#stable isotopes

ITA version ESP version

Instagram Facebook Linktree

In Nebraska, there is a unique place called Ashfall Fossil Beds, where around 12 million years ago, a massive volcanic eruption—650 km³ of expelled material—covered a wetland in ash, trapping and fossilizing a large number of animals, including over 100 rhinos. Since its discovery in the 1970s, researchers have wondered why so many animals were found in a single location. Did they live in herds? Was it a seasonal gathering linked to a varied diet that encouraged movement? Or had they come together seeking refuge from the ash?

A recent study attempted to answer these questions by closely examining the fossilized teeth of one of those rhinos: Teleoceras major. This species, endemic to North America during the Clarendonian—part of the North American land mammal age—was a rhino with a single horn, a stocky body, and short legs. All Teleoceras species exhibited sexual dimorphism in their lower second incisors: females had smaller, rounded tusks, while males had larger, pointed ones, possibly used in fighting or displays to gain mating privileges.

The study focused on the remains of 13 adult individuals (8 ♀ and 5 ♂), specifically the M2 and M3 molars. From the tooth enamel, researchers extracted material to measure three stable isotopes: carbon (δ¹³C) to determine diet, oxygen (δ¹⁸O) to infer climate and water intake, and strontium (⁸⁷Sr/⁸⁶Sr), which reflects local geology and helps detect potential geographic movement.

Results showed a diet based almost entirely on C3 plants, typical of humid, wooded environments, with no signs of C4 plant consumption. Oxygen values varied with seasonal water changes, but strontium values remained homogeneous, indicating constant use of the same territory, with no signs of migration from geologically distinct areas. More detailed analyses on serial samples confirmed this ecological stability.

T. major lived within a limited area of a few dozen km² and did not migrate regionally. It was sedentary and closely tied to aquatic environments, much like modern hippos. The lack of isotopic evidence for immigration supports the idea that all sampled individuals were local—likely part of a large herd—killed in the same catastrophic ashfall event.

See You Soon, and Good Science!

[This paper is freely available, by the way!]

Hi all! This is the first in a new weekly series I’m going to be doing, in which I will be doing a short paper review each Thursday. We’re starting off big this week, with a delve into the world of stable isotopes. 

But Hayley, you say, what in the world are stable isotopes? 

Okay, time for some backstory!

The part with the backstory

Isotopes, as you may be aware, are atoms of the same element that have different numbers of neutrons, and therefore different atomic weights. If you think back to chemistry class, or forward to chemistry class if you haven’t taken it, in which case this might help you get a leg up, you’ll remember or just be learning that each element is defined by the number of (positively charged) protons in its nucleus. So each atom of carbon has 6 protons, and if it has more or less, then that’s a problem, because then it’s not carbon. But within the same element they can vary in numbers of (neutral) neutrons. Neutrons are important for holding together an atom’s nucleus, because if you get a bunch of positive charges together in a nucleus they start to repel, just like if you get a bunch of elementary school-aged children together. Different isotopes act basically the same, and I can guarantee you that right now some of the carbon atoms you are using in your body have six neutrons, some have seven, and some have eight. These are referred to as carbon-12, carbon-13, and carbon-14, respectively, and that number refers to the number of (protons+neutrons) in the nucleus. [Sometimes some isotopes are unstable and undergo radioactive decay. Carbon-14 does this. But that’s not really relevant to what we’re doing today.]

(Image: This is a carbon-12 atom. The yellow and red bits in the middle are the protons and neutrons. Not remotely to scale.)

Different isotopes of the same element act mostly the same (besides sometimes being radioactive and going berserk), so animals can take them up and use them. But they don’t act entirely the same, and that information can be used by palaeontologists to learn really cool stuff about past life. 

A big thing that isotopes can do differently is something called mass-dependent fractionation. That’s fancy-people-speak for “it’s easier to move things that are less heavy”, and it means that it’s easier to move things that are less heavy. 

