Traces of former body heat: Using tooth material with a "thermometer function", researchers were able to prove that the megalodon shark was at least partially warm-blooded. He kept his body temperature at about seven degrees Celsius above the surrounding water temperature. Presumably this made the predator more agile, but generating heat also came at a cost. It is possible that this calculation no longer worked out in the end and that warm-bloodedness could have played a role in the giant's extinction, the scientists say.
Next to him, the great white shark looks like a dwarf: A monstrous shark once roamed the oceans of the earth, which according to estimates could reach a length of over 15 meters. This is evidenced by the hand-sized teeth of the fish called megalodon (Otodus megalodon), which have been discovered at many sites around the world. Bite marks on fossil whale bones suggest that this, arguably the largest shark of all time, was the top predator of its time. But this position was no guarantee of survival: About 3.6 million years ago, Megalodon disappeared from the oceans of the earth. Apparently he was not up to certain challenges of this era and died out.
With few remains of megalodon found apart from the teeth, some aspects of the exciting predator still remain unclear. Based on the known characteristics, paleontologists suspect that it was a representative of the mackerel sharks. An interesting peculiarity is known from this group: While most fish are cold-blooded, whose body temperature corresponds to that of the water surrounding them, today's mackerel sharks keep their body temperature partly or completely warmer. This form of warm-bloodedness is also known as mesothermia. In their study, the researchers led by Robert Eagle from the University of California in Los Angeles (UCLA) have now investigated whether and to what extent Megalodon could also have been warm-blooded.
Temperature signature in teeth
They looked for answers in the giant's fossil tooth material. As the researchers explain, tooth enamel is made up of minerals, the components of which include carbon and oxygen atoms. