Mega-Jaws — Donald Prothero on the Rise and Fall of the Giant Shark
As the title The Story of Life in 25 Fossils suggests, each chapter of Donald R. Prothero’s book focuses on how fossils reveal something about the history of our planet. Surely, one of the most fascinating moments in this history is when Carcharocles megalodon or “Mega-Jaws” ruled the seas. In the following excerpt from the book, Prothero explains what the fossil record reveals about the fate of these giant sharks:
Monster of the Seas
The sheer size of Carcharocles megalodon raises a question: Why did it grow so big? The most common answer seems to be that sharks were responding to the great abundance of large prey in the Miocene, especially the huge radiation of many types of whales and dolphins that developed in the early and middle Miocene. C. megalodon was bigger than all but the largest whales known from the same beds, so it was a true “super-predator,” capable of killing and eating almost anything that swam in the Miocene oceans.
There is abundant fossil evidence of this behavior. Deep gouges and scratches that could have been produced by only the huge teeth of C. megalodon have been found on many fossil whale bones, suggesting that the sharks scratched the bones as they tore flesh from the carcasses. The list of whales with traces of C. megalodon attacks is very long, including dolphins and other small whales, cetotheres, squalodontids, sperm whales, bowhead whales, and rorquals like the fin whale and blue whale, plus seals, sea lions, manatees, and sea turtles (which were three times the size of the largest extant sea turtles). A C. megalodon tooth was found associated with the bitten ear bone of a sea lion. There were also several finds of C. megalodon teeth embedded in whale backbones, and numerous cases partially scavenged whale carcasses (especially at Sharktooth Hill) have been found surrounded by shed C. megalodon teeth.
Of course, this does not exhaust the list. Most sharks (especially great whites) are indiscriminate, opportunistic feeders and attack anything that moves that they can catch. This is why so many modern sharks have ocean trash (including road signs, boots, and anchors) in their stomachs when they are cut open. So C. megalodon certainly would have eaten smaller fish and most other sharks when it could catch them. But its large size is primarily an adaptation to attacking large prey like whales, which no other marine predator could threaten until C. megalodon came along.
The bite marks on one particular whale specimen about 9 meters (30 feet) long suggests how C. megalodon preferred to attack. The marks seem to focus on the tough bony areas (shoulders, flippers, rib cage, upper spine) rather than on the soft underbelly, which modern great whites target. This suggests that C. megalodon tried to crush or puncture the heart or lungs of the whale, which would have killed it quickly. This, in turn, explains why the teeth of C. megalodon are so thick and robust: they were adapted for biting through bone. Another common strategy focused on the flippers, since fossils of the hand bones have the highest frequency of bite marks of all. A big bite to crush, cripple, or rip off one flipper would have been sufficient to disable the prey and allow the shark to finish it off with several more bites.
The predatory behavior of these mega-sharks gives us additional clues as to why they slowly vanished over the late Miocene to Pliocene. Even though they were at the top of the food chain in the middle Miocene, by the early Pliocene there were even bigger whales that they could not attack and more large predatory whales, such as squalodontids and sperm whales. The late Miocene sperm whale Livyatan melvillei was truly gigantic (18 meters [60 feet] long), the largest mammalian predator ever to swim the oceans (the genus name is a homonym of “Leviathan,” and the species name honors Herman Melville, the author of Moby-Dick). This monster could have eaten C. megalodon if it wanted to.
Then as the global oceans got colder during the Pliocene (especially after the Arctic ice cap formed about 4 to 3 million years ago), C. megalodon teeth seem to get scarcer and scarcer. When they last appear, in rocks of the late Pliocene, they are extremely rare, suggesting that a combination of the competition from very large predatory whales and the increasingly colder oceans was too much for them. Whatever the cause, they are truly extinct.