Megalodons were the ultimate apex predators, a new method reveals

Since many of the large marine carnivores that might have made for delicious meals are relatively recent arrivals (in evolutionary terms), it seems likely that smaller megalodons were on the menu. "Cenozoic marine food webs might have been dominated by giant cannibals," they write.

Interesting Engineering sat down with paleoecologist Dr. Emma Kast, the study's lead author, to hear more about the research and learn if the megalodon was really as fearsome as everyone says.

This interview has been edited for length and clarity.

Interesting Engineering: Why are teeth so important to understanding the megalodon?

Emma Kast:  When we're looking at ancient sharks, all we have to go on is the fossil record, and we only have their teeth, generally. That's because unlike us, they don't really have bones. Their entire skeleton is made of cartilage, like your nose. That doesn't preserve very well in the fossil record.

Essentially, everything we know about the megalodon and its ancestors is from these gigantic teeth in the fossil record. You look at one of those teeth and it's just stunning. You wonder what they were eating. That was really the starting point of this entire study: What the heck were these guys doing? What were they eating?

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IE: What does a megalodon tooth look like?

Kast: Gosh, I wish I had one on my desk. They're the size of a hand and triangular.  They have serrations on the side, like a bread knife, which is amazing. They're millions of years old, and they still have the detail to preserve those serrations.

IE: Are they found all over the world?

Kast: Yeah, they're found all over the world.  There are some great papers by a researcher named Catalina Pimiento assessing how they are distributed across the globe. They really were like a global species. 

For this paper, we looked at a couple of different locations. We have a decent number of teeth from North Carolina, some teeth from Japan, and a smattering of teeth from other places — whatever we could get our hands on.

IE: How have modern scientists made sense of the huge, triangular teeth found all over the world?

Kast: Until this point, really all that we knew about them was by looking at the shape of the tooth and asking "What does this morphology mean in terms of what it was doing?" We've taken this leap into geochemistry by measuring the composition of the teeth, and asking what that tells us about their diet. It really is a shift in terms of what we can investigate.

IE: What had researchers learned from the shape of the teeth?

Kast: The megalodon had these triangular teeth with the serrated edges, and what people think is that they have those teeth because they needed to rip and tear whatever they were eating. So, people think that shape means they're eating marine mammals. In contrast, sand tiger sharks, which tend to eat fish, have these really pointy teeth, almost like toothpicks. That's the level of information you can get just from the shape. 

IE: What kind of prey would've been available for predators living between 20 million and 3 million years ago, when megalodons lived?

Kast: In the modern ocean, we have a decent understanding of what the food web looks like. We can go out and observe interactions between species, and we can do things like look at stomach contents and see what certain animals are eating. A lot of us can probably visualize in our mind that you have some kind of like plankton at the bottom of your food web. Then you might have some small fish, some medium-sized fish. Then you get up to large fish, like tunas and sharks, that are eating those fish. And at the very top, we have stuff like orcas and great white sharks, who are eating small marine mammals and large fish. That's kind of like the picture we have of the modern ocean.

Then if we go into the past, into the Miocene — that's 23 million years ago to 5 million years ago, roughly — when megalodon was swimming around, what we know about food webs is also in the fossil record. So, what kind of fossilized bones and teeth and shells and stuff do you find? We know who was there, but fitting those ancient animals into a food web is difficult because you need the extra information about who was eating who. That's what we're after, really. Who are these giant sharks eating? What was their diet? Where exactly did they fit in the food webs of these ancient oceans?

IE: How can you tell who was eating who by looking at these teeth?

Kast: What we went after is called the nitrogen isotope ratio. There are two facts that will help understand how it works. So the first is that nitrogen is this nutrient that all animals need to get from their diet. So, all of the nitrogen that's in our bodies —  or the shark's body — is from their food. The other thing is that nitrogen has these two isotopes. I kind of think of them as flavors of the same thing. There is 15 nitrogen and 14 nitrogen. Animals generally hold on to the 15 nitrogen flavor a little bit better than 14 nitrogen.

So, if you combine those two facts, what ends up happening is, as you move up the food web, you get progressively more of the 15 nitrogen relative to the 14 nitrogen at each step. A really tiny fish would have a low 15-nitrogen-to-14-nitrogen ratio. And at the very top of the food web, you'd have a pretty high 15-nitrogen-to-14-nitrogen ratio.

IE: So, when an animal consumes nitrogen, it proliferates through their entire body including the teeth?

Kast: The short answer is yes. These ratios are definitely influenced by physiology and metabolism. There can be overlaying complicated details, but the broad picture is that yes, then the isotope ratio that's in the tooth reflects the isotope ratio of the whole animal. And that gives you a sense of its position in the food web.

IE: What did you find when you looked at these ratios in the megalodon teeth?

Kast: The ratios are crazy high! That's the headline: super high nitrogen isotope ratios, which means that the megalodon was at this really high position on the food web. To get a little quantitative, if we make some assumptions, it's actually telling us that if you go back to that modern food web that I described, the megalodon would actually be sitting above our top marine predators. 

If we imagine the orcas and the great whites at the top of the food web, the megalodon would actually be above them, and above them twice: two positions higher up in the food web. We're predicting extraordinarily high trophic levels, and high positions in the food web, from these really high values.

IE: Higher than any predator alive today?

Kast: Yes, it's really extraordinary. It means a lot of questions because if they really are two positions higher, it means there's a kind of gap in between. What were they eating? It couldn't have been orcas or great whites or equivalent-type species.

IE: Because the ratios are too high?

Kast: Yes, they're too high. One thing to interject here is that there's actually a wide spread. Some individuals — some teeth —  have more moderate values, and then some extraordinarily high values. On average, they're about two positions higher. If they were eating things like orcas and great whites, they'd just be one position higher. There are some possible explanations that are cool, but we can't really say which one or the other [is more accurate].

Maybe there are some extinct marine mammals that filled in that gap. There are raptorial sperm whales (they're like toothed whales) that might have been at a pretty high trophic level. We just don't know what their position in the food web would have been. Maybe there are some other extinct species that we haven't looked at yet, that can fill that gap. They could also have been doing crazy stuff like cannibalism, which tends to push you up the food web because if you eat yourself, then your diet is higher in 15 nitrogen.

Those are some possibilities we just don't know yet.

IE: With every step up the food web, a lot of energy is lost, right? Does that mean the total energy input must have been much higher in the Miocene ocean?

Kast: I love that question because that's what I think about most with this. Sure, it's really cool that megalodons were at this really high position in the food web, but it brings up a lot of questions in terms of the overall ecology of the ancient oceans that allowed for the megalodon to exist.

I'm not sure if it could indicate — and this is definitely just hypothesizing — some differences in productivity at the base of the food web, differences that provide a lot more energy all the way up. Or maybe there's something different in the way the food web is structured. So, if you think of the food web like a triangle, could the triangle have been pointier for some reason?

Learning more about this megatooth shark might tell us about the ecosystems in the past.