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Earth’s fossil record is loaded with the remains of ocean creatures. But ancient life didn’t exist in a vacuum, and a key part of understanding these species is understanding the environments they lived in.

Fortunately, there’s more to paleontology than just identifying old, dead stuff. Fossils can also provide clues to the conditions those species lived in, even in environments as complex and variable as the oceans. And these clues can give us more info than you might think, like what their environment felt like or even how long the Sun was out.

We’ll start with one of the biggest concerns anything might have about its environment: How hot is it? You might think ancient temperatures are beyond our understanding since there are no fossilized thermometers, but there kind of are. You see, lots of marine organisms build shells or skeletons out of minerals like calcite, which is a combination of calcium, carbon, and oxygen.

This process relies on chemical reactions, and a lot of chemistry is temperature-dependent. For example, oxygen atoms in ocean water come mainly in two forms, or isotopes: oxygen-16 and oxygen-18. The difference is the number of neutrons per atom.

Since O-16 atoms have two fewer neutrons, it’s lighter than O-18. And as you might expect, the lighter version of water tends to evaporate more easily, while the heavier form tends to precipitate first. So, depending on the temperature, some areas of the planet will have a different ratio of light to heavy water.

The key is, marine critters inherit that ratio when they build their bodies, so paleontologists can measure the ratio of isotopes in fossils to calculate ancient water temperature. And other elements can help, too! Like, some marine organisms also incorporate magnesium into their bodies.

And that reaction is endothermic, meaning it requires heat, so warmer water means more magnesium in fossils. These techniques are all part of the practice of paleothermometry, and they’ve revealed some big changes in Earth’s history. For instance, a big study from 2000 used the magnesium and oxygen content in the shells of single-celled creatures called foraminifera to estimate global temperatures over the past 50 million years.

They used forams to track global temperature changes as the Earth transitioned from hothouse conditions without polar ice caps to modern icehouse conditions, where polar ice caps remain year-round. This study provided some fine detail on this time in history, revealing multiple phases of cooling. The researchers were even able to match some of those phases with other events from the geologic record, like major changes to ocean ecosystems, and even the growth of the ice sheets that now cover Antarctica!

All this from some old shells! But, fossils can let us get even more detailed, too. Like, there’s another field called paleobathymetry, which uses things like fossils to study ancient water depths.

Just like some species are picky about temperature, some are picky about the depth they live at, since depth is related to things like tides, currents, and oxygen saturation. That includes forams! In today’s oceans, foram communities look different at different depths, thanks to those varying conditions.

And these community structures go back millions of years! So, by comparing foram species in a fossil deposit with today’s communities, paleontologists can get an idea of whether an ancient habitat was a shallow coast, a deep bay, or down in the abyss. And this can help us track how depths change over time, sometimes, in pretty extreme ways.

For example, a study from 2004 used fossil foram communities to track the sinking of a part of New Zealand! Around 20 million years ago, tectonic shifts caused the Waitemata Basin of northern New Zealand to sink under the sea. It’s back above the water now, but its history is still something scientists are interested in.

In their study, these researchers examined how the fossils changed throughout the geologic record of the basin, and it allowed them to document the stages of its sinking. The foram communities showed how, over several million years, the basin became a flooded land of bays and inlets, then sank to shallow ocean, and settled in waters as deep as 2000 meters! At least, for a time.

And finally, as if that isn’t enough, a study published in February 2020 used marine fossils to calculate how many hours were in a day 70 million years ago. Because the days really are getting longer! As Earth spins, the Moon’s gravity causes the oceans to slosh back and forth, a.k.a. the tides.

And that motion robs the Earth of some of its rotational momentum, slowing it down ever so slightly. So today, one rotation is about 24 hours, but marine fossils prove that that wasn’t always the case. This research looked at relatives of clams from the Late Cretaceous Period.

Like many living species, these bivalves grew their shells throughout their lives, but that growth came in cycles, as their development was affected by changing seasons, tides, and light levels. The end result is layers within layers: Daily layers of growth within bigger, seasonal ones. That meant the researchers could literally count the daily cycles within a full season cycle, that is, the number of days in a year.

It’s a practice called sclerochronology. In this case, they counted 372 days. And since models tell us that the length of a year hasn’t changed, that means that each day back then was only 23.5 hours long!

Another study did the same thing with fossil corals, which grow in similar ways. And it showed that about 430 million years ago, there were about 21 hours per day! Now, the implications of that aren’t totally clear, but that does mean life back then had less time to grow and photosynthesize.

So maybe it had to be more efficient or change its behavior somehow. Really, though, this all just goes to show how much we can learn about the past when we ask the right questions about the right fossils. And speaking of questions: Around this time of year, you might be asking yourself, “What do I get my friends and family for the holiday?” Well we’ve got the answer: SciShow magnets!

If you’re looking for the perfect gift for the science fan in your life, you’re in luck:. Over on DFTBA we’ve just released a brand new SciShow Magnetic Words set! It’s full of science-y words curated by the whole SciShow team, like ‘Mitochondria,’ ‘Supermassive’ and ‘Quantum,’ and it comes packaged in a handsome, SciShow-branded tin.

You can use the set to declare important scientific discoveries such as:. Researchers Have Found Microscopic Butt Hair On Mars. Or, whatever you think is the funniest.

You can find the magnetic words and lots of other SciShow merch over on DFTBA.com/SciShow! [♪ OUTRO].