Megalodon, a prehistoric shark that lived between 3.6 and 15 million years ago, is believed to be the largest shark — indeed, the largest fish of any sort — ever to swim the seas. Reaching up to 50 feet long, megalodons were more than twice the length of their modern relative, the great white shark.

A new study suggests that even a baby megalodon (born “live” like present-day sharks) was outsized, about six and a half feet long — longer than most adult humans are tall. A growing baby that big needed a lot of nutritional energy, and there’s a strong suspicion that a baby megalodon got nutrition the way some infant sharks still do: by eating its siblings.

These weren’t just giant baby sharks. They were cannibal sharks.

Jaws, the Prequel

Appropriately enough for a monster shark, the name megalodon simply means “megatooth,” according to Inverse. As EarthSky notes, their teeth could grow up to seven inches long — and until recently, those teeth were practically all we knew about the megalodon. Sharks’ skeletons are made of cartilage, not bone, which means they rarely survive in the fossil record.

But according to EcoWatch, a single well-fossilized megalodon spine was found in Belgium in the 1860s. Now housed in a Belgian museum, the authors of the new baby megalodon study describe it as the “only reasonably preserved vertebral column of the species in the entire world.”

While this fossil has been known for a century and a half, much newer technology has now been applied to studying it. CT scans of the vertebrae revealed growth rings, similar to the rings of trees, that can be used to determine the age of the specimen at death.

The Belgium specimen was about 30 feet long, which was modest for a megalodon. But by working backward from the growth ring pattern, researchers were able to determine that it was about 6.6 feet long at birth.

The Ultimate Sibling Rivalry

That’s a lot of baby megalodon to feed, and it must have required a rich, high-energy diet. This is where the cannibal part comes in. Megalodon was a lamniform shark, like today’s great white and mako sharks, as Smithsonian Magazine reports. Instead of simply laying eggs, as most fish do, lamniform shark eggs hatch inside the mother’s body, and the young sharks remain there until they’re large enough to survive on their own.

During that prenatal growth period, as another Inverse article explains, baby lamniform sharks survived by oophagy: They eat eggs. So do many of us, but the eggs the baby sharks are eating are their own prospective siblings. And if some of their siblings have already hatched? They’ll eat them, too. Sharks have a reputation as eating machines, and baby sharks are no exception.

As of yet, there is no direct confirmation from the fossil record to confirm that baby megalodons were cannibal sharks like their modern relatives. But the inference that they were is strengthened by the adult megalodon’s enormous size, which was likely both cause and consequence of evolutionary pressures acting on the prehistoric ocean ecosystem.

You’re Gonna Need a Bigger Boat

Sharks, as Smithsonian magazine explains, are relatively warm-blooded compared with most fish, though not so fully warm-blooded as mammals. In technical terms, they’re mesotherms. The higher energy level associated with warmer blood has two important and related effects. It means that sharks need a lot of food to keep themselves warm, so they are very, very hungry.

But it also means that sharks have the speed and power to catch their prey and (for the moment) satisfy their appetite.

In the megalodon’s day, the oceans offered a particularly rich food source thanks to the arrival of mammals such as seals, dolphins and whales. This ecosystem provided an ideal environment for a shark that was big enough to even dine on whales. Its huge size helped it keep warm, giving it more energy to hunt.

And instead of releasing large numbers of eggs — most of which would be eaten by other fish — megalodon eggs were hatched internally, where a growing baby megalodon could dine on them. The nutritional value stayed in the family, so to speak. Thus, the young megalodon could emerge already more than six feet long, ready to dine on seals if not quite yet on whales.

Evolution may not always make for the happiest family relationships, but the megalodon’s survival strategy allowed it to thrive as terror of the seas for more than 10 million years.