A Triassic Mass Extinction Mystery That Led To Dinosaur Dominance : Short Wave Dinosaurs ruled the earth for many millions of years, but only after a mass extinction took out most of their rivals. Just how that happened remains a mystery — sounds like a case for paleoclimatologist Celina Suarez! Suarez walks us through her scientific detective work, with a little help from her trusty sidekick, scientist-in-residence Regina G. Barber.

Rise Of The Dinosaurs

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EMILY KWONG, BYLINE: You're listening to SHORT WAVE from NPR.

REGINA BARBER, HOST:

Out in South Africa, there's a team digging for clues to unlock one of the most elusive murder mysteries of all time.

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BARBER: Regina Barbara here. And I've been called to help on a new case. And it begins with our detective, Dr. Celina Suarez.

CELINA SUAREZ: We find our clues in rocks and fossils within those rocks.

BARBER: Well, a detective of a different kind. Celina is a paleoclimatologist at the University of Arkansas, and she spends her days either in the lab or out in the field hunting for answers.

SUAREZ: A paleoclimatologist's goal is to reconstruct the climate in the past and understand how that climate has changed over time.

BARBER: Celina's unraveling the mystery behind a mass extinction on Earth. There have been five mass extinctions in the history of Earth, and Celina's team of wily investigators is focused on the fourth one. This is not the asteroid that wiped out the dinosaurs. It's the one before that, back when Pangea was still around. It's called the end-Triassic Extinction.

SUAREZ: So that was a mass extinction that happened about 201 million years ago. This mass extinction was just before the dinosaurs basically came to dominate the globe.

BARBER: But there's our mystery.

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BARBER: Because no one really knows why the dinosaurs took over. The dino ancestors didn't look like much. They were no bigger than modern-day chickens. And for a minute, there was a bunch of other big, bad Triassic predators out there vying for power. And then...

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BARBER: ...It's all dinos - huge ones. But how? That's the case Celina is trying to crack. And her detective dig took her all the way to South Africa, to a group of rocks called the Elliot Formation.

SUAREZ: We were staying in a little town called Imeha (ph). And basically, it is fairly rugged. And so we were following up streambeds and dongas and up the sides of the mountain, collecting rock as we went.

BARBER: For Celina, the clues to unraveling this whodunit lies within these rocks. They can give you a timeline of what happened more than 200 million years ago.

SUAREZ: We were collecting rocks from the bottom of the formation to the top of the formation to get an estimate of age, as well as collecting rocks to analyze remnant organic carbon. Basically, if you think of plants and they die and they get buried and they decay, but they still leave a remnant of themselves in the sediment. And the reason why we're looking at organic remnant - because we're looking at major shifts in the carbon cycle, the global carbon cycle.

BARBER: Celina's using the carbon isotopes you can find in these really old rocks to piece together a timeline when all the other Triassic creatures died and find out perhaps why.

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BARBER: Today on the show, a cold case of epic proportions. We're solving the mystery of how dinos took over the world. You're listening to SHORT WAVE, the daily science radio noir from NPR.

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BARBER: Before we dive into this mystery, let's set the scene. We're starting out in the Triassic era around 250 million years ago.

SUAREZ: It was pretty bleak for the first few million years of the Triassic, but eventually, life started going again. The tropics were probably very dry and warm and maybe seasonal with monsoons. And then, like, the mid-latitudes were probably wetter and cooler.

BARBER: That's the stage of our murder mystery. Now let's meet the characters. There are a few key players vying for power, all competing against our dino ancestors for territory. First up, the phytosaurs.

SUAREZ: We had kind of these crocodile-looking organisms called phytosaurs. They lived, probably, in the freshwater ecosystem, in rivers and lakes. And they got pretty big. Some of their skulls were like, you know, three feet long.

BARBER: Next up, the aetosaurs. These were one of the dominant, vertebrate land animals.

