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Megalodon sharks grew biggest in colder waters
The biggest ever shark might not always have been so large.
A study found that megalodon tended to grow larger in cooler environments, and that proposed nurseries may just have been warm-living communities.
An ancient shark living millions of years ago could offer insights into the impact of climate change on its modern relatives.
A team of American researchers suggest that the size of megalodon (Otodus megalodon) may have varied depending on how where it lived. Their research found that the sharks living in cooler waters would have grown larger than those in warmer areas.
Growing larger may have helped megalodon reduce heat loss, could have improved their ability to cover large distances and allowed them to hunt bigger prey. This could help scientists assess how sharks living today will change as water temperatures are altered by a warming world.
Co-author Professor Michael Griffiths says, 'The results of this study have important implications for understanding how modern climate change is rapidly accelerating marine habitat shifts to more polar latitudes in apex predators such as sharks.'
The results of the study were published in the journal Historical Biology.
Though they have previously been thought of as close relatives, scientists now believe megalodon and the great white shark are more distantly related. Image © Mark_Kostich/Shutterstock
What do megalodon's teeth reveal?
Megalodon is believed to have evolved around 20 million years ago during the Neogene Period.
For millions of years, the sharks were one of the ocean's top predators, potentially growing up to 18 metres long. This would have made it one of the largest fishes to have ever lived, equivalent in size to species like the whale shark and only slightly shorter than a bowling lane.
However, only estimates can be made about the species' size. This is because nearly all megalodon fossils are teeth, measuring up to 18 centimetres long. For comparison, this is around three times the length of a great white shark tooth.
Based on comparisons with modern and fossil relatives, the position of the teeth in the shark's mouth is used to estimate just how big megalodon could be. The equations used to do this were updated last year using a largely complete set of teeth when it was discovered their previous estimates could vary wildly.
In some areas, megalodon teeth significantly smaller than the average have been found in large deposits, which has been suggested as evidence of nursery grounds for the ancient shark. A 2020 study found evidence these nurseries could have been located in what is now Spain, the USA and Panama.
However, these estimates were taken using lateral teeth that the authors of the current study say are more likely to produce uncertainty in the size estimates they produce. As a result, they decided to reassess the findings using the upper anterior teeth, which the scientists say produce better estimates, as well as using new methods of estimating body length.
Previous studies had suggested that large amounts of small megalodon teeth in sites such as Yorktown were evidence of nurseries, but this study questions that finding. Image © Ryan Somma, licensed under CC BY-SA 2.0 via Flickr.
How did megalodon's size vary with temperature?
Around 80 megalodon teeth were used to estimate the body length of the sharks they came from, and then compared to the estimated sea surface temperatures at the time the animals were alive.
Though the sample size was limited due to the use of only certain teeth, the broader patterns of length in the results were similar to studies using other methods of analysis.
These results showed that sharks living in cooler waters tended to be larger than those living in warmer waters. This correlates with a principle known as Bergmann’s rule, where animals found in colder climates are often larger as a greater size allows them to retain more heat.
While this rule hasn't been documented in other shark species, the researchers say megalodon is very different to its modern relatives and so cannot be directly compared.
Lead author Professor Kenshu Shimada says, 'Our findings suggest a previously unrecognised body size pattern for the fossil shark, notably following Bergmann's rule.'
'The main conclusion of this study is that not all geographically different megalodon individuals grew to gigantic sizes equally. The common notion that the species reached 18 to 20 metres long should be applied primarily to populations that inhabited cooler environments.'
The scientists' research may also undermine the concept of nursery areas for megalodon, as these sites tended to be located in warmer waters.
Dr Henry Maisch, another co-author, says, 'It is still possible that megalodon could have utilised nursery areas to raise young sharks. But our study shows that fossil localities consisting of smaller megalodon teeth may instead be a product of individual sharks attaining smaller overall body sizes simply as a result of warmer water.'
Changes in body size as a result of different temperatures around the world may have affected how megalodon lived. Swimming distances, predation success and oxygen demands are all impacted by temperature in living animals, making it possible megalodon was affected similarly.
Discovering how temperature changes affect these ancient sharks may also help scientists studying the impacts of climate change on their modern relatives, assisting in the creation of better plans to conserve these species.
