The largest shark alive today, reaching up to 20 meters in length, is the whale, a sedate filter feeder. As recently as 4 million years ago, however, sharks of that size likely included the fast predator megalodon, famous for absolutely huge cheeks and correspondingly large teeth.
Due to incomplete fossil data, we are not sure how big megalodon was and can only make conclusions based on some of its living relatives, such as the great white and mako shark. But thanks to some new research on its fossilized eggs, we are now fairly confident that it shares something else with these families: it was not completely cold-blooded and obviously kept its body cold above the sea area.
Taking the temperature
Most sharks, like many fish, are ectothermic, meaning that their body temperature matches that of the surrounding water. But some species, part of a group called the mackerel shark, have a specialized form of blood flow that helps retain some of the heat their muscles produce. This allows them to keep some body parts at a higher temperature than their environment. A species called salmon can maintain a body temperature that is 20 °C warmer than sub-Arctic waters.
Megalodon is also a mackerel shark, and some scientists have suggested that, too, it must have had at least an endothermic part to have maintained its growth rates in the different environments it lived. But, as mentioned, the megalodon remains we have aren’t even enough to let us know how big the animal was, much less whether it had the kind of specialized circulatory system needed for shark endothermy.
So, a group of researchers decided to directly test whether there are signs that regulate your body temperature using things we actually have: your teeth.
The work is based on a phenomenon known as isotope clumping. If the environment is hot enough, the small weight differences between atomic isotopes are not important, as the heat is hot enough to mix the isotopes well within a material. But as the material dissolves, the heavier isotopes tend to clump together, forming clusters within a material. We now have equipment that can monitor the distribution of isotopes within a material at high resolution, obtaining a direct measure of its clumpiness. That, in turn, can be used to generate an estimate of the temperature the material is created from.
(Scientists have used this technique to calculate ancient temperatures to monitor our changing climate.)
The new work relies on fossil beds that contain at least three types of fossils. One is obviously megalodon eggs. But others are needed to provide some degree of external reference for the statistics obtained from sharks. These include the bones of known cold-blooded fish, which provide the basis for environmental temperatures. They also took samples of the ear bones of whales to have known warm-blooded control. Importantly, they collected these samples from widely distributed sites in the Atlantic and Pacific Oceans, ensuring that any differences are not simply a matter of local environmental conditions.
Summer, move quickly
Samples of ectotherms show the kind of regional differences you’d expect from ocean temperatures, with estimates ranging from a low of 17°C in California to a high of 23°C in the Mediterranean. Megalodon samples, in contrast, are always warmer, with an average temperature of about 7°C compared to cold-blooded samples.
This is not as hot as whale samples. But, as the researchers point out, whale samples come from their inner ears, which are normally removed from the environment, and it is possible to show the temperature inside the animal. In sharks, on the contrary, the eggs are exposed to the environment and therefore can be intermediate between the body temperature and the external temperature. The temperature of mackerel sharks also tends to vary across different body parts.
So why was a high body temperature selected for the megalodon? There are two potential reasons. One is, as noted above, that temperatures may be necessary to maintain the growth rates needed to allow something like megalodon to evolve in non-tropical environments. The second is fast. Thermal muscles may be necessary to power the animal through water quickly enough to be an effective predator. The mako shark, for example, is the fastest and partially endothermic shark.
Megalodon’s large body size may also make heat retention a little easier, as it increases the ratio of body volume to surface area, meaning there is less space to lose heat compared to the amount of muscle that produces it.
The authors of the new paper, however, suggest that the megalodon may also have been vulnerable to climate change. The metabolic demands involved in maintaining its endothermy may have made the megalodon sensitive to changes in the ecosystem. And, near the time of its destruction, the Earth generally cooled, causing sea levels to fall, which would have destroyed aquatic ecosystems. And the megalodon seems to have depended on beach nurses during its early years.
PNAS, 2023. DOI: 10.1073/pnas.2218153120 (About DOIs).