One useful aspect of this deals with oxygen isotope fractionation. You are breathing oxygen as you read this, and if you are not then you will not likely be reading this for very much longer. Oxygen is also present in water, though, and water covers 70% of the Earth’s surface and makes up 70% of your body. It’s kind of a big deal, if you’ve never heard of it, and for our purposes it comes in two main flavours - water with oxygen-18, and water with oxygen-16. 

Oxygen-16 makes up 99.76% of all oxygen on earth, and oxygen-18 makes up 0.2% (the rest is oxygen-17, which no one cares about). This means that about 0.2% of water is heavier than the rest of water. This water is harder to move, because it’s heavier, and when evapouration occurs, oxygen-16 is more likely to evapourate because it takes less energy to move it.

But wait! Putting in more energy makes it easier to move heavy things, and this is exactly what happens. If temperatures are warmer, more oxygen-18 gets evaporated than does at colder temperatures. 

We need something to compare this to, because otherwise we’d just be measuring samples and it’d be hard to know what different numbers are referring to. For oxygen, we use the standard of Vienna Standard Mean Ocean Water, or V-SMOW. It’s not super important to know what that is. 

What is important is delta notation: δ18O (delta-eighteen-O) refers to how different the ratio of oxygen-18/oxygen-16 is from the standard. We use the symbol ‰, “per mil”, which is fancy people talk for 0.1%, for this, because the differences are really little. +1‰ means that there’s 0.1% more oxygen-18 in the sample than normal - that is, it’s 0.1% “heavier” than normal water. 

So, to get that all neatly said: Higher temperatures = More heavy water evapourates = the water that is left behind is lighter.

The part with the actual paper

Oh yeah, the paper! That’s why we were here. Okay, so mosasaurs were a group of big marine lizards (not dinosaurs) in the Cretaceous period. I wrote a little about them here. They looked kinda like this, and could get up to 17m/55ft long.

(Image: A mosasaur, a streamlined marine reptile with a large head, flippers, and a vertical tail fin. Image by Dimitry Bogdanov.)

Being lizards, mosasaurs have been traditionally thought to be cold-blooded - perhaps a reasonable assumption, as all living lizards are cold-blooded. However, it has been suggested various times in the literature as well as informally that mosasaurs may have been warm-blooded, owing to their assumed ecology as fast-moving pursuit predators and the fact that other marine reptiles have been demonstrated to be warm-blooded. 

In order to quantitatively test this, teeth from three species of mosasaur from the Mooreville Chalk in Alabama were collected and analysed to find the isotopic composition of oxygen. We need a control sample, though, to ensure that some geological process or change in atmospheric oxygen won’t mess up our data by adding or removing some heavy oxygen. In order to do this, the authors also tested samples from fish, turtles, and aquatic toothed birds that lived at the same time and place. That way they could place the mosasaur fossils in a reliable context. 

And the results they found are really interesting!

(Image: a graph showing δ18O values of different fossils from the Mooreville Chalk. I’ll do my best to explain what’s going on in the following paragraphs.)

Okay, so first things first. The fish and turtles tested have very similar δ18O values to each other. They both fall around the +21.2 to +22.0‰ range, or 2.12% to 2.2% heavier than “normal” water (because the “lighter” water has evapourated and left). This corresponds to a temperature of about 26-29°C (79-84°F). This is consistent with estimates of the temperature of the formation. This is what we expect from cold-blooded animals. Good! The method works!

Okay, next step. We can pretty safely assume that these birds were warm-blooded, and thus had a higher body temperature than their environment. Those tested had a δ18O value of about +18.5 to +19.5‰. That’s still heavier than normal water, but it’s less heavy than the cold-blooded animals (because the higher body temperatures led to more of the “heavy” water evapourating and leaving the body). This corresponds to body temperatures in the range of 36-39°C (97-102°F). This is right in the range of modern warm-blooded animals, and it’s another verification that the method works.

Okay, so what do the data say about mosasaurs? Well, it’s kind of a broad scatter. The δ18O values of the samples fall in the range of +19 to +21‰, which corresponds to a range 30-38.5°C (86-101°F). In other words, we’re getting body temperatures that are consistently warmer than the environment or cold-blooded animals, and in some cases are as warm as birds from the same environment!