Ancient Shell Growth Rings Show Days Were 30 Minutes Shorter Millions Of Years Ago

Scientists have found that millions of years ago, near the end of the reign of the dinosaurs, the Earth turned much faster than it does today. At that time scientists say that the Earth rotated 372 times per year compared to the current 365. The information comes from a new study of fossilized mollusk shells from the Late Cretaceous. The faster rotation of the Earth would mean that days lasted only 23.5 hours rather than the 24 hours we see today.

The agent mollusk shell that the scientist studied is from an extinct and highly diverse group known as rudist clams. These creatures grew very fast and laid down daily growth rings. Scientists use lasers to sample minute slices of the shell and count the growth rings more accurately than human researchers using microscopes have been able to do in the past.

The growth rings allow the researchers to determine the number of days in a year and to more accurately calculate the length of the day 70 million years ago. The study found evidence that the mollusks harbored photosynthetic symbionts that could've fueled reef-building on the scale of modern-day corals. The study was able to obtain high enough resolution that could be combined with the fast growth rate of the ancient bivalves to reveal new detail about how the creature lived and the water conditions that grew and down to the fraction of the day.

The team says that it has about 4 to 5 data points per day, something almost ever found in geological history. Scientists can look at a day 70 million years ago. The chemical analysis performed on the shell showed that ocean temperatures were warmer in the Late Cretaceous than previously understood with temperatures reaching 104 degrees Fahrenheit in the summer and exceeding 86 degrees Fahrenheit in the winter.

The team says that the high temperatures in the summer approached the physiological limits for mollusks. The scientists focus a laser on small bits of shell making holes that were 10 micrometers in diameter. Trace elements inside those channels reveal information about the temperature and chemistry of the water at the time the shell formed.