A 24-hour day seems so natural here on Earth, but 70 million years ago, that would have felt leisurely to creatures accustomed to making do with just 23.5.
Ancient clam fossil reveals Earth's day was shorter 70 million years ago : Read more
Near the end of the article we read "But that rate can't be steady across geologic history. If it were, Earth and the moon would have been occupying the same space about 1.4 billion years ago — which is pretty awkward, given that scientists know the moon formed more like 4.5 billion years ago. So the drift must have been slower in the past. The fossil shell represents a first step toward pinning down the timeline of how Earth's spin slowed and the moon drifted away. This lone clam won't be enough, particularly since it is so young in the context of the moon's age, but the scientists hope to be able to conduct similar analyses with other, older fossils that would round out the timeline.”
Note *lone clam* so a single data point to plot or a single clam sample used. Over the last 30-40 years, there are various studies to show the lunar month was shorter and Earth's length of day shorter too extrapolated back into deep, geologic time. An 18 hour day reported 1.4 billion years ago in the Precambrian strata, ""Summary: A new study that reconstructs the deep history of our planet's relationship to the moon shows that 1.4 billion years ago, a day on Earth lasted just over 18 hours.",
https://www.sciencedaily.com/releases/2018/06/180604151200.htm
Some 4.5 billion years ago according to the giant impact model for the origin of the Moon, the proto-earth after the giant impact had a 2 or 2.5 hour day, "We present a tidal evolution model starting with the Moon in an equatorial orbit around an initially fast-spinning, high-obliquity Earth, which is a probable outcome of giant impacts." My observation, a 2.5 hour day in the attachment is reported. "In Fig. ED6 we started Earth with a spin period of only 2 h (as opposed to 2.5 h in Fig. 1). Just like in Fig. 1, we changed the tidal properties of Earth halfway through the simulation, leading to two different outcomes (see Fig. ED6 caption). While the qualitative evolution of the system is similar to that shown in Fig. 1, there are some qualitative differences.",
https://ui.adsabs.harvard.edu/abs/2016Natur.539..402C/abstract
A key comment here "we changed the tidal properties of Earth...". Tidal properties for the Earth-Moon system are subject to wide interpretation in efforts to reconcile with the geologic time scale in use. There is some 2800 years of solar eclipse records from Assyria and Babylon, including current solar eclipse measurements (as well as lunar laser ranging measurements) showing a *different tidal property* for the slow down in Earth's day and rate of lunar recession that will not match with the *lone clam* study or other studies like the 18 hour day report or the 2.5 hour day for the proto-earth some 4.5 billion years ago.
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