Megalodon and other extinct giant sharks started life in nurseries

What makes a nursery?

The idea that a large number of fossilized teeth represents the presence of a large number of sharks is, according to Gibson and co-author Dr. Robert Boessenecker of the Carcharocles angustidens paleo-nursery paper, partly an assumption.

 

“Although sharks do lose and regrow teeth throughout their lives,” Gibson explained by email, “it seems less likely that a single shark stayed in [one place] and lost teeth at such a regular rate that the teeth all became fossilized and did not get moved elsewhere by ocean currents and tidal action. What is more likely is C. angustidens teeth are just common enough at these localities due to how many sharks were living there and shedding teeth. If a population is present in the locality regularly, the probability of deposition and fossilization of shed teeth would have been higher, even after some are moved elsewhere by currents.”

 

Dr. Boessenecker, a research fellow at the Mace Brown Museum of Natural History, added that “we can't ever really prove that separate teeth came from the same individual unless we find an associated tooth set. So we assume they all represent different individuals. That being said, some sharks lose multiple teeth per feeding event, but the odds of these teeth representing dramatically less than that number of individuals is probably quite low.”

 

Other important clues to determining whether a location is the site of a paleo-nursery originate from today’s shark species and their behavior. Some existing shark nurseries are found within areas that offer both protection and food for growing babies. These are two key aspects that paleontologists find mirrored in the fossil record: shallow ancient marine environments filled with nutritional resources.

The presence of certain fossil invertebrates, such as mollusks, provides evidence that the ancient marine environments were shallow, and fish fossils suggest potential food for growing sharks. The absence of mammal fossils such as seals or sea cows in these areas is also an important clue, according to more than one paper, as some of today’s sharks only consume mammals once they reach adulthood.

 

Protection from ancient predators in these environments seems incredibly important, particularly if neonates and juveniles lived in areas with other ancient sharks. Such was undoubtedly the case for megalodon in Panama, where over 400 teeth from at least 16 different species of fossil shark were also uncovered. Young megalodons were not tiny. Neonates in this megalodon paleo-nursery were estimated to be as long as 2 meters (approximately 6 feet); juveniles—whose fossil teeth were more plentiful in this assemblage—were estimated at 2 to 10.5 meters (about 6 to 34 feet).

 

Co-existing predators, however, were likely capable of handling babies that size. Among the ancient predators found were the great hammerhead shark (Sphyrna mokarran) and an extinct form of weasel shark (Hemipristis serra).

Competing with amateurs

The team working in South Carolina faced a non-scientific challenge when identifying paleo-nurseries: trophy-hunting collectors. These collectors impact the information paleontologists can obtain about a fossil shark tooth assemblage.

 

“Shark tooth hunting is a big hobby here,” Gibson wrote about South Carolina, “and I have personally visited collectors that have shark teeth prominently displayed throughout their house. Collectors, in my experience, are more likely to donate the ‘small’ or broken specimens than ‘the big ones.’ However, this does create a bias so they can’t really be used for this kind of study.”

 

“It's a real problem whenever collectors find fossils that are scientifically significant and also have perceived financial or prestige value,” Dr. Boessenecker agreed in a separate email. “Collectors love their shark teeth, and it can be very hard to convince some collectors of the benefit of relinquishing fossils. Sometimes it's not even worth trying—but you never know until you ask.”

When collectors take bigger teeth out of a site, leaving only smaller teeth in place, this changes the true fossil representation of that location. So if both scientists and collectors are working on a site, the collectors, according to Dr. Boessenecker, “are almost always going to score the big teeth before we ever get a chance to make it out there—they outnumber us about 1,000 to 1 in the Charleston area. In the case of the first sample from the 1970s,” he wrote, referencing one of the assemblages he and the team studied, “the teeth were all collected during a controlled excavation. In our case, private collectors were not allowed by the landowner, and we had exclusive access to the site.”

Why nurseries?

All these findings suggest that sharks have been congregating in nurseries for many millions of years. And researchers argue they have good reasons for doing so.

 

Dr. Catalina Pimiento is a paleontologist at the Paleontology Institute and Museum of the University of Zurich in Switzerland and Swansea University in Wales, UK. She was lead author on the paper announcing the first megalodon paleo-nursery in Panama and co-author of the paper on the first ancient great white shark paleo-nursery in Chile. Since 2010, she has done further work on ancient sharks, including estimating the actual size of megalodon, which may have been as large as 15–18 meters (approximately 48–59 feet). “There is a misconception about giant sharks,” she wrote, addressing a notion that arises every Shark Week, that they are "able to ‘hide’ in parts of the ocean, where they can live unnoticeable.” They do not.

 

Looking back on work since 2010, she wrote, “I think that we now better understand how sharks have used nursery areas as a strategy to protect their young over millions of years; showing how ancient this behavior is evidence how effective and widespread it was.”

 

But how and when nurseries were used involves a balancing act. Dr. Sora Kim, paleoecologist and assistant professor at the University of California, Merced, has done a lot of research on ancient sharks and is part of a team that just released a paper on how shark size might be impacted by both ecological and environmental pressures.

 

In it, the team argues that there is a specific size that juvenile sharks need to become to safely live and survive as adults. If juvenile sharks move from a nursery to an adult site and are smaller than this ideal size, it impacts their survival.

 

Dr. Kim explained by email that the “results of this study support the idea of thresholds related to movement patterns in and out of juvenile nurseries. There are costs and benefits being in the nursery and leaving, so it is a delicate balance. I think this balance can also shift based on environmental conditions, resource availability, and competition/predation... both in and out of the nursery site!”

 

In other words, the insight we are beginning to pull from the fossil record—how certain shark species survived for millions and millions of years—offers a warning to us today. Shark conservation, and conserving the environments in which they develop and grow, is important.

 

Dr. Pimiento is also “very interested in the factors that determine [ancient shark] extinction susceptibility and the extent to which we can apply that to modern systems.”

 

“I hope future research reveals the ecological consequences of the extinction of megalodon,” she added. “As an apex predator with a wide distribution, you would expect for its extinction to have caused many changes in the trophic webs. We haven’t quantified [these] changes yet.”

 

We do, however, know how today’s sharks impact their ecosystems. And we are seeing a worrying decline in shark populations overall.

 

“I feel like we’re almost coming into this new era of shark paleontology where we can start having more insight into their actual ecology based on these quantitative methods to give them context,” Dr. Kim said in a video interview.

 

“It does seem that there are paleo-nurseries in the fossil record. And I think the longevity of nurseries in the evolutionary history of sharks indicates that they’re very important,” she said. “And it should be something that we conserve today, because nurseries are usually near shore, and that’s where humans have the heaviest impact.”