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A shark's teeth usually says "stay away", but we can learn a lot from them, including what type of parents they were.

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Megalodon was the largest shark to ever live. At 18 meters long, it was three times longer than a great white and larger than a humpback whale!

And yet, this heavy hitter may have had a major weakness: its nurseries. But sharks don’t fossilize well, so how can we tell? The secret may be in their teeth.

Megalodon was an apex predator that lived in warm and temperate waters around the world from the early Miocene to the Pliocene, roughly 20 to 2.6 million years ago. It ate fish, including other sharks, and even whales. We’ve found ancient whale bones with the tips of Megalodon teeth broken off in them.

But Megalodon didn’t start out quite so fearsome. Like all animals, it had to be a baby at some point. In a paper published in 2020, scientists announced that they’d found a bunch of potential Megalodon nurseries.

Nursery areas are common among marine animals. They improve the number of babies that survive and grow into adulthood by providing things like more food or a better chance to avoid predators. Picture something like baby fish growing up in a sheltered lagoon full of food before venturing out to sea as adults.

It turns out baby Megalodon may have had similarly cozy childhood homes. Now, like I said, Megalodon didn’t fossilize well. Like all sharks, its body was mostly made up of soft, squishy cartilage, with the major exception being its teeth.

Most of the Megalodon fossils we have are, in fact, teeth. And you might think that’s way too little evidence to say we’ve found a whole baby shark nursery. But, it turns out you can tell a lot from a tooth.

The scientists collected teeth at two quarries in Spain. By using the tooth-to-body proportions of existing great white sharks as a guideline, they were able to estimate the size of the sharks the teeth once belonged to. Then, by taking the frequency of different sizes and crunching some numbers, they were able to estimate what the population looked like in the area.

They also repeated this with fossils found in eight additional geological formations. Overall, they found that five sites matched what they’d expect in nurseries. That is, there were a lot of baby or juvenile-sized animals.

Supporting their idea is the fact that, at least in that main quarry in Spain, the area would have been a shallow, protected bay with lots of potential prey, like marine mammals. The perfect place for a young mega-shark to grow up. This is really cool, but unfortunately, these nurseries might have been a key weakness in the Megalodon life cycle.

We think these huge sharks took a while to become sexually mature and finish growing. They may have taken 25 years to reach full size. And a lot can happen to a baby shark in that time.

In humans, we depend on parents and care-takers to protect us. In sharks, these nursery areas could have protected them. This means that nurseries may have been part of the reason the sharks even had the evolutionary leeway to grow so big and so slow in the first place.

But this may have also kind of locked the sharks into this evolutionary strategy, making the nurseries necessary to keep the numbers of new adults coming into the population steady. And, like animals who can only eat a certain kind of food, depending so heavily on one strategy can be a weakness. Toward the end of the Megalodon’s reign, for instance, many of the coastal habitats that made good nurseries may have vanished.

And increasing competition from juvenile great whites may have made the remaining nurseries less safe or bountiful. This may have meant that there would have been fewer babies making the many-years-long journey to adulthood and eventual parenthood. Too few to replace the ones that died.

Over time, the population numbers would have dwindled. Combine that with other potential factors, and by around 2.6 to 3.6 million years ago, Megalodon was megalo-gone. Thanks for watching this episode of SciShow, and thanks as always to our patrons for making it possible.

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