Join us for part two of our conversation with shark researcher Dr. Catalina Pimiento, where we find out why this fifty-foot-long super predator disappeared from the oceans. This Pulsar podcast is brought to you by #MOSatHome. We ask questions submitted by listeners, so if you have a question you'd like us to ask an expert, send it to us at sciencequestions@mos.org.

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ERIC: Imagine a shark the size of a city bus - nearly three times the size of any shark living today. This prehistoric predator, called megalodon, cruised our oceans millions of years ago. Last time, we found out what it ate and how we know so much about it with Dr. Catalina Pimiento, a researcher at the Paleontology Museum and Institute at the University of Zurich in Switzerland and Swansea University in the United Kingdom. On today's episode of Pulsar, the second part of my conversation with Dr. Pimiento, focusing on her research into the fate of the megalodon. So yeah, that was the next question. Why did they go extinct? What made them disappear?

CATALINA: When I started to study this species, there were different hypotheses out there about these. And one of them was, for example, it became too large to be sustainable, and to be able to live and it became extinct. So a friend of mine, when I was a grad student, Meghan Balk and I, we did a study in which we analyzed whether or not the species was increasing in size. And we found that actually, there's no increase in size through time. So sometimes it was bigger, smaller, bigger, smaller, it was static at the end of the day. So body size didn't change.

ERIC: So it didn't simply evolve to be too big to survive - what other ideas were out there?

CATALINA: Another hypothesis was that it lived in warm waters, and during the Pleistocene the glaciations made the water too cold for the species to live and then prey moved, and megalodon couldn't follow them. But we also show that temperature didn't really affect the distribution of this species, and a person who I collaborated with once and that I think is a really great researcher in particular on a couple of other studies in which he tested the hypothesis of the species being mesothermic. That means being able to regulate the body temperature of its most important parts, like body parts, like the brain, the stomach, the eyes. That makes sense because other species like big, large macro predatory sharks in the same order have that ability. And he used stable isotopes, and made different comparisons and long story short, multiple lines of evidence suggested that the species was mesothermic. So when you're mesothermic and you can regulate your body temperature, you can live in cold waters. And that's why the great white can also go to colder waters.

ERIC: So it wasn't colder water temperatures, either. What hypothesis did you move on to next?

CATALINA: We proposed initially, the causes of the extinction had to do with biotic or biological reasons. One could have been declining prey that has been studied for a long time. And a lot of people have proposed before, that around the time that megalodon became extinct, there was a decline in the diversity of cetaceans.

ERIC: So a decline in cetaceans - less whales to eat - I could see how that could lead to megalodon's extinction.

CATALINA: And the other cause that we thought of could have been the appearance of potential competitors, like the orca, or the evolution of the great white.

ERIC: So maybe it was out-competed for prey. How did you go about testing these ideas?

CATALINA: When I did my first postdoc, I decided that I couldn't be so focused on one species anymore. And I needed to look at the whole community. We did a study, actually here in Zurich, where we looked at first if megalodon had gone extinct, or if it was part of an extinction event in which other species went extinct. So we knew that cetaceans were also declining. And we knew that there was once upon a time a giant aquatic sloth - imagine that!

ERIC: An aquatic...sloth?

CATALINA: Giant! Massive! And it also became extinct. There were also see turtles and seabirds that went extinct. So we're like, it looks like the marine megafauna was in some trouble. So let's explore this hypothesis.

ERIC: So maybe there was a bigger event that caused many extinctions. And the megalodon's disappearance was part of whatever that event was.

CATALINA: Exactly. We collaborated with a team of brilliant people. And one of them was Daniele Silvestro who is also here in Switzerland. And he develops software to analyze possible occurrences, accounting for sample size and preservation. And he's very, very clever with these things. And he has this method where we can calculate extinction rates based on fossil occurrences. So we used his method, and it turns out that the Pliocene had very high extinction rates compared with the background Cenozoic value. So there was a spike in the extinction rates. Not quite a mass extinction event, but significant.

ERIC: Significant enough that you can measure in the fossil record after millions of years. How many species went extinct?

CATALINA: One third of the genetic diversity of the marine megafauna became extinct around this time.

ERIC: Wow, that's still a lot of extinctions.

CATALINA: That is quite a lot. Not only megalodon, but then many, many species. And that includes mostly species of marine mammals, as we had predicted, some sharks, sea turtles, and so on.

ERIC: That definitely sounds like something that greatly affected a whole series of ecosystems. What could have caused these extinctions?

CATALINA: So one of my collaborators and actually one of my mentors in paleontology, Carlos Jaramillo, who is at the Smithsonian. He said, I think it has to do with area. I think it has to do with area because, you know, at that time, there were changes in temperature. But those changes translated into changes in sea level. So why don't you assess how much area was lost, whenever you have increases or decreases of sea level. So these changes in sea level around the same time that these extinctions took place are well known. And there are many curves out there that scientists have used for many years. And they just show how striking it was, the changes of sea level at the time - it was crazy. So you can imagine that the sea level was increasing, decreasing, increasing, decreasing and when the Pliocene started, it was just crazy. It was mad, it was violent increases and decreases, and it was completely crazy. So I collaborated with Sara Varela who is at the Naturkunde Museum in Berlin. And she did something really cool and really clever - she took the ocean for each time in which we had this sea level curve. And she calculated the depth of each cell in the ocean at that particular time. And then she calculated how much area would have been lost having these changes in sea level. And what we found is that the aftermath of these violent sea level oscillations translated into area loss. So area underwater - coastal habitats that are so productive, where all of these animals live - was not available anymore.

ERIC: So tracing this extinction spike to coastal shallow water areas disappearing led you to the cause of megalodon's extinction. Loss of habitat...sounds a lot like what we're trying to prevent today with human activity and climate change affecting our entire planet, including habitats like the ocean.

CATALINA: So the exact same reason why we lost a third of the marine megafauna is what we're doing to habitats today. Not in the same way because what you rightly say climate change, but what it does is increases sea level and so on and that's terrible. And that is causing many, many extinctions. But habitat destruction is something that really directly affects species. And we saw it only a few million years ago, where the marine megafauna became extinct. Having a place to live, a habitat, is fundamental. And that's something that we learn from this extinction of animals. But also, something that was quite revealing was that we used to think of the animals that we have today, or the marine megafauna that we have today, as something that we inherited in a pristine way. So this is the marine megafauna we have it and now we're starting to destroy it. But actually, they should be recovering from an extinction event that happened only a few million years ago, because this extinction event took place between three and two million years ago. So you know, it takes millions of years to recover from an extinction event. In the absence of humans, these animals who survived these extinctions so not megalodon but the ones that did survive - should be recovering, but obviously, they're not because we're facing an even bigger extinction today. So I guess the other lesson is that this foundation already fragile, is already vulnerable, because it's already depleted.

ERIC: Well, definitely lots of reasons to try and reverse the impacts we're seeing not just for today's sharks, but for the entire ocean.

CATALINA: Absolutely. They are so important, right? I mean, the extinction of these animals produces cascading effects that go down in the food web. So the loss of a shark or the loss of a big mammal, or an apex predator is catastrophic. So we really don't want this to happen. So we really hope that the action is taken.

ERIC: Absolutely. Well, thank you, Dr. Pimiento for answering our many questions about the megalodon.

CATALINA: Thank you for having me. It was really, really fun to talk about all these questions.

ERIC: That's it for this episode. Until next time, keep asking questions.

If you liked this episode, be sure to check out:

What Happened to Megalodon, The Biggest Shark Ever?

How Do We Know What Extinct Animals Ate?

Why Did Dinosaurs Have Feathers?

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