Megatooth sharks, including megalodon, seem to have had the highest position in food webs ever occupied by marine predators
Megalodon and other megatooth sharks may have eaten other predators – and each other – meaning they occupied an unusually high position in the food web.
“It is very likely that megatooth sharks were at a higher trophic level than any other marine predator,” says Zixuan Rao at Princeton University.
Rao and her colleagues made the discovery by analysing nitrogen isotopes in shark teeth. There are two natural stable isotopes of nitrogen – nitrogen-15 and nitrogen-14 – both of which are found in animal tissue. However, because nitrogen-14 is preferentially excreted from living organisms, animal tissue is generally richer in nitrogen-15 than it would otherwise be.
This means that when a predator eats an animal, the flesh it consumes is richer in nitrogen-15. This richer nitrogen-15 signal is incorporated into the predator’s own flesh and becomes further enriched as the predator also preferentially excretes nitrogen-14. If that predator is ultimately eaten, the second predator will incorporate an even richer nitrogen-15 signal into its tissues. Because this process continues up food chains, researchers can use the ratio of nitrogen-15 to nitrogen-14 in fossils to estimate how high up a food web an ancient animal would have been.
Rao and her colleagues analysed nitrogen ratios in a tooth material called enameloid extracted from five extinct species of megatooth sharks. These species ranged in size from the 3.5-metre-long Otodus auriculatus to the 15-metre-long Otodus megalodon – known to many people simply as megalodon.
The researchers also measured the nitrogen isotope ratio in samples from existing marine mammals such as dolphins, seals, walruses and polar bears as well as from modern sharks, including the great white.
It was the megatooth sharks that had the highest nitrogen ratios – higher than any in living marine predators. “We’ve never seen nitrogen ratio values this high until this project. We expected high values, but not this high,” says Rao.
The results suggest not only that megatooth sharks were at the top of food webs, but that they ate other predators near the top of the food web too.
Exactly which predators the sharks ate isn’t completely clear. The nitrogen isotope ratios in existing marine mammals weren’t high enough to account for the unusually high nitrogen-15 levels found in the megatooth fossils. It may be easiest to explain the signal if megatooth sharks ate other, smaller megatooth sharks.
The results also indicated that megatooth sharks made this shift to eating other predators early in their evolution, when they were relatively small animals measuring about 3.5 metres in length.
“This is really interesting,” says Rao. “It suggests the huge size of the biggest megatooth sharks were not necessary for them to reach the top of the food web.”
Understanding the evolution and behaviour of megatooth sharks can help us to understand how past climate events impacted the marine ecosystems they lived in, says Rao. “Looking at the past is the key to the future, if we can understand how the climate impacted ecosystems in the past, it can help guide us to protect life in the future.”
“Previous studies have suggested that megalodon occupied a higher position in the food web than the great white shark. This study better supports this hypothesis using a much larger sample size and using cutting-edge methodologies,” says Catalina Pimiento Hernandez at Swansea University in the UK.
Journal reference: Science Advances, DOI: 10.1126/sciadv.abl6529
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