New research has confirmed that the iconic megalodon shark was indeed the mighty-toothed giant we all hoped it to be, far larger than other predatory sharks alive in its time. A new study looked at body, jaw, and dentition measurements from specimens of all 13 species of extant macrophagous lamniformes (aka, big hungry sharks) to form accurate predictions of extinct sharks’ body size simply from their fossilized teeth. Published in the journal Historical Biology, these analyses allowed researchers to understand the size limitations of certain sharks and which processes appear to enable enormous adult body size.
Sharks are cartilaginous fish meaning that other than their teeth they don’t leave much in the way of fossil evidence behind. This makes understanding the true size of them difficult, as there are no tell-tale skeletons to dig up on the seafloor. To overcome this, the researchers looked to shark species still alive today to find out the limitations of certain species’ growth and what physiological processes could explain this.
Their findings showed that most sharks in the Mesozoic and Cenozoic were relatively small, with only four genera containing members that exceeded 6 meters (20 feet) in length. The largest species was indeed our hero, Otodus megalodon, which by the researchers’ estimations reached at least 15 meters (50 feet), which rivals the current record-holder for world's biggest fish: whale sharks.
While megalodon’s reputation in modern science makes this finding unsurprising, the species’ enormous size is unusual in the context that all other macrophagous lamniformes at this time were about half that size. Exactly how megalodon was able to jump ahead of the race so significantly isn’t clear, though endothermy (being “warm-blooded”) is one theory that has been suggested in the past. The researchers on this study, however, had a different idea.
Lamniforme sharks that are still alive today include the sand tiger, thresher, basking, mako, and great white sharks, many of which can grow to enormous sizes. All of these sharks exhibit a reproductive strategy known as ovoviviparity where multiple eggs hatch inside the shark and grow inside the mother, a little bit like human twins. A big difference from human pregnancy however occurs when "intrauterine cannibalism” is seen within these animals, where early hatching embryos snack on their siblings.
So how could this encourage an enormous size in adulthood?
“The outcome is that only a few embryos will survive and develop, but each of them can become considerably large in body size at birth,” study author Kenshu Shimada told IFLScience, “Although energetically costly for the mother to raise such large embryos, newborns have an advantage as ‘already-large’ predators with reduced chances of getting eaten by other predators. The exact number of pups and their size at birth are species-specific, likely influenced by a multitude of biological and environmental factors, and our new study suggests that such variation could have also served as additional evolutionary sources for some lamniformes to become gigantic, including Megalodon."
However, another question has arisen from attempting to answer the riddle of megalodon's size.
“Much of the attention has been given to the question, 'Why did Megalodon evolve to be so large?'" Shimada said. "However, our new study has provided another way of thinking about the scientific question, "Why do all other non-planktivorous lamniformes have a general size limit of 7 meters?". That needs further investigation.”