We ask ancient shark researcher Catalina Pimiento all of the best questions we've ever received about Megalodon, a prehistoric fifty-foot-long shark with teeth the size of a human hand, during this Pulsar podcast 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: Sharks are awesome. They have existed on the earth for almost half a billion years.

Today on Pulsar we're featuring questions we've been asked at the Museum of Science about the megalodon, the largest shark that ever existed.

Answering these questions is shark expert, Doctor Catalina Pimiento, a researcher at the paleontology museum and Institute at the University of Zurich in Switzerland, as well as a researcher at Swansea University in the United Kingdom.

Doctor Pimiento, thank you so much for joining me all the way from Switzerland today.

CATALINA: Thank you so much for having me. I'm very excited.

ERIC: I'm excited too because, as I mentioned just a second ago before we started recording, I love ancient sharks, particularly the megalodon. We got so many questions on our social media, this is such a popular topic. So why don't we start with what everybody wants to know, exactly how big was this ancient giant shark?

CATALINA: Well, there has been some controversy about this, but let's say that it's between 15 and 18 meters of total length. Really, really big.

ERIC: That's enormous. Can you put that into perspective? It's as big as a--?

CATALINA: I usually say as big as a school bus. I am from Colombia and I am from Bogota, which is the capital city, and over there we have these articulated buses. They are much bigger than a normal school bus, so as big as those.

ERIC: So big. And as far as when they were cruising Earth's oceans, we get asked pretty often, did they exist at the same time as the dinosaurs?

CATALINA: No, no they weren't at the same time with the dinosaurs or with humans. So it was in between.

ERIC: So dinosaurs went mostly extinct 66 million years ago. When exactly was the megalodon alive?

CATALINA: Between 23 million years ago and 3 to 2 million years. It's difficult, sometimes, to pinpoint the exact time when it appeared because megalodon is part of a chronospecies. The ancestor of megalodon was very similar and lived right before it. Like a sequence of a lineage that ended or finished with this giant shark.

ERIC: Right it didn't just suddenly pop into existence. It evolved from something similar to it before that?

CATALINA: Yeah it was a kind of a gradual evolution from an ancestor that was smaller, and also had a different dentition. And then as time passed it grew, but also it developed serrations and lost some of the cusps in the teeth and some modifications but it's all part of the same lineage. We call it the megatooth lineage.

ERIC: So if you're able to go back in time 15 million years, and swim alongside this gigantic shark, what would it look like? Would it look like a great white shark but bigger or would it look different?

CATALINA: That's a great question and I was very lucky to have found a student that was as interested in these questions, as we are at the moment, and he wanted to explore it further. It has been long assumed that the megalodon was like a fat, bigger version of the great white. So whenever you see illustrations of the species, it's basically a fatter, bigger version of the great white. But we don't have this time machine that you're proposing to go to 50 million years ago and check that.

So Jack Cooper, my PhD student, He has been trying to assess this very question. He did a series of analyses last year and that he recreated like a two dimensional model of the species by assessing the dimensions of the body parts. For example, we talked about the maximum size and I said between 15 and 18 meters. But then Jack was asking, how big was the tail? The caudal fin. How big was the dorsal fin? What was the distance between one fin and the other? What is the distance between the head? How big was the head?

He used an analysis in which he not only used the great white, but older species that are as related to the megalodon as the great white. So the model was informed by other species, so that we didn't assume that it was just a bigger version of the great white. The thing is that sharks have a very consistent body plan. And this has been shown before. When you look at all the sharks that are pelagic and they're macropredators.

They have more or less the same body plan, even if they're from very different families or orders or something like that. So it turns out it looks just like a shark, but what we found that was very interesting, is that it was stockier then you would have reconstructed, only based on the great white. Jack is going to take it a step further and I don't want to say too much about this because it's still cooking in the oven. But he wants to take a step further and kind of present a more complete model of what this body could look like but that's a great question. And that's something that we're very interested in.

ERIC: So not exactly just a bigger great white shark, it was its own shark related to a couple of the ones we have today, but exactly like none of them.

CATALINA: Exactly like none of them, but very much a shark. Because at the end of the day, all of these sharks share the same body planning. It's very interesting to find how previous studies have shown that sharks maintain these proportional distances between body parts through time. And that's why we were able to extrapolate all of these measurements because the relationship between body parts is isometric, unlike humans.

