What caused this massive megalodon’s mega-toothache?

The enormous, extinct shark megalodon dominated the seas millions of years ago, but even this fearsome ocean carnivore wasn't immune to tooth trouble. 

Recent analysis of a megalodon (Otodus megalodon) tooth with a rare abnormality — a groove all the way down the middle — hints that the dental deformity may trace its roots to an injury to the giant shark's jaw, perhaps caused by prey that fought back. In an illustration showing one possible encounter, a fish pierces the pursuing predator's jaw with its sharp bill, which may have set the stage for the megalodon growing a split tooth. 

Another possibility is that the big shark was impaled by a spine from a stingray, scientists wrote in a new study.

In humans and other mammals, genetic factors, disease or injury can sometimes affect tooth buds and cause a dental abnormality known as "double tooth pathology," in which a single tooth grows with a split running lengthwise down the center, the study authors wrote. Such teeth can represent two tooth buds that combined to form one tooth, a process known as fusion, or a single tooth bud that divided, known as gemination. However, little is known about this pathology in sharks. 

For the study, researchers analyzed a 4-inch (10-centimeters) long split megalodon tooth alongside split fossil teeth from other sharks, to determine what may have caused the deformities. The scientists concluded that a traumatic injury was the most likely cause of the prehistoric split teeth, and the encounter that damaged the megalodon tooth may have affected how the shark hunted and fed.

Related: Biggest sharks in the world

Megalodon was one of the biggest predators that ever lived, measuring at least 50 feet (15 meters) long and as much as 65 feet (20 m) long by some estimates, Live Science previously reported. To put that into perspective, modern great white sharks (Carcharodon carcharias) typically grow to be no more than 20 feet (6 m) long. 

Though the megalodon thriller "The Meg" (Warner Bros Pictures, 2018) proposed that solitary megalodons could still lurk in the ocean depths, most megalodon fossils date to around 15 million years ago, and all evidence of the giant sharks vanished from the fossil record after they went extinct, about 2.6 million years ago.

Sharks have skeletons made of cartilage, which isn't as robust as bone and typically doesn't fossilize well, so most of the megalodon fossils that survive to the present are teeth. Like modern sharks, megalodon was constantly losing teeth and growing new ones, with a constantly regenerating supply stored in its jaws. Thanks to this so-called tooth conveyer belt, some sharks can lose and replace tens of thousands of teeth over the course of their lifetimes, study co-author Haviv Avrahami, a doctoral candidate in the Department of Biological Sciences at North Carolina State University in Raleigh, told Live Science in an email.

"That'd be like us losing ~20 baby teeth every month," Avrahami said.

To find out what might be at the root of the split megalodon tooth, the researchers pored over hundreds of fossil shark teeth in the collection of the North Carolina Museum of Natural Sciences in Raleigh. They found only two more examples of the unusual split-tooth deformity: two teeth from the bull shark Carcharhinus leucas, a species that lived alongside megalodon millions of years ago (but was much smaller, reaching a maximum body length of around 12 feet, or 3.7 m) and is still around today. 

They measured the deformed teeth alongside normal teeth from the two shark species, then conducted computed X-ray tomography, or CT scans, to map the pulp cavities in the bull shark teeth and the vascular networks inside the megalodon tooth and see how they may have developed. 

Their analysis suggested that gemination was likelier than fusion to have produced the double teeth, based on where the pulp cavities and networks split "and the lack of major root abnormalities" in all the double teeth, according to the study. The split teeth were also very symmetrical, which would not be the case if they formed from the fusion of two tooth buds in different developmental stages, the scientists reported.

The root of the matter

As for what may have caused the pathology, traumatic damage to the tooth bud from a stab wound is a more probable cause than disease or infection, which sharks typically shrug off, Avrahami explained. "Sharks are weird in that they seem to be particularly resistant to getting sick with infections," he said. For that reason, "other tooth deformities in sharks are thought to be more likely due to trauma," he added. Modern bull sharks are known to feed on spiky prey that can cause such injuries, including sawfish, rays and sea urchins, and while megalodon is thought to have preyed mostly on marine mammals — and perhaps fish and turtles — its diet may have been more diverse than expected, including sea life bearing pointy defensive weapons. 

With about 300 teeth in its mouth, the megalodon likely wasn't too troubled by a single split tooth. But if its tooth bud had been injured by a barb or spine that then lodged in the big shark's jaw, "it probably would have caused the animal a great deal of pain, possibly making it more difficult for it to hunt," Avrahami noted.

The study's closer look at megalodon teeth not only offers new insights into dental deformities in sharks; it also raises questions about how common double-tooth pathologies may have been in other animal lineages that also had continuous tooth replacement, such as dinosaurs (including toothed birds) and crocodilians, Avrahami said.

"I've seen a lot of hadrosaur teeth in my life, which are dinosaurs that also have extensive tooth battery conveyor belts, and not a single one had a double tooth look to it. Why??" he asked. "So, I really hope future researchers explore this more."

The findings were published May 11 in the journal PeerJ.

Originally published on Live Science.