The megalodon, a huge shark that was the scourge of the ancient oceans and is a star in modern movie theaters, is named for its "large tooth" - and for good reason. Its serrated teeth - up to about 7 inches long (18 cm) - could tear through any prey in the deep blue sea.
Those teeth now are providing a fuller understanding of this extinct predator, with an analysis of the mineral makeup of their enamel-like tissue confirming that megalodon was warm-blooded - a trait scientists suspect contributed both to its tremendous success and eventual downfall.
Researchers estimated that megalodon, which reached at least 50 feet (15 meters) and possibly 65 feet (20 meters) long while hunting marine mammals including whales, boasted an overall average body temperature of about 81 degrees Fahrenheit (27 degrees Celsius) and could keep it at about 13 degrees F (7 degrees C) above that of the surrounding seawater.
This may have made megalodon a more dynamic predator - a strong swimmer able to digest food in an energetically efficient manner and, importantly, tolerate colder water, letting it broaden its range to nearly worldwide.
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Most fish are cold-blooded - ectothermic - with body temperatures matching the surrounding water. But a few are warm-blooded - endothermic - generating their own body heat. Examples include certain sharks including the largest modern one, the great white.
"The only comparable living species today in terms of both diet and body temperature are the great white shark and, to a lesser extent, the mako shark. Though, as shown in our study, megalodon was quite a bit warmer than both of these modern apex predators, which makes megalodon unique," said geochemist and paleoclimatologist Michael Griffiths of William Paterson University in New Jersey, lead author of the research published in the journal Proceedings of the National Academy of Sciences.
The study found that megalodon, while warm-blooded, had a lower body temperature than whales.
"One theory is that they were regionally endothermic - that some parts of their body were warmer than other parts, whereas body temperature is higher and more uniform across the body in most large mammals," UCLA atmospheric and oceanic scientist and study co-author Robert Eagle said.
Megalodon, perhaps the largest shark of all time, appeared about 23 million years ago, then disappeared about 3.6 million years ago amid declining ocean temperatures and sea levels.
Warm-bloodedness could have been useful for megalodon in cooling waters.
"Yet, the fact that the species became extinct suggests the probable vulnerability - or the cost - of being warm-blooded because warm-bloodedness requires constant high food intake to sustain high metabolism," paleobiologist and study co-author Kenshu Shimada of DePaul University in Chicago said.
"It is quite possible that there was a shift in the marine ecosystem due to the climatic cooling that caused the sea level to drop, altering the habitats with the populations of the types of food megalodon depended on, such as marine mammals, possibly becoming scarce, leading to the extinction of megalodon," Shimada added.
Scientists previously had suspected megalodon's warm-bloodedness but the study provided the first empirical evidence. The researchers analyzed geochemical characteristics in fossil megalodon teeth to determine the temperature at which minerals in enamel-like tissue formed - an indicator of body temperature.
After being overshadowed for decades in popular culture by the great white - think the 1975 blockbuster Jaws and its endless progeny - megalodon is now in the spotlight thanks to the 2018 film The Meg and its upcoming sequel Meg 2: The Trench.
"Megalodon is primarily represented only by teeth and a handful of vertebral specimens in the fossil record," Shimada said. "Contrary to novels and movies that portray megalodon as a super-sized, monstrous shark, the fact is that we still don't even know exactly how it looked or how it lived. This is exactly why the 'science of Megalodon' continues to be an exciting academic field."
