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About 70 million years ago, during the late Cretaceous period, a year lasted roughly 372 days rather than 365, meaning each day was around half an hour shorter than it is today. The research team examined the growth layers of a single fossilised rudist bivalve, Torreites sanchezi, recovered from the Al-Hajar Mountains in Oman. Using laser-based sampling, the researchers counted daily growth increments in the shell with far greater accuracy than earlier microscope-based methods, calculating both the length of the Cretaceous day and the distance between the Earth and the Moon at the time. According to the study published in Advancing Earth and Space Sciences, titled ‘Subdaily-Scale Chemical Variability in a Torreites Sanchezi Rudist Shell: Implications for Rudist Paleobiology and the Cretaceous Day-Night Cycle’, the shell also preserved evidence of ocean temperatures several degrees warmer than today, and of a likely relationship between the mollusc and photosynthetic symbionts, similar to those in modern giant clams.
Laser analysis reveals 372 daily growth rings in ancient shell
The specimen analysed in the study lived for more than nine years on a shallow seabed in the tropics, in an area that is now dry land in Oman. According to the Advancing Earth and Space Sciences press release, the mollusc “grew fast, laying down daily growth rings,” which the researchers were able to identify and count using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). This method focused a laser on shell sections roughly 10 micrometres wide, about the diameter of a red blood cell, to measure trace element concentrations linked to daily and seasonal growth.The approach combined “point-by-point analysis” with line-scanning LA-ICP-MS and stable isotope measurements taken from the same shell, allowing chemical variation to be studied on both seasonal and daily scales. The team used three separate counting methods, cycle-length estimation, visual layer counting under microscopy, and chemical layer counting from the trace element records, to cross-check their results. These combined to produce a composite estimate of 372 growth laminae per year, which the authors interpreted as representing 372 daily cycles of shell deposition.
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Earth’s days were 23 hours and 31 minutes long 70 million years ago
Dividing the length of a year, which has remained constant over geological time, by 372 days gave a Campanian day length of around 23 hours and 31 minutes. Because the length of a day depends on how fast the Earth spins, and this spin is gradually slowed by tidal friction from the Moon, the researchers also used the day-length figure to estimate the Earth-Moon distance during the late Cretaceous. Applying established models of the Earth-Moon system, they calculated a distance of approximately 3.83 × 10⁸ metres, a result the authors describe as consistent with existing models of how the Moon has been retreating from Earth over geological time, currently at a rate of about 3.82 centimetres per year.
Ancient shell reveals evidence of photosymbiosis in Torreites
The study also found that daily chemical variation in the shell exceeded both seasonal and tidal variation, a pattern the researchers linked to the presence of photosynthetic symbionts living within the mollusc, similar to the relationship seen in modern giant clams. De Winter said the shell showed “a very strong dependence on this daily cycle,” which the team took as evidence of the day-night light cycle being recorded directly in the shell’s chemistry.This finding was described by Peter Skelton, a rudist specialist not involved in the study, as the first paper to provide convincing evidence for photosymbiosis in Torreites, though he noted the conclusion applies specifically to this genus rather than to rudists more broadly.
