Seventy million years ago a day was a half-hour shorter.

A chunk of rock that had been buried in the ground for millions of years has become a new clock for understanding Earth's rotation. Analysis of a fossilized Cretaceous-era bivalve shell has revealed that our planet's days were half an hour shorter 70 million years ago.

In turn, knowing this can now help scientists to more accurately piece together the rate at which the Moon is slowly moving away from our planet.

Understanding how Earth's rotation has changed over time is a pretty interesting challenge. We can't exactly go back and experience it or record it; instead, we have to rely on how our planet has recorded those changes over time.

For instance, by studying how changes in solar radiation recorded in ancient rock matched up with the Sun's cycles over tens of thousands of years, scientists were recently able to determine that Earth's days were just 18 hours long around 1.4 billion years ago.

But obtaining information on a more granular scale has proven somewhat challenging. This is where an extinct bivalve called Torreites sanchezi comes into play. T. sanchezi comes from a group of bivalves called rudists that were wiped out in the Cretaceous-Palaeogene extinction event 66 million years ago, and nothing like them exists today.

They were sort-of shaped like a vase, with a lid at the wider end; these bivalves dominated reef ecosystems. But they did have a few things in common with modern clams - one of which is that their shells grew at the rate of a layer per day.

You can probably guess where this is going. Just as tree rings contain information about the year in which they grew, those shell rings can be analysed, too. Specifically, they can reveal water conditions, such as temperature and chemistry, on sub-daily timescales, showing us how these animals lived.

"We have about four to five data points per day, and this is something that you almost never get in geological history," explained geochemist Niels de Winter of Vrije Universiteit Brussel in Belgium. "We can basically look at a day 70 million years ago. It's pretty amazing." Read the full article.