Megalodon babies ate their shark siblings in the womb, leading them to be the size of adult humans at birth

• Megalodons were more than 6.5 feet long at birth. As adults, the huge, prehistoric sharks reached lengths of 50 feet and had heads the size of cars.
• That's far bigger than any other meat-eating shark, living or extinct.
• A new study suggests the sharks grew to such large sizes because megalodon babies ate each other in the womb.

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

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

According to a new study, baby megalodons set themselves up to get super-sized before they were born.

Meg eggs hatched inside the womb, where the babies grew bigger and hungrier before their mothers ever gave birth. But not all embryos survived the gestation process: Some were eaten by their womb-mates.

"'Early-hatched' embryos will begin to eat surrounding unhatched eggs," Kenshu Shimada, the 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."

Megalodon pups were larger than most adult humans at birth, Shimada's team found, reaching lengths of more than 6 feet. That gave them an advantage - a fin up, perhaps - against hungry predators that preyed on newborn sharks.

Hatching first has its advantages

Megalodons went extinct more than 3 million years ago. All we have left are their 6-inch, serrated teeth and a smattering of bones. That paucity of fossils makes it challenging for scientists to figure out how how large these creatures got and what enabled them to reach such enormous sizes.

So researchers make educated guesses about megalodons based on their modern counterparts.

In an earlier analysis, Shimada's group looked at how megalodons' modern relatives, called lamniforms, live today. This group includes great whites, makos, and sand tigers, which have the same diet and body type as megalodons did.

This comparison is how the researchers determined that megalodon babies likely ate each other. Current-day female lamniform eggs still hatch inside the mother's body, then the pups develop in her womb before being born. The embryos that hatch early sometimes eat their unhatched brothers and sisters - a process called intrauterine cannibalism.

"I believe it really is a matter of which hatches first, but more work is needed to ascertain this," Shimada said.

Sandtiger shark pups sometimes eat their litter mates after they're born, too.

For megalodon shark moms, having cannibalistic progeny in the womb would have been tough, since the sharks had to expend a lot of energy to nourish pups so large.

"But the cost must have been offset by the benefits of newborns having an advantage of being 'already-large' predators with a reduced risk of being eaten by other predators," Shimada said.

Megs gave birth to enormous newborn sharks

To determine how big meg pups were when they were born, Shimada's group analyzed 6-inch-wide megalodon vertebrae from the Royal Belgian Institute of Natural Sciences.

Scientists can use vertebrae and teeth to estimate a creature's body size. While megalodons' body proportions are unknown, Shimada's group used the vertebrae-to-body size ratio of another lamniform, a great white shark, to estimate the megalodon's size.

Based on that logic, the researchers determined that the fossils likely came from a shark that was about 30 feet (9 meters) long.

A CT scan also showed that the vertebrae had 46 growth bands - each of which represents a year of a meg's life, like rings in a tree trunk. Shimada's team pinpointed the smallest of those bands, what he calls "a birth ring," and calculated what size the shark would have been when its vertebrae were that size. That's how the researchers concluded this megalodon was 6.6 feet (2 meters) long at birth.

That makes it the largest live-born baby shark in history, Shimada said.

Currently, the world's largest live-born babies come from megamouth and basking sharks, both of which are lamniforms. Those baby sharks reach lengths of 5.6 feet.