Dinosaurs kept warm — and accidentally survived a mass-extinction event

Interesting Engineering sat down with paleontologist Paul Olsen, a co-author of the Science Advances paper, to talk about how researchers study ancient geography and climate, what that information reveals about dinosaurs, and how knowing what dinosaurs looked like can answer questions about the history of life on Earth.

This interview has been edited for length and clarity.

Interesting Engineering: You and your colleagues are weighing in on some long-running debates in this paper. What new data are you bringing?

Paul Olsen: Our primary discovery was empirical evidence of freezing in this part of China in the early part of the Mesozoic and the Triassic, early Jurassic. Such evidence has been lacking. It wasn't clear that this part of China was in the Arctic, north of the paleo Arctic Circle. 

IE: How did you determine where your site was located millions of years ago?

One of our co-authors, Dennis Kent, is very famous for his work on paleomagnetics. He determined by using this circuit plate technique exactly what the paleolatitude was. He came up with 71 degrees north, which is above the Arctic Circle. Then, we happened upon this website, which his colleagues developed, and got exactly the same result. So, we could have done it from 2014. Anybody could have done it. The information was out there just wasn't clearly stated. Even though it kind of shocked most people who've worked in that area, it's a pretty clear conclusion. 

IE: What method did Kent use to determine your site's paleolatitude?

It's based on what's called the plate circuit method. The Earth is a finite size, and all the tectonic plates are next to each other. If you move one, something else has got to move. So, if you solve the geometry for some area that's really well known — where the latitude has been determined by paleomagnetics — then you know where everything else was.

IE: How does paleomagnetics work?

Iron particles orient themselves like a compass needle to Earth's magnetic field, especially in lake sediments or ocean sediments. From that, you can rather easily deduce latitude.

IE: You and your co-authors used direct evidence from the site in China to determine that it had seen freezing temperatures in the past, right? 

Our primary discovery was lake ice-rafted debris. That's the climatic indicator that we found. My student, Clara Chang, demonstrated that the distribution of grain sizes was identical to what you see in modern aquatic systems in which there is seasonal ice that forms. 

The seasonal ice freezes against the edge of the lake. When spring comes, and the ice breaks up, chunks of ice carrying coarse grains from the shoreline drift out to the middle and drop down. There's no hydrodynamic way to get those big grains out in the middle of the lake unless they are carried out there by an ice raft. The other discoveries we made were dinosaur footprints.

IE: So you were able to show that this area with dinosaurs was once north of the Arctic circle and that it froze. Does that mean our popular idea of dinosaurs only living in hot, steamy jungles is a misconception?

It's a total misconception. Dinosaurs in high latitudes and in polar regions — living in places where it's freezing in the winter times — that's not marginal: that's what they did. Most of the age of dinosaurs, the Mesozoic, was a time of very high CO2, and there were no ice caps per se. But that doesn't mean it didn't freeze in the wintertime.

The mindset — and this started in the 1800s — was that dinosaurs are found with palms in places like Wyoming, which doesn't have palms today. And therefore, since they were reptiles and cold-blooded, they required that warmth. And in fact, 1906 was the first discovery of an Arctic dinosaur. Because it was a dinosaur, that was actually used as evidence that it was warm and never froze.

IE: That's interesting in its own right, but it's also useful in explaining how dinosaurs survived several volcanic winters, correct? How does the fact of dinosaurs living in freezing regions explain mysteries in the fossil record?

If you imagine that this is normal for these insulated dinosaurs to be present in these high latitudes or polar regions, and then you have a catastrophic event that exports cold to the tropics, then you have a mechanism for preferentially wiping out large, non-insulated animals — the pseudosuchians [relatives of modern-day crocodiles] — and that's the pattern that you see.

IE: How do you know that all dinosaurs had insulation and not just those species that lived in colder climates?

What we did in our paper was a little bit of a rehash of things that other people have said. We used what's called a phylogenetic bracket method. We can use the distribution of feather-like structures to determine what dinosaurs in general would have. So, birds have feathers. We've known since 1996 that a lot of theropods have feathery structures, or, in some cases, filaments instead of branching feathers like modern birds. We've known since the 2000s that some ornithischian dinosaurs (the bird-hipped dinosaurs) also had these filaments. And it's been known, believe it or not, since the 1830s that pterosaurs have filaments.

And since then, every pterosaur that's been properly looked at has these filaments, including very large ones. Since pterosaurs are close relatives of dinosaurs, and these filaments are present in pterosaurs and dinosaurs, the simplest argument that you can make in terms of the origin of these structures, is the common ancestor of pterosaurs and dinosaurs had the filaments.

IE: Does that mean they were better adapted than the pseudosuchians for surviving the mass extinction event that wiped out so many species 220 million years ago? 

They were pre-adapted by accident — not by evolution, but by accident — to the two volcanic winters. And they survived those at the end-Triassic, they survived other volcanic winter episodes through the Jurassic, and into the Cretaceous. And they also survived the impact winter of the end of the Cretaceous, because we still have birds.

There are twice as many species of birds on Earth as there are mammals. So, in a sense, in terms of diversity, it still is the age of dinosaurs.