Megalodons may have grown to the size of school buses by eating their shark siblings in the womb, new research suggests

• Megalodons — huge, prehistoric sharks — reached lengths of 50 feet, with heads the size of cars.
• The predators are super-sized compared to other sharks, both living and extinct. But how megalodons achieved that massive size is a mystery.
• A new study suggests the sharks' size could be explained in part by the fact that they ate each other in the womb.
• Scientists aren't sure how megalodons hunted, but a decline in prey and an increase in competitors may have caused their extinction.

A megalodon's head alone was the size of a car.

The prehistoric monster was largest meat-eating shark to ever roam the oceans: It reached lengths of 50 feet, with dorsal fins that jutted 5 feet out of the water.

The ancient predator has been thrown into a cultural and scientific spotlight in the last few years after nearly a dozen new studies and a 2018 Hollywood blockbuster revitalized interest in the "Meg."

Researching the creature is difficult, since all we have left of the "Meg" are its 6-inch long, serrated teeth. That makes it challenging for scientists to figure out how how large these creatures actually got and why they were able to reach such enormous sizes.

But new studies that examine Megs' descendants are helping us understand the prehistoric shark's size in new ways.

One analysis, published on Monday, suggests that baby megalodons may have set themselves up to get super-sized before they're even born.

Meg eggs hatch inside the womb, where they grew ever bigger and hungrier before their mothers give live birth. But not all embryos survive the gestation process: Some are eaten by their womb-mates.

"'Early-hatched' embryos will begin to eat surrounding unhatched eggs," Kenshu Shimada, lead author of the study and a professor of paleobiology at DePaul University, told Business Insider. "The consequence is that only a few pups will survive and develop, but each of those pups can become considerably large in body size at birth."

Elusive fossil evidence makes the Meg's size 'hard to pin down'

Shimada's group first wanted to pin down how big megalodons could get.

So the researchers looked at how Meg's modern relatives, called lamniforms, live today. These sharks — which include great whites, makos, and sand tigers — have the same diet and body type as Meg did. So Shimada measured the size ratio between those sharks' teeth and body proportions, then applied that comparison to fossilized megalodon teeth.

That revealed that the maximum length a Meg could get is about 50 feet (15 meters) — more than double the length of a great white, the biggest carnivorous shark alive today.

"This does not mean that Megalodon individuals larger than 15 meters did not exist, but their existence has not been substantiated based on scientific specimens in museum collections," Shimada said.

Indeed, a group of UK researchers did similar tooth-body comparisons in a September study, and found that megalodons could have been 52 feet long. That research also showed that a 52-foot (16-meter) megalodon had a head and tail 13 feet (4 meters) long, and a dorsal fin equal to the height of an average woman.

"It has been hard to pin down these dimensions because we only have teeth remains," Catalina Pimiento, an author of that September study, told Business Insider.

Scattered vertebrae exist, but scientists have yet to find any other bones, since sharks' soft, cartilaginous skeletons rarely survive fossilization.

"It may be we will never find a complete skeleton," added Mike Benton, Pimiento's co-author.

Cannibal megalodons

Given the paucity of fossils, scientists are forced to make educated guesses about what influenced megalodons' lives and diets using their modern counterparts.

There are, in theory, two ways a shark can get enough food to become as gigantic as the Meg: either they become filter-feeders and munch on abundant plankton, like whale sharks do, or they gain the ability to control their own body temperatures, which improves their hunting skills.

A January 2019 study from Pimiento's group suggested that megalodons followed the latter route: They were mesothermic, meaning they could regulate the temperature of their organs. This enabled the sharks to live in colder waters and swim faster — and therefore catch more prey.

Mesothermy was "a key driver in the evolution of its gigantic size," Pimiento said.

What drove these sharks to become mesothermic? Shimada thinks the key to answering that question has to do with the way lamniforms give birth.

A female lamniforms' eggs hatch inside her body, then the pups develop in her womb before being born. The babies that hatch early have the benefit of getting to eat their unhatched brothers and sisters — a process called "intrauterine cannibalism."

Sometimes, like in sandtiger sharks, hatched pups will eat their litter mates after they're born, too.

Given that lamniforms are related to megalodons, Shimada said the same rules likely applied to the prehistoric predators.

In the new study, his group suggested that such cannibalism could've kickstarted megalodons' mesothermy: The task of nurturing a small number of gigantic offspring — well fed on their siblings' blood — in their wombs may have prompted Meg moms to eat more. And in order to eat more, they needed to either switch to a plankton diet, or crank up their internal temperatures to hunt better.

'Its hunting style was likely a single-strike tactic'

The prehistoric shark's teeth can tell us more about how Megs preferred to hunt.

One study suggested in March 2019 that the sharks liked to bite hard then step back. Adult megalodons packed a bite force of 11 tons.

"Megalodon's teeth suggest its hunting style was likely a single-strike tactic, designed to immobilize its prey and allow it to bleed out," Victor Perez, the lead author of that study, said in a release. He added: "A shark wouldn't want to grab and hold onto a whale because it's going to thrash about and possibly injure the shark in the process."

Researchers have discovered megalodon bite marks on marine mammal bones, but whether those remarks represent predatory attacks or scavenging activities is still unknown, according to Shimada.

Dwindling prey and competition with other sharks may have caused the Meg's extinction

The biggest megalodon mystery has little to do with its size, however. Paleobiologists like Shimada and Pimiento still aren't sure how the shark went extinct.

Megalodons dominated the oceans for 20 million years before disappearing from the fossil record between 3.6 and 2.6 million years ago. Theories about their abrupt extinction run the gamut.

A 2018 paper posited that fallout from a supernova 150 light-years away dosed megalodons in deadly radiation. Another study suggested the newly evolved great white shark out-competed them.

Pimiento said megalodons' extinction did indeed coincide with the appearance of great whites and orca whales, but at the same time, there was a decline in marine mammals — their preferred prey — as global sea levels dropped.

As the Meg jockeyed with more competitors for less prey in a shrinking ocean habitat, its fate was sealed.

Some people think the Meg still roams the oceans, but scientists disagree

In the 2018 film "The Meg," Jason Statham battles a megalodon that, according to the story, appears in modern times after swimming out of the depths of the Marianas Trench in the Pacific Ocean. The movie was based on the 1997 book, "Meg: A Novel of Deep Terror."

Benton said that, although the idea that a Meg could suddenly appear in today's oceans isn't believable, the on-screen shark's dimensions match his recent study's findings.

"I guess the book and movie got the size about right, and much of the behaviour was OK being based on the modern great white shark," he said.

Steve Alten, the author of "Meg," told Business Insider his fans constantly ask him whether he believes the creatures are still alive.

"My response is always the same," he said. "Because so much of the oceans remain unexplored, the possibility remains they could be out there in deep water."

Benton agreed it's "perfectly reasonable" for people to ask the question.

"Oceanographers drag all sorts of amazing things out of the deep oceans. Think about the coelacanth, long thought to be extinct, but found alive for the first time in 1938," he said.

But he and Pimiento both said that if a megalodon was out still out there, researchers probably would have found it already.

"Most sharks live in shallow, coastal waters — not deep ocean, like the coelacanth," Benton said. "It was a big beast, so I'd be amazed if we had missed it."