Megalodon was the biggest predatory shark that ever lived, and its young were also gargantuan; at birth, they were as big as the average basketball player. A new study published online on January 11 in the journal Historical Biology, suggested that Megalodon's babies promoted their growth by gobbling up their smaller siblings while still in the mother's womb, a survival strategy shared by some modern sharks.
During the study, a US research team calculated the size of megalodon babies by analyzing skeletal fossils of a megalodon that measured about 30 feet (9 meters) long when it died. The scientists then looked at "growth rings" in pieces of the shark's preserved skeleton, similar to the rings in tree trunks used to determine a tree's age.
Megalodons belong to a class of fishes called Chondrichthyes, which have skeletons made of cartilage rather than hard bone. Extinct cartilaginous fish like megalodon and other megatooth sharks are therefore known mostly from their teeth, which were made of calcium and therefore survive in the fossil record longer than these fishes' delicate cartilaginous skeletons. But there is a rare collection of 150 megalodon vertebrae whose cartilage had mineralized, and the only reasonably preserved vertebral column of the species in the entire world is kept at the Sternberg Museum.
Using computed X-ray tomography (CT) scans, the scientists counted 46 regularly-spaced growth rings in three of the megalodon's vertebrae. They then applied a mathematical growth curve equation that's commonly used to calculate growth patterns in modern sharks, as each ring represents a year of growth.
The examined shark would have been about 46 years old when it died. By working backwards to the earliest growth ring — the band at birth — the scientists calculated the shark's length as a newborn, estimating it to be around 6.6 feet (2 meters) long — bigger than any known newborn sharks.
"Nourishing such enormous young would have carried high energy costs for the mother, suggesting that her babies supplemented in-utero nutrients with a side helping of unborn sibling cannibalism," co-author Kenshu Shimada, a professor of paleobiology at DePaul University in Chicago and research associate at the Sternberg Museum in Kansas, said in a report published on the Live Science website.