This rudist clam from the United Arab Emirites is similar to one found further south on the Arabian Penisula that has been used to reveal the length of the late Cretaceous day. Wilson44691 – Public Domain via Wikimedia commons

The Earth is turning more slowly every year, thanks to the Moon's gravitational influence. As a consequence, days are getting longer. Although astronomers have known this for a long time, they've struggled to calculate the exact rate of change over millions of years. Now they've got help from the shell of a late Cretaceous clam, which has helped geochemists work out that the length of a dinosaur day was half an hour shorter than it is today.

Rudist clams were an order of mollusks that were very successful right up until they died out alongside the non-avian dinosaurs 66 million years ago. Some were very fast-growing species, depositing an extra layer to their shell every day, which can be distinguished visually like tree rings.

Dr Niels de Winter of Belgium’s Vrije Universiteit Brussels vaporized small patches of a 70 million-year-old Torreites sanchezi shell with lasers. This way he could count the 40 micrometer-wide (0.0016 inches) increments with much greater precision than microscopes, while analysis of the product revealed how the clam's chemistry changed through the year.

In Paleoceanography and Paleoclimatology, de Winter calculates there were 372 days between seasonal peaks 70 million years ago. Since we know the length of the year has barely changed, each day must have lasted 23.5 hours.

De Winter was able to detect unprecedented detail about how the clam lived and water conditions it lived 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," he said in a statement. "We can basically look at a day 70 million years ago. It's pretty amazing.”

This fine-grained record hints at the clam's ecosystem and how it grew. Temperatures in the late Cretaceous were known to be higher than today, and the clam spent nine years living in a shallow seabed in modern-day Oman, which was then equatorial. Nevertheless, de Winter was surprised the clam's chemistry is consistent with summer water temperatures as high as 40ºC (104ºF). Even in winter, it was above 30ºC (86ºF). De Winter doubts mollusks could survive in waters much warmer than this, and the paper considers the possibility high evaporation and low rainfall distorted the results.

Daily and seasonal layers are visible in a cross section of a Torreites sanchezi clam. The red box highlights well-preserved parts of the shell. Microscopic images in the inserts show daily laminae which are bundled in groups likely linked to the 14/28 day tidal cycles. Credit: AGU

Modern tropical corals only appear in the fossil record 23 million years ago, but T. sanchezi and related species have been proposed as the base of analogous reefs. "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."

De Winter has shown his clam had cycles of fast and slow growth over the course of a day. He thinks this means that, like modern corals, it formed a symbiosis with photosynthesizing organisms, using the nutrients delivered during daylight hours to grow, as giant clams do today. Most modern molluscs, which live by filtering food from the water column, are much less influenced by cycles of light and dark. Such a symbiosis would have helped clam-based reefs approach the productivity of corals today.