of about 3.8 cm (1.5 in) per year, and the changing tidal forces are slowing down the speed of the Earth’s rotation. And a slower rotation, of course, means a longer day.

Previous studies have shown that about 1.4 billion years ago, a day lasted just 18 hours. That’s because the Moon was closer, making the Earth spin faster.

On the new study, the team was able to precisely measure the length of a day much later – about 70 million years ago. This was right at the height of the dinosaurs’ reign, 5 million years before the cataclysm that wiped out roughly three quarters of life on Earth.

Specifically, the team determined that at that time, a day was only 23.5 hours long. And because each day was half an hour shorter, more days could fit into one year – there were 372 days in a year back then, making it one week “longer” than we’re used to.

So how did the scientists figure this out? By studying a fast-growing, ancient mollusk. Torreites sanchezi is an extinct species of bivalve, a clam-like creature that lived in the oceans right up until that history-changing asteroid struck.

These things grew extremely quickly, adding several new calcite layers to their shells per day. Scientists can study these growth rings to get clues about the environment the mollusk at each stage. It’s similar to trees or layers of rock in the ground, but instead of years or millennia, the changes occur over mere hours.

“We have about four to five datapoints per day, and this is something that you almost never get in geological history,” says Niels de Winter, lead author of the study. “We can basically look at a day 70 million years ago. It’s pretty amazing.”

The team focused the study on one particular specimen, which appeared to live for about nine years in a shallow tropical seabed. They used a laser to take tiny samples of the shell, and analyzed the trace elements found inside. This told them about the temperature and chemistry of the water that the bivalve lived in, and how that changed over time.

The analysis showed that the composition of the shell changed layer by layer, and it grew faster during daylight hours than at night. The researchers were able to see groups of layers that repeated in clear patterns, which indicated changing seasons. By carefully counting these layers, the team determined that there were 372 days in each yearly cycle.

They also discovered some more details about the kind of world these creatures lived in. For example, the oceans reached temperatures of up to 40 °C (104 °F) in summer and 30 °C (86 °F) in winter. That’s obviously much warmer than they are now, but it’s even warmer than previously thought for the time.

The research was published in the journal Paleoceanography and Paleoclimatology.

Source: American Geophysical Union

Jose Gros
Perhaps what happens in the system Earth-Moon is the slowdown of Earth rotation transfers energy to the Moon, and this is why it is going away. A body in orbit that moves faster along time is a falling object, an object that slows down the turning speed is going away
Yes, but even though there were more weeks in a year millions of years ago, the actual period of time in a full year was shorter millions of years ago, because the earth has been slowly moving away from the sun for millions of years and the actual time for a full orbit is now longer and larger, and this slow distance away has been calculated at about 15 centimeters per year. This calculation also takes in the average of it's perihelion and aphelion too.
"the oceans reached temperatures of up to 40 °C (104 °F) in summer and 30 °C (86 °F) in winter. That’s obviously much warmer than they are now, but it’s even warmer than previously thought for the time." And we're worried about global warming!? Sounds like a pretty nice climate!
How do the scientists explain that these bivalves survived temperatures of 40C when there is no more oxygen in seawater at 35C?
A beautiful analysis of the shells' days and years with which I have no reason to disagree. However I question that the years which have passed is certain and all caused by the receding moon and tidal changes. Two other things may be a cause for the now longer days and our years presently having fewer days. Firstly there is good evidence for Noah's flood which changed the earth's mass distribution somewhat and brought more water to the surface if the hydroplane theory of Dr. Walt Brown Ph.D. ( is correct. Further, his implication is that water (mass) was also lost making the rotational angular momentum less. Secondly, it is also well known that Africa and South America were closer and even touching in the past. Moreover, there is a theory that all the continents were first one total ball before breakup, called Pangea but without an ocean, just all one big continent. This is called the Growing Earth Theory or Expanding Earth Theory. A very very strong video is found here. ( I know this is a very scorned theory yet there are good arguments in its behalf. The continental area is now only maybe 30% of the earth's surface (70% oceans) that is a strong argument for those who say the earth has grown materially from one continental ball, and now has a much larger diameter and thus has slowed significantly due to its larger diameter. This will likely be scorned by many but without the best evidence. I think there are other questions in the universe which fit this theory nicely also, but that is too long for here.
Alan McNaughton
I agree with you John. Dr Walt Brown gave a far more acceptable hypothesis as to the earths extra diameter pre-flood. (Pf) The only major shifts in the hydroplates which include earthquakes were and are due to the eroded walls of the now plates that came crashing together, forming under sea trenches and mountain ranges,once this erosion removed enough material to the surface for this occur. The pillars that pf supported the 10 to 20 miles of rock came crashing down and are still settling as the earth quakes. The pangea hypothesis is beyond my comprehensive powers. Continent's have to do hopscotch for this theory to work. According to Walt Brown the diameter of earth would be a mile less this would have to change the length of days and fossils should be able to confirm this. It totally adds up.