Body size of the extinct Megalodon indeed off the charts in the shark world

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IMAGE: SCHEMATIC DRAWING SHOWING THE DISTRIBUTION OF MAXIMUM POSSIBLE SIZES OF ALL KNOWN 70 NON-PLANKTIVOROUS GENERA (GROUPS) IN THE SHARK ORDER LAMNIFORMES, COMPRISING MODERN (IN GRAY) AND EXTINCT (IN BLACK; WITH... view more 

CREDIT: KENSHU SHIMADA

A new study shows that the body size of the iconic gigantic or megatooth shark, about 15 meters (50 feet) in length, is indeed anomalously large compared to body sizes of its relatives.

Formally called Otodus megalodon, the fossil shark that lived nearly worldwide roughly 15-3.6 million years ago is receiving a renewed look at the significance of its body size in the shark world, based on a new study appearing in the international journal Historical Biology.

Otodus megalodon is commonly portrayed as a super-sized, monstrous shark, in novels and films such as the 2018 sci-fi thriller "The Meg," but it is known that the scientifically justifiable maximum possible body size for the species is about 15 meters (50 feet). Nonetheless, it is still an impressively large shark, and the new study illuminates exactly how uniquely gigantic the shark was, according to Kenshu Shimada, a paleobiologist at DePaul University in Chicago and lead author of the study.

Otodus megalodon belongs to the shark group called lamniforms with a rich fossil record, but the biology of extinct forms is poorly understood because these cartilaginous fishes are mostly known only from their teeth. Based on measurements taken from present-day non-planktivorous lamniforms, the study presents an equation that would allow estimations about the body length of extinct forms from their teeth. The study demonstrates that O. megalodon that reached about 15 meters (50 feet) is truly an outlier because practically all other non-planktivorous sharks have a general size limit of 7 meters (23 feet), and only a few plankton-eating sharks, such as the whale shark and basking shark, were equivalent or came close to the size. The study also reveals that the Cenozoic Era (after the age of dinosaurs, including today) saw more lamniform lineages attaining larger sizes than the Mesozoic (age of dinosaurs) Era.

Warm-bloodedness has previously been proposed to have led to the gigantism (over 6 meters, or 20 feet) in multiple lamniform lineages. The new study proposes their live-bearing reproductive strategy with a unique cannibalistic egg-eating behavior to nourish early-hatched embryos to large sizes inside their mother to be another possible cause for the frequent evolution of gigantism achieved by lamniform sharks.

Understanding body sizes of extinct organisms is important in the context of ecology and evolution. "Lamniform sharks have represented major carnivores in oceans since the age of dinosaurs, so it is reasonable to assert that they must have played an important role in shaping the marine ecosystems we know today," said Shimada.

"This is compelling evidence for the truly exceptional size of megalodon," noted co-author Michael Griffiths, a professor of environmental science at William Paterson University in Wayne, New Jersey. Co-author Martin Becker, also a professor of environmental science at William Paterson University, added, "this work represents a critical advancement in our understanding of the evolution of this ocean giant."

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