SUAREZ: And those organisms were kind of like cows. They kind of had, like, an armored back. And they had little heads and little teeth, and they were plant eaters.

BARBER: And it starts to get a little spicy with our big carnivore characters, the Rauisuchians.

SUAREZ: That kind of look like dinosaurs but walked on all fours that had big, sharp teeth, but, like, their limbs were underneath the body. So they could probably, like, gallop and run. So you had this, like, allosaurus-looking skull on, like, a four-legged, galloping organism running at you. You know, that would have been pretty scary.

BARBER: Like a T-Rex horse?

SUAREZ: Kind of like a T-Rex horse, yeah (laughter).

BARBER: And compared to those creatures, the dinosaur ancestors running around at this time were kind of...

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BARBER: ...Wimpy.

SUAREZ: Those first dinosaurs were probably small theropods. Again, the small, two-legged chicken to, like, dog-sized theropods.

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BARBER: OK. They're the underdogs in this story.

SUAREZ: Yeah. They were the underdogs.

BARBER: So how did these tiny, little chicken T-Rexes become...

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BARBER: ...The fearsome predators that populated the whole Earth?

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BARBER: I'm putting my tweed jacket on and will play the Watson to Celina Sherlock because it's time to meet our suspects. So at this time, there was one, giant continent, Pangea, sitting on tectonic plates that were starting to move apart. And this would lead to a series of chaotic, violent, massive volcanic eruptions.

SUAREZ: You had this big rift happening. And as a result, you had massive amounts of volcanism and these major basalt flows and eruptions happening.

BARBER: There's lava, fire, brimstone, destruction everywhere. And as a consequence, the climate is extremely altered over the whole planet, almost immediately.

SUAREZ: Like, years to tens of years after the eruption.

BARBER: And basically, all those rival big reptiles - the aetosaurs, the phytosaurs and the Rauisuchians - they all go extinct.

SUAREZ: And dinosaurs come to inherit the Earth. But we don't know the details quite yet.

BARBER: That's our mystery, right?

SUAREZ: That is our mystery, yeah.

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BARBER: So we have our victims. Now for the suspects. There are basically two hypotheses on who to pin this mass extinction on - a cold snap, followed by warming, and a hot-only scenario, where it's just relentless warming. First, we'll interrogate the whiplash theory.

SUAREZ: When you have lots of volcanic eruptions, you release a lot of ash and aerosols, sulfur dioxide, which actually causes cooling.

BARBER: All this ash creates a haze and blocks out the sun. In the higher latitudes closer to the poles, there might have been even ice.

SUAREZ: And those organisms that could internally metabolize and keep their bodies warm - it is thought maybe that their metabolism and the fact that they may have had proto-feathers to keep them warm allowed them to exist during this cooling time period.

BARBER: So you have cooling in the short term. But in this scenario, after the cold, came the opposite - intense global warming.

SUAREZ: There are several major pulses of volcanic activity. And that series of pulses, over time, built up CO2 in the atmosphere, to where we had a warming event. And so the pendulum would swing now to warming. And organisms that cannot thermoregulate well have a real hard time dealing with these major shifts.

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BARBER: That's the whiplash. Animals that couldn't regulate their body heat by shivering or panting or puffing up their fur or feathers - we're talking the aetosaurs, the phytosaurs and the Rauisuchians - well, they just couldn't hang.

SUAREZ: But happy dinosaurs are, like, hey, I could deal with this. And they're like, great. I can eat this plant that you no longer are eating. And so they start to thrive.

BARBER: So this is camp one, that this pendulum shift from cool to warm just wrecked almost every species in the Triassic, allowing the more nimble dinos to rise victorious. Now onto the second theory of the case.

Then there's the second school of thought where there's this volcanic eruption. It triggers this huge greenhouse effect. No pendulums, just boom - greenhouse.

SUAREZ: Right. That school of thought is more along the lines of there was major thermal damage to plants, so basically too hot for the plants - they wither. So then the thought for that is, like, oh, well, there was just no food for herbivores to eat. And the ecosystem kind of collapsed in that way.

BARBER: And slowly, one by one, the climate killer squeezed out our nondino players.

SUAREZ: So for, like, the nondinosaurian plant eaters, like the aetosaurs, they probably didn't regulate their temperature, at least not as efficiently as dinosaurs did. They also probably were not nearly as mobile as their dinosaur cousins, like the prosauropods. I mean, if you look at a crocodile, they can move - but not nearly as efficient for moving around on land. And so these probably were handicaps for them when it comes to rapid environmental climate change.

BARBER: So whodunit?

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BARBER: The key is figuring out the timing. That's why Celina's in the field, digging around for rocks and fossilized teeth from the Triassic period to piece together this mystery.

SUAREZ: One of the big mysteries, or one of the big problems that need to be solved in order to answer these questions is to get high-resolution dating of these rock units - like, for example, in South Africa, where we were at - to track this record of climate change to a higher resolution so we can start to answer these questions. Was it the cold or was it the hot? Or was it both? Or was it the fact that it may have pendulumed (ph) back and forth between the two, and that caused this mass extinction to the organisms who couldn't keep up with that pendulum?

BARBER: After Celina collects all these rock samples, she uses carbon dating - or measuring the ratios of different carbon isotopes - to nail down the timeline.

SUAREZ: And so we're going to look for that shift within the rock record. What did the climate do before, during and after that event? What organisms live before, during and after that event? How did they change?

BARBER: As a paleo-detective, what are you seeing with your data right now? Or is it still inconclusive?

SUAREZ: Some preliminary data - we definitely see a warming trend, don't see an immediate cooling trend, but that may be because our resolution of the rock record is not fine enough to resolve a short-term cooling.

BARBER: That doesn't rule out the whiplash theory. It just means we need more evidence. But if you ask Celina which theory she suspects...

SUAREZ: It was probably a little bit of both. I mean, yes, you - when you look at volcanic eruptions, you do have immediate cooling. Whether or not that immediate cooling was enough to cause a mass extinction...

BARBER: That's going to require more detective work. OK. So it's a little ambiguous, and it might not stand up in court yet. But according to Celina, there probably was that pendulum, both cold and heat. And to really know what part they each played, researchers like Celina will need to keep drilling down into the geological record. OK. This is a huge mystery, right? But it was in the past. So why even search for these clues? Like, why study mass extinctions?

SUAREZ: When we look at the end-Triassic mass extinction event, the amount of carbon released to the atmosphere is on par with the amount of carbon - it's actually little less than the amount of carbon - that humans are releasing to the atmosphere today through the burning of fossil fuels. And so one of the mysteries that we just don't really know is, is this amount of carbon going into the atmosphere that we see today going to cause a mass extinction?

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SUAREZ: The closest analogue we will have to conditions we'll see in about a hundred or 300 years were last seen in millions of years in the past. And so that's why it's important to solve these mysteries, to understand not only how the Earth acts as a major system but also what may be in store for us in the future.

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SUAREZ: The Earth has done the experiment for us in the past, so we just need to understand it.

BARBER: This has been a doozy of a mystery, one that we can't fully solve today. But for more clues and more detective work, led by Celina and her team, check out our show notes.

We want to take a second to thank all of our Short Wave Plus subscribers. Y'all are the best. Subscribers help support our show and they get sponsor-free versions of all of our episodes as a thanks. You can subscribe at plus.npr.org/shortwave.

This radio mystery was produced by Margaret Cirino, edited by Gabriel Spitzer and fact-checked by Brit Hanson. The audio engineer was Stu Rushfield. Gisele Grayson is our senior supervising editor. Beth Donovan is our senior director. And Anya Grundmann is our senior vice president of programming. I'm Regina Barber. Thanks for listening to SHORT WAVE from NPR.

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