We just can’t let Otodus megalodon rest. From Peter Benchley’s JAWS to the dreck that regularly bobs up to the surface of basic cable TV, we can’t seem to resist invoking the specter of a shark so large that it could easily engulf a person without a drop of blood spilled into the sea.
Despite our fascination with this enormous, extinct relative of today’s great white shark, though, there’s still a great deal we don’t know about the life and death of the biggest shark that ever lived. For starters, we still don’t know why the last of the megatooths died over 2.5 million years ago.
In the entire history of cartilaginous fish, Otodus megalodon was a huge success story. And that’s not just because of the predator’s size and inferred ferocity. This species patrolled the coasts of the Atlantic, Pacific, and Indian Oceans for about 20 million years. Few creatures can claim such a record. And that only makes the disappearance of the shark all the more puzzling.
Changes brought on by a cooling climate have been the focus of the traditional explanation for the monstrous shark’s demise. O. megalodon has often been thought of as a warm-water hunter, and so, the argument goes, as sea temperatures dipped at the end of the Pliocene the whales, seals, and other fatty mammals the shark relied upon migrated to chilled seas where the shark couldn’t follow. The pitiful selachian was simply left behind as cetaceans spouted off for the poles.
But was the great shark so restricted by temperature? To find out, paleontologist Catalina Pimiento and colleagues drew from the Paleobiology Database to analyze occurrences of O. megalodon over time in relation to climate. Contrary to what had previously been thought, temperature probably didn’t freeze the shark into extinction.
The big picture looks something like this. During the shark’s early years, around 20 million years ago, O. megalodon primarily swam through waters of the northern hemisphere. Populations expanded around 15 million years ago to include every major ocean basin on the planet, the researchers write, but from there the sharks populations steadily declined.
All of this happened irrespective of climate. During times of major temperature spikes and dips, Pimiento and coauthors note, O. megalodon occurrences didn’t seem to show any direct response. Not to mention that the shark seemed fully capable of coping with a range of temperatures from 53 to 80 degrees Fahrenheit, and there have been waters in this range from the shark’s time until today. As Pimiento and coauthors write, “[O.] megalodon would have not been affected significantly by the temperature changes during the Pleistocene, Holocene and Recent.”
So if it wasn’t cooler waters, what drove the shark to extinction? There’s still no definitive answer. Even today, when we can witness species disappear, it’s often difficult to precisely retrace the road from the vanishing point back to the first signs of trouble. In the case of O. megalodon, though, Pimiento and coauthors have some ideas about possible killswitches.
Through hindsight, we can see that the road to extinction for the megatooth shark started in the middle of the Miocene. This coincided with two major events, as previously pointed out by paleontologist Dana Ehret as well as the authors of the new study. Against a background of crashing whale diversity during this time, the world saw the evolution of some stiff competition for O. megalodon: large sharks close to the ancestry of the great white as well as sperm whales that behaved like today’s orcas. This trend continued only through the Pliocene, with fewer big baleen whales and an increasing array of predators that young megatooth sharks would have struggled against to get enough food down their throats. There was less protein to go around for an expanding guild of predators who relied upon warm, blubbery prey.
The case isn’t closed yet, though. So much of what’s known about O. megalodon comes from teeth, the occasional vertebra, and some bite marks. Those pieces only reach so far in revealing the massive shark’s biology, including how much the fish actually relied on filter-feeding whales for food or the other predators it was striving against to survive.
We can be sure the megatooth shark is dead. The fish’s fossil record taps out by 2.5 million years ago, and we surely wouldn’t miss populations of fifty-foot-long sharks patrolling the global coastlines. But why the shark vanished is a secret still waiting to be dredged from the fossil record.
Pimiento, C., MacFadden, B., Clements, C., Varela, S., Jaramillo, C., Velez-Juarbe, J., Silliman, B. 2016. Geographical distribution patterns of Carcharocles megalodon over time reveal clues about extinction mechanisms. Journal of Biogeography. doi: 10.1111/jbi.12754
[This post was originally published at National Geographic.]