The megalodon, a giant shark that went extinct some 3.6 million years ago, is famous for its utterly enormous jaws and correspondingly huge teeth. Recent studies have proposed that the megalodon was robust species of shark akin to today’s great white sharks, only three times longer. And just like the great white shark inspired Jaws, the megalodon has also inspired a 1997 novel and a blockbuster film (2018’s The Meg)—not to mention a controversial bit of “docu-fiction” on the Discovery Channel. But now a team of 26 shark experts are challenging the great white shark comparison, arguing that the super-sized creature’s body was more slender and possibly even longer than researchers previously thought in a new paper published in the journal Paleontologia Electronica.
“Our study suggests that the modern great white shark may not necessarily serve as a good modern analogue for assessing at least certain aspects of its biology, including its size,” co-author Kenshu Shimada, a palaeobiologist at DePaul University in Chicago, told The Guardian. “The reality is that we need the discovery of at least one complete megalodon skeleton to be more confident about its true size as well its body form.” Thus far, nobody has found a complete specimen, only fossilized teeth and vertebrae.
As previously reported, the largest shark alive today, reaching up to 20 meters long, is the whale shark, a sedate filter feeder. As recently as 4 million years ago, however, sharks of that scale likely included the fast-moving predator megalodon (formally Otodus megalodon). Due to incomplete fossil data, we’re not entirely sure how large megalodons were and can only make inferences based on some of their living relatives, like the great white and mako sharks.
Thanks to research published last year on its fossilized teeth, we’re now fairly confident that it shared something else with these relatives: it wasn’t entirely cold-blooded and apparently kept its body temperature above that of the surrounding ocean. Most sharks, like most fish, are ectothermic, meaning that their body temperatures match those of the surrounding water. But a handful of species, part of a group termed mackerel sharks, are endothermic: They have a specialized pattern of blood circulation that helps retain some of the heat their muscles produce. This enables them to keep some body parts at a higher temperature than their surroundings. A species called the salmon shark can maintain a body temperature that’s 20° C warmer than the sub-Arctic waters that it occupies.
Megalodon is also a mackerel shark, and some scientists have suggested that it, too, must have been at least partially endothermic to have maintained its growth rates in the varied environments that it inhabited. The 2023 study measured isotope clumping—which can provide an estimate of the temperature at which a material formed—in mastodon teeth. They confirmed that the megalodon samples were consistently warmer, with an average temperature difference of about 7° C compared to cold-blooded samples.
Of particular relevance to this latest paper is a 2022 study by Jack Cooper of Swansea University in the UK and his co-authors. In 2020, the team reconstructed a 2D model of the megalodon, basing the dimensions on similar existing shark species. The researchers followed up in 2022 with a reconstructed 3D model, extrapolating the dimensions from a megalodon specimen (a vertebral column) in Belgium. Cooper concluded that a megalodon would have been a stocky, powerful shark—measuring some 52 feet (16 meters) in length with a body mass of 67.86 tons—able to execute bursts of high speed to attack prey, much like the significantly smaller great white shark.
However, Shimada et al. outlined several concerns about Cooper et al.’s 2022 reconstruction, citing questionable accuracy of their reconstructed vertebral column (many of the vertebrae appear to be missing); a discrepancy in jaw size relative to its vertebrae compared to the great white shark; and the fact that Cooper et al. scanned a juvenile white shark specimen for their reconstruction when one could expect significant body changes as such a shark matured, among other concerns regarding the 3D reconstruction methods.
Shimada et al. felt the time was right to revisit the issue in light of several studies last year, including the aforementioned confirmation that the megalodon was endothermic, as well as another study (which Shimada also co-authored) that concluded the megalodon’s cruising speed was likely slower than that of great whites and similar sharks. So they re-examined the fossil record and compared vertebral fossils, and Cooper et al.’s 2022 reconstruction, to a CT scan of the whole vertebral skeletons of a living great white shark.
The team concluded that based on the spinal column, it was much more likely that the megalodon was more slender than a great white since the combination of a great white build with the megalodon’s much longer length would have simply proved too cumbersome. They believe the body shape of the modern mako shark would be a more accurate model. “It was still a giant, predatory shark,” said co-author Phillip Sternes, a biologist at the University of California, Riverside. “But the results strongly suggest that the megalodon was not merely a larger version of the modern great white shark.
“It would almost certainly not be feeding like modern great whites,” co-author Charles Underwood of Birkbeck University of London told New Scientist. “It wouldn’t be hovering over the sea floor waiting for a whale to move above it and then slamming into it from below, basically shredding it. It would involve more straight pursuit, a longer pursuit, less of an ambush predator. Because it doesn’t have the top speed; it doesn’t have the acceleration.”
Cooper, meanwhile, stands behind his 2022 paper and thinks Shimada et al.’s conclusions are overly simplistic. “The long and short of it is that no matter which hypothesis you support about its body shape, it was a very big shark,” he told New Scientist. “Of course, a complete skeleton would go a long way to helping us find out more. But I also don’t think we should assume this would settle all debates on this very charismatic animal.”
Paleontologia Electronica, 2024. DOI: 10.26879/1345 (About DOIs).