Extinct giant shark gave birth to 'cannibal' babies over 6ft 6in long

Megalodon reconstruction at the Museo de la Evolución de Puebla in Mexico.

Although Otodus megalodon is typically portrayed as a super-sized, monstrous shark in novels and films such as the 2018 sci-fi film "The Meg", scientific data support a more modest but still impressive estimate of about 50 feet (15 meters) for the presently known largest individuals.

"As one of the largest carnivores that ever existed on Earth, deciphering such growth parameters of O. megalodon is critical to understand the role large carnivores play in the context of the evolution of marine ecosystems", Dr Shimada said. Not surprisingly, Megalodon's babies were also on the hefty side.

"The new study is really the first of its kind for megalodon", Kenshu Shimada, a paleobiologist at DePaul University in Chicago and research associate at Kansas's Sternberg Museum in Kansas, wrote in an email.

Despite being one of the most familiar faces among ancient animals, certainly among ancient sharks, precious little is known about the megalodon. This means the fossil record is very poor for this animal.

Megalodon vertebrae are like trees - for each year of age, the shark's bone gets another concentric ring of calcified bone.

Identified annual growth bands in a vertebra of the extinct megatooth shark Otodus megalodon along with hypothetical silhouettes of the shark at birth and death, each compared with size of typical adult human. Credit: DePaul University/Kenshu Shimada.

Also, scientists took CT scans of a rare megalodon vertebra kept in a collection at the Royal Belgian Institute of Natural Sciences in Brussels.

The images revealed the vertebrae to have 46 growth bands, meaning that the almost 30ft (9m) Megalodon fossil died at the age of 46. By analyzing how each band formed in the giant vertebrae, the researchers could calculate the shark's body length during each year of its life, including at its birth. "The detection of the very fine growth bands is literally the breakthrough of this study", Shimada told ZME Science.

This back-calculation suggests that Megalodon gave live birth to the largest babies in the shark world, measuring about 2 meters (6.6 feet) in length. But that's not the study's most interesting conclusion.

Some sharks lay eggs, while others give birth to live young. And like modern sharks, these baby Megalodons were killers well before they left their mum's womb. "The mother's womb in present-day lamniform sharks secretes a lipid-rich fluid, so I envision embryos to be developing in a nutrient-rich environment slurping soft eggs and the fluid".

"The outcome of the egg-eating behaviour is that only a few pups will survive and develop, but each of them can become considerably large in size at birth", Professor Shimada said. Although likely to be energetically costly for the mother, this process gives newborns an advantage by reducing the chance of being eaten by other predators, Dr Shimada said.

According to the data, the study also shows that the shark grew without significant "growth spurts" at an average rate of about 6.3 inches (16 centimeters) per year, at least during the first 46 years of its life. The researchers measured the vertebra of a megalodon from the Miocene epoch, which lasted from five to 23 million years ago.

The two-metre measurement is fairly accurate and aligns with other findings at megalodon nurseries in Panama and Spain, said Jack Cooper, a fossil shark researcher and doctoral student at Swansea University's department of biosciences in Wales, United Kingdom, who was not involved in the study.

Though the lower bound of the estimated lifespan was 88 years, Shimada's team acknowledged the shark could potentially get much older, though, given the harsh conditions of ocean life several million years ago - even for an apex predator - most megalodons would not have reached such an age.

The findings were reported in the journal Historical Biology.