The study analysed a shell of the mollusc Torreites sanchezi, which lived for more than nine years in a shallow tropical seabed, a location that today is dry land in the mountains of Oman. The ancient molluscs had two shells, or valves, that joined together in a hinge, like asymmetrical mussels, and grew in dense reefs, as oysters do. They thrived in water a few degrees warmer than modern oceans. In the late Cretaceous period, they dominated reef building in tropical waters around the world and played the role that corals play today. They disappeared like the dinosaurs, 66 million years ago.
The fossil shells grew rapidly and made growth rings every day. The new study used lasers to sample tiny slices of shell and was able to count the rings more accurately than had been possible with microscopes before. With the new method, a laser was focused on small pieces of shell, creating holes 5 micrometres in diameter, or about as wide as a red blood cell. Trace elements in these small samples reveal information about the temperature and chemistry of the water at the time the shell was formed. The analysis yielded accurate measurements of the width and number of daily growth rings and seasonal patterns. The researchers used seasonal variations in the fossilised shell to identify years. The high resolution obtained in the new study, combined with the rapid growth of the shells, revealed the living conditions of the creature in periods as precise as a fraction of a day.
“We have about four to five data points a day, and this is something you hardly ever get in geological history. We can basically look at a day 70 million years ago,” says Niels de Winter, lead author of the study. “That’s a resolution you can only dream of as a geologist, of course. This type of shell, known as rudists, that lived in the late Cretaceous period, has some special properties and has no equivalent today. The intricate resolution of the daily layers shows that the shell grew much faster during the day than at night.”
Due to the shell’s daily growth, the researchers were able to determine that at that time there were 372 days in a year. The orbital period of the Earth around the sun was no different than today, which means a year lasted as long as it does now, but there were more and shorter days in a year.
“That has to do with the mutual attraction of earth and moon,” says de Winter. “That gravitation caused a gradual slowing down of the Earth’s rotation around its own axis, combined with a slow distancing of the moon from the Earth.”
Chemical analysis further indicated that during late Cretaceous, days were about 40° Celsius in the summer and around 30° in the winter, roughly the liveable limit for shellfish, according to de Winter. He and his colleagues now hope to repeat the research for shellfish that lived at other times in our distant geological history. Climate reconstructions from the distant past usually describe long-term changes that occur on the scale of tens of thousands of years. What is new about this type of study is that it can tell us something on a time scale of living things. They therefore have the potential to bridge the gap between climate and weather models.
Niels de Winter
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