Pop science
When dinosaurs still left fresh footprints on the mud, our planet twirled around faster than it does today.
Chronicled in the rings of an ancient timekeeper is a story of days half an hour shorter and years a week longer than they are today. That ancient timekeeper is an extinct rudist clam, one of a group of molluscs that once dominated the role that corals fill today in building reefs. The clam belonged to the species Torreites sanchezi and lived 70 million years ago in a shallow tropical seabed that is now dry land in the mountains of Oman in the Middle East.
This ancient clam grew extremely fast from its home in a dense reef, creating a growth ring on its shell for every day of the nine years that it lived. A group of researchers analysed the clam’s shell to get a snapshot of what time and life was like in the Late Cretaceous period, about 5 million years before the story of both the dinosaurs and these clams ended.
Scientists used a laser to pierce microscopic holes in the shell, then examined them for trace elements. These provided information on the temperature and chemistry of the water in which this mollusc lived. “We have about four to five data points per day, and this is something that you almost never get in geological history,” said lead author Niels de Winter. “We can look at a day 70 million years ago.”
Analysis of the shell, which is made up of two parts connected by a natural hinge and is known as a ‘bivalve’, revealed that the ocean temperatures were warmer during that time than previously thought. They reached 40 degrees Celsius in the summer and more than 30 degrees Celsius in the winter.
They also found that the shell grew much faster during the day than during the night, suggesting that these clams may have had a relationship with another species
that fed on sunlight and fuelled reef-building. This type of one-way or two-way relationship in which organisms help each other is called symbiosis and is also present in some giant clams and algae.
Because this ancient mollusc also showed great seasonal variations, or changes in the shell in different seasons, researchers were able to identify different seasons and count the years. They found that years during that time were 372 days long and days were 23-and-a-half hours long. It was previously known that days were shorter in the past, but this is the most accurate count found for the Late Cretaceous period. While the number of days in a year has changed, the length of a year has been constant over time, as Earth’s orbit around the Sun doesn’t really change.