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

An illustration of a megalodon. Shutterstock
Megalodons - giant prehistoric sharks - reached 50 feet in length with heads the size of cars.
The predators are very large compared to other living and extinct sharks. But how megalodons reached this massive size is a mystery.
A new study suggests that the size of the sharks could be partly explained by the fact that they ate each other while in the womb.
Scientists aren't sure how megalodons hunted, but a decline in prey and an increase in competitors could have caused their extinction.
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The head of a megalodon alone was the size of a car.
The prehistoric monster was the largest carnivorous shark to ever roam the oceans: it reached 50 feet in length with dorsal fins protruding 5 feet out of the water.
The old predator has seen a cultural and scientific spotlight in recent years after nearly a dozen new studies and a 2018 Hollywood blockbuster that revived interest in the "Meg".
The creature is difficult to research because all we have of the "Meg" is her 6 inch long, serrated teeth. This makes it difficult for scientists to figure out how big these creatures actually got and why they could reach such enormous sizes.
However, new studies examining Meg's offspring are helping us understand the size of the prehistoric shark in new ways.
Analysis released Monday suggests that baby megalodons may have set up to become oversized even before they were born.
Meg eggs hatch in the womb, where they grew bigger and hungrier before their mothers give birth alive. But not all embryos survive the pregnancy process: some are eaten by their uterine mates.
"Early hatched embryos will begin to eat unhatched eggs," Kenshu Shimada, lead author of the study and professor of paleobiology at DePaul University, told Business Insider. "The result is that few puppies survive and develop, but each of these puppies can grow to be considerably large at birth."
Elusive fossil evidence makes it difficult to determine the size of the Meg.
The paleobiologist Kenshu Shimada is holding a tooth of an extinct shark Otodus megalodon or the so-called "Meg". Courtesy of Kenshu Shimada / DePaul University / Jeff Carrion
Shimada's group first wanted to find out how big megalodons could get.
The researchers examined how Meg's modern relatives, so-called lamniforms, live today. These sharks - which include whites, macos, and sand tigers - share the same diet and body type as Meg. So Shimada measured the proportions between the teeth of these sharks and the proportions of their bodies, and then applied that comparison to fossilized megalodon teeth.
That revealed that the maximum length a Meg can reach is around 15 meters - more than twice as long as a great white, the largest shark still alive today.
"This does not mean that megalodon individuals taller than 15 meters did not exist, but their existence has not been proven by scientific specimens in museum collections," said Shimada.
In fact, in a September study, a group of British researchers made similar tooth-to-body comparisons and found that megalodons may have been 52 feet long. This investigation also showed that a 16-meter-long megalodon had a head and tail that was 4 meters long and a dorsal fin that was the size of an average woman.
Comparison of a reconstruction of the dorsal fin of an adult megalodon with a 5 foot diver. Oliver Oliver E. Demuth
"It was difficult to pinpoint these dimensions because we only have tooth bits left," Catalina Pimiento, author of this September study, told Business Insider.
There are scattered vertebrae, but scientists have yet to find any other bones, as the sharks' soft, cartilaginous skeletons rarely survive fossilization.
"We may never find a full skeleton," added Mike Benton, Pimiento's co-author.
Cannibal megalodons
Given the scarcity of fossils, scientists are forced to make educated guesses about what influenced the life and diet of megalodons with their modern counterparts.
In theory, there are two ways a shark can get enough food to become as gigantic as the Meg: either they become filter-eaters and eat copious amounts of 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 by Pimiento's group suggested that megalodons followed the latter path: they were mesothermal, which meant they could regulate the temperature of their organs. This allowed the sharks to live in colder waters and swim faster - and thus catch more prey.
Mesothermia is "a key factor in the development of its gigantic size," said Pimiento.
What made these sharks go mesothermal? Shimada believes the key to answering this question has to do with the way lamniforms give birth.
Jaws of Carcharocles megalodon, an extinct species of shark that lived 23 to 3.6 million years ago. Florida Museum photo by Kristen Grace
A female lamniform's eggs hatch in her body, then the pups develop in their womb before they are born. The babies that hatch early have the advantage of being able to eat their unhatched brothers and sisters - a process known as "intrauterine cannibalism".
Sometimes, as with sand tiger sharks, hatched pups will eat their littermates even after they are born.
Given that lamniforms are related to megalodons, Shimada said the same rules likely apply to the prehistoric predators as well.
In the new study, his group suggested that such cannibalism could have fueled megalodon's mesothermia: the task of feeding a small number of gigantic offspring - who feed well on their sibling's blood - in the womb could prompt meg-mothers have to eat more. And to eat more, they either had to switch to a plankton diet or increase their internal temperatures to better hunt.
"His hunting style was probably a single strike tactic."
The prehistoric shark's teeth can tell us more about how Megs preferred to hunt.
A study in March 2019 suggested that the sharks like to bite hard and then step back. Adult megalodons had a bite force of 11 tons.
The blade-like, serrated teeth of a megalodon were ideal for hunting down meaty marine mammals such as whales and dolphins. Florida Museum photo by Kristen Grace
"Megalodon's teeth suggest that his hunting style was likely a single strike tactic aimed at immobilizing and bleeding his prey," said Victor Perez, lead author of the study, in a press release. He added, "A shark does not want to hold onto a whale because it will beat up the shark and possibly injure it."
Researchers have discovered megalodon bite marks on marine mammal bones, but whether these 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 to become extinct
The biggest megalodon puzzle, however, has little to do with its size. Paleobiologists like Shimada and Pimiento are still unsure how the shark became extinct.
Megalodons ruled the oceans for 20 million years before disappearing from the fossil record 3.6 to 2.6 million years ago. Theories about their sudden extinction determine the range.
A 2018 paper found that the fallout of a supernova 150 light years away dosed megalodons in deadly radiation. Another study suggested that the newly developed great white shark outperformed them.
A great white shark on Guadalupe Island, Mexico. Gerald Schömbs / Unsplash
Pimiento said the megalodon extinction coincided 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 fell.
When the Meg fought for less loot with more competitors in a shrinking ocean habitat, her fate was sealed.
Some people think the Meg is still roaming the oceans, but scientists disagree
In the 2018 film "The Meg", Jason Statham battles a megalodon that, according to history, appears in modern times after swimming from the depths of the Mariana Trench in the Pacific. The film is based on the 1997 book "Meg: A novel of deep terror".
Benton said that while the idea that a Meg might suddenly appear in today's oceans isn't believable, the dimensions of the on-screen shark are consistent with the results of his most recent study.
"I think the book and film are about the right size and much of the behavior was fine as it is based on the modern great white shark," he said.
A shot of the megalodon in the 2018 movie "The Meg". Warner Bros.
Steve Alten, the writer of Meg, told Business Insider his fans kept asking him if he believed the creatures were still alive.
"My answer is always the same," he said. "Because so many oceans remain unexplored, there is a chance they are out there in deep water."
Benton agreed that it was "perfectly reasonable" to ask the question.
"Oceanographers pull all kinds of amazing things out of the deep oceans. Think of the coelacanth, long thought to be extinct but first found alive in 1938," he said.
But he and Pimiento both said that if a megalodon was still out there, researchers would likely have found it by now.
"Most sharks live in shallow coastal waters - not in the deep ocean like the coelacanth," Benton said. "It was a big animal so I would be amazed if we missed it."
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