Megalodon was the biggest shark in the world — 50 feet long or more — and one of the largest fish ever to exist. It roamed most of the world’s oceans from 23 million to 3.6 million years ago.
A new study by paleoecology Professor Sora Kim and colleagues shows the shark’s body temperature was considerably higher than previously thought and provides clues to the species’ demise.
Kim, professors Michael Griffiths and Martin Becker from William Paterson University in New Jersey, Kenshu Shimada, a paleobiologist at DePaul University in Chicago, Rob Eagle from UCLA and others used fossil teeth to determine megalodon was endothermic. The high metabolic cost of maintaining that endothermy as the climate changed might have helped doom the species. A new paper in the Proceedings of the National Academy of Sciences details their findings.
Previous studies have suggested that megalodon was likely warm-blooded, or regionally endothermic, like some modern-day sharks, such as makos and great white sharks. But those findings were based on inference, the researchers said. The new study provides the first empirical evidence of warm-bloodedness in the extinct shark.
Megalodon has a rich fossil record, the researchers said, but its biology remains poorly understood like most other extinct sharks because no complete skeleton of the cartilaginous fish is known in the fossil record. Its abundant teeth are found in every ocean except the Antarctic.
The research team used those fossil teeth to test the “Megalodon Endothermy Hypothesis” employing a novel geochemical technique involving clumped isotope thermometry, as well as phosphate oxygen isotope thermometry.
Eagle worked on the clumped isotope thermometry, which is based on the thermodynamic preference of two or more “heavy” isotopes of a particular element to form bonds in a mineral lattice based on the mineralization temperatures. The degree to which these isotopes bond can reveal the temperature at which the mineral formed. Phosphate oxygen isotope thermometry is based on the principle that the ratio of stable oxygen isotopes in phosphate minerals depends on the temperature of the water from which they formed.
“Studies using these methods have shown them to be particularly useful in inferring the thermophysiologies of fossil vertebrates of 'unknown' metabolic origins by comparing their body temperature with that of co-occurring fossils of 'known' metabolisms,” Griffiths said.
Isotopic analysis indicates megalodon fed on animals that were high up in the food chain, such as other sharks or marine mammals, Kim explained.
But during the time megalodon went extinct, there were climate shifts and a restructuring of ocean circulation patterns that also altered the size and availability of potential food sources.
“Megalodon had an elevated body temperature compared to white sharks and the surrounding water, but not as high as marine mammals,” Kim said. “With warmer body temperatures, they could likely swim faster, go deeper and have access to different prey. But they needed a lot more energy to be able to survive.”
“If its prey sources are getting smaller, but it needs all that energy to survive and maintain its body temperature, those two things are working against each other,” she said.
This precarious energetic balance was perhaps put in peril when productive coastal shelf habitats diminished and there were accompanying shifts in prey landscapes due to Pliocene sea-level changes, the researchers wrote.
The new findings show that despite having traits that allowed them to be a commanding presence in the ocean, large marine apex predators such as Otodus megalodon were not immune to the effects of climate change, highlighting the need for conservation efforts to protect modern shark species, including the great white shark.
Given current stresses on modern sharks including on the somewhat equivalent great white — due to factors including climate change, ocean acidification and overfishing — the researchers’ work has implications to the future of marine apex predators that play a critical role in the ocean food web, the researchers said.
“Otodus megalodon was one of the largest carnivores that ever existed, and deciphering the biology of the prehistoric shark offers crucial clues about the ecological and evolutionary roles large carnivores have played in marine ecosystems through geologic time,” Shimada said.
The research team hopes to make more discoveries about megalodon.
“We are currently extending this work back in time through the Otodus (‘megatooth’ shark) lineage with the hopes of deciphering when endothermy first appeared in these apex predators and what role it played in the evolution of gigantism in Otodus,” Griffiths said. “In addition, we are measuring various other isotope systems to probe the diet and trophic level hierarchy of these sharks, which may also hold clues to their paleo habitats, ecology and biology.”
