The largest fish alive today, reaching up to 20 meters in length, are the whale sharks, which feed on filters. As recently as 4 million years ago, however, this type of shark may have included the megalodon, famous for its enormous jaws and equally large teeth.
Due to insufficient fossils, we don’t know how big the megalodon was and can only speculate about its living relatives, such as the great white and mako sharks. But thanks to some new research on its fossilized teeth, we’re now confident that it shared something with these relatives: it wasn’t completely cold and apparently kept its body temperature above the surrounding ocean.
Heat
Most fish, like most fish, are ectothermic, meaning that their body temperature matches that of the surrounding water. But only a few species, including mackerel sharks, have a special circulatory system that helps prevent their muscles from overheating. This helps them to keep other parts of the body at a higher temperature than the place where they live. The salmon shark can maintain a body temperature that is 20°C higher than the sub-Arctic waters they live in.
Megalodon is a mackerel shark, and some scientists have suggested that it, too, must have been a little smaller to maintain its size in the different habitats it lived in. But, as we have said, the megalodons we have are not enough to tell us the size of the animal, regardless of the fact that it had a special type of shark endothermy.
Therefore, a group of researchers decided to test directly if there were signs that control his body temperature by using the things we have: his teeth.
This work relies on a phenomenon called isotope clumping. If the environment is warm enough, the small differences between atomic isotopes do not matter, because the temperature is hot enough to mix the isotopes within the material. But when the material cools, the heavier isotopes bond together, forming strings within the material. We now have tools that can monitor the distribution of isotopes within high-risk materials, allowing a direct measurement of their clumpiness. The same, can be used to make an estimate of the temperature that was created.
(Scientists have used this technique to measure past temperatures to predict climate change.)
The new project relied on fossils that contained three different types of fossils. One was apparently the teeth of a megalodon. But the others were required to provide external evidence of the numbers of these fish. These include the bones of the famous cold-blooded fish, which provided the basis for the warming environment. He also found samples of whale ear bones for the known heat capacity of blood. In short, they obtained these samples from widely distributed areas in the Atlantic and Pacific Oceans, ensuring that any differences are not just a matter of environment.
Warm up, walk fast
Models of ectotherms showed the range of environments you would expect from seawater temperatures, with estimates ranging from a low of 17°C in California to a high of 23°C in the Mediterranean. The megalodon samples, in contrast, were always hot, with a temperature difference of about 7°C compared to the cold-blooded samples.
These are not as hot as whale models. But, as the researchers point out, the samples of whales came from their inner ears, which are well removed from the environment, and thus show the internal temperature of the animal. Unlike sharks, toothfish are more adaptable to the environment and can live between body temperatures and outside of the world. The temperature of mackerel sharks is also different in different parts of the body.
So why can body temperature be selected in megalodon? There are two reasons. One is, as mentioned above, that heat would have been necessary for the growth necessary for something as large as megalodon to exist in non-tropical environments. The second is speed. Warm muscles may be necessary to get the animal into the water quickly to become a predator. For example, the mako shark is a very fast shark and one part of the shark is dangerous.
Megalodon’s large body size would also have made heat retention easier, as it increased the amount of body mass to the surface, meaning there was less room for heat loss compared to the amount of muscle it produced.
The authors of the new paper, however, suggest that they may have left the megalodon vulnerable to climate change. The high metabolic rate involved in maintaining endothermy would have made megalodon sensitive to environmental changes. And, around the time of its extinction, the Earth often cooled, causing sea levels to drop, which would have disrupted coastal ecosystems. And the megalodon seems to have relied on coastal habitats in its early years.
PNAS, 2023. DOI: 10.1073/pnas.2218153120 (About DOIs).
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