For example, if you see a human baby, those proportions of the size of the legs, of the hands, of the arms, are totally different to what it's going to be like when we're adults. So we couldn't really extrapolate the baby to a human in the same way, the same linear way. But with sharks we can, because they maintain this proportion. So that made it easier, in a sense, because we were able to use these extrapolations. But at the same time, it looks like it should look, which is a shark.

ERIC: And talking about all these models and reconstructions brings us back to our next common question, which is, how do we know about megalodon? We see evidence of dinosaurs as it's often, full fossilized skeletons, but we don't really see that with sharks. So what fossils and evidence do we have?

CATALINA: That's a great question as well. Yeah, so our only evidence of this species is their teeth. And that is the case for all sharks, so we all know that sharks are made of cartilage and cartilage doesn't preserve very well and the fossil record was very rare. So what we have is teeth only. However, teeth are very, very abundant because, unlike when you find a skeleton of a dinosaur or of a whale, the individual has to die and then the perfect conditions have to be provided so that this body fossilizes and gets preserved.

But with sharks, they shed their teeth all the time. They're losing their teeth constantly. So they leave behind like a very, very abundant fossil record without it having to die. So that's kind of a trade off. Yet, yeah we only have teeth and although teeth are very good proxies of body size, of diet, of feeding mechanism. And by calculating those you can infer other things, like habitat and at some degree some sort physiology and behavior, still is the case that the teeth are the only evidence we have.

But there are also other kinds of fossils that I think a lot of people don't know about. And it's their vertebral column, or the vertebral centra of sharks. These structures are harder because they calcify and they have fossilization potential. The problem is that, in this case, the animal has to die.

ERIC: Right. You're not going to shed your spine and then grow a new one.

CATALINA: No, if only. No, I would love to have a bigger sample size for this, but there are some. You can find these vertebral centra and in fact, there are a couple of vertebral columns in some museums in Europe. In fact, there is one in Belgium that has been there for 100 years or so. And for a while it was displayed in the Museum of Natural History in Belgium and is no longer displayed, but is in the collections.

And Jack and our collaborators in the museum in Belgium went over there and did 3D scans of these vertebral columns. So we're using that to kind of know more about their body size. So stay tuned because we are using this very rare specimen that, although, has been around for a while, it hasn't been used yet to kind of answer this question of what it will look like. But in summary, we have teeth and very rare vertebral centra.

ERIC: And you mentioned that by looking at the teeth we can learn things about their diet. So what did a shark the size of a school bus eat?

CATALINA: The Megalodon has the kind of tooth morphology that suggest that it was a predator, that he was cutting and slicing flesh because of the serrations and because of the shape and such a big size. It's the size of a human hand. A tooth can be really, really big.

ERIC: So one tooth the size of your entire hand?

CATALINA: Yeah, one tooth. Based on these teeth, we also know that they were very big because these teeth are huge. And that's why it's very easy to identify the species when you find these teeth because they're unique in their size, and the shape, and the serrations, and so on. You see a tooth and you say, this shark was massive, it was a giant. There's no debate about this animal being a giant. And as a giant, it had to eat a lot. And it had to eat a lot of prey that provided enough food to satisfy those energetic requirements.

So stay tuned because we're going to be able to provide more details on exactly how much he needed to eat, and things like that. But we think that he ate animals that provided a lot of energy. So marine mammals have a lot of fat, and just like a great white and other big sharks, they would go for what provides them with the most level of energy. So in the case of megalodon, it will only make sense that it will go for high-calorie prey.

ERIC: So lots of whales, things like that?

CATALINA: Yeah. Everything blubber, yeah.

ERIC: And so we got another question about where they were found. So around the world, as well as what kind of environment. Like shallow water, deep water?

CATALINA: That's a fundamental question that has allowed us to better understand many of the aspects of the species. We actually were able to map all the fossil occurrences of the megalodon. And we mapped it through time, and we found that he had a cosmopolitan distribution. He lived in almost all the oceans of the world. Interestingly also in waters that were perceived as being too cold for such an animal to live.

We even did some species distribution model to assess, if the conditions had changed, where would megalodon have moved? Like what conditions were the necessary ones for this species to live, and we found that actually it could live anywhere. Water temperature didn't really affect its distribution directly. This allow us to also propose that temperature changes wouldn't have affected the species to the point of driving it to extinction.

ERIC: My final question about what did cause the megalodon to go extinct led to another entire episode's worth of conversation. So tune in next week to find out what happened to the biggest shark to ever swim the seas. Until then, keep asking questions.

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