Off the prehistoric coast of Panama, a mega-toothed shark nursery

My early elementary reading school choices often got me into trouble. Every week I would pass over the recommended, grade-appropriate sections for the few shelves containing the books about dinosaurs, sharks, and alligators - if it was big and hard sharp teeth, I wanted to learn about it. The school librarian was not too pleased with this, even calling my parents in on one occasion to insist that I read something fit for younger children, but I just could not get enough of theropods, crocodylians, and enormous sharks.

Given my love for "cold-blooded killers" (as so many titles described them) it was not very long before I learned about "Megalodon" (formally known as Carcharocles megalodon), an immense shark which disappeared just a million-and-a-half years before human bathers began to wade into the shallows. Perhaps, some books hinted, the giant sharks still lurked in some unknown ocean recess, and a photograph of an array of American Museum of Natural History scientists inside the restored jaws of the shark drove home the point that it could have made of meal of just about anything it wanted. It was one of the most fascinating and terrifying images I had ever seen.

But the AMNH jaws were a bit out of proportion. As directed by Bashford Dean, the famous jaws were reconstructed by assuming that the teeth of the extinct shark would have had the same proportions to the jaw as in the living great white shark (Carcharodon carcharias), yielding a maw that would have fit a shark 100 feet long or more. As scientists learned more about the giant fish, though, they realized that this method had overinflated the size of the shark, with present estimates placing it at the more modest (but still gargantuan) 60 feet +. Despite the large size of C. megalodon, however, the young of this shark would still have been vulnerable to other seagoing predators of their time, and according to a new study published in PLoS One the "mega-toothed shark" may have protected its young by delivering them in nurseries.

Large sharks, such as the great white shark, are apex predators as adults, but they start off small and relatively vulnerable. It is for this reason that some species deposit their young in shallow habitats full of fish and other pint-sized fare, and it appears that C. megalodon did the same. Over the years paleontologists have identified several sites relatively rich in juvenile C. megalodon teeth, including the 10 million year old (Miocene) Gatun Formation of Panama. The marine fossil site preserves a shallow habitat containing numerous C. megalodon teeth and a relative paucity of whale bones (prey for adult sharks that their young probably would not have fed on), and for the first time a team of paleontologists have tested the idea that his place was once a haven for the young predators.

In order to make the case that the small teeth truly represented juvenile sharks paleontologists Catalina Pimiento, Dana Ehret, Bruce MacFadden, and Gordon Hubbell had to rule out several alternate hypotheses. It might be argued, for example, that individuals of C. megalodon evolved increasing body size over time, meaning that the teeth from the Gatun Formation belonged to small adults rather than juveniles. When the teeth from Panama (both from adults and juveniles) were compared to C. megalodon teeth from the older Calvert Formation of Maryland (14 million years old) and the young Bone Valley Formation of Florida (5 million years old), however, they did not find any sign that C. megalodon was becoming larger over time.

Another possibility was that, because tooth size and shape varies within the jaws of individual C. megalodon, the teeth were just the smaller teeth of large animals. To test this the authors compared the teeth from the Gatun Formation to associated tooth sets of adult and juvenile C. megalodon to make sure they were properly interpreting the position of the teeth and the life stage they represented. What they found was that the teeth truly did come from small individuals and fell within the range expected for juvenile sharks.

To further check their hypothesis, the authors used a previously-published model based upon growth in great white sharks to estimate the length of the Gatun Formation C. megalodon individuals. The results showed that most individual sharks (21) could be categorized as neonates or juveniles as they were under 35 feet in length (with the smallest being about seven feet long), while there were only approximately seven individuals which would have exceeded that length and been categorized as adults. So long as this sample is a true signal of the kind of animals which lived at the site, it would seem that juveniles significantly outnumbers adults, just as would be expected in a nursery.

That a nursery would have provided some protection for the young sharks is inferred from the presence of other teeth found in the Gatun Formation. A seven foot long newborn C. megalodon is still a pretty big shark, but the hammerhead sharks and extinct snaggletooth sharks which lived in the area were even bigger, growing to 20 feet or more. Young C. megalodon would have run the risk of becoming prey for adult members of the species, as well, so despite the fearsome reputation of the species, these sharks were quite vulnerable as newborns. The use of nurseries allowed the young sharks to begin their lives in relative safety until they were large enough to make it on their own, and now that such a site has been identified scientists can use similar criteria to pinpoint other nurseries at fossil sites elsewhere.

Pimiento, C., Ehret, D., MacFadden, B., & Hubbell, G. (2010). Ancient Nursery Area for the Extinct Giant Shark Megalodon from the Miocene of Panama PLoS ONE, 5 (5) DOI: 10.1371/journal.pone.0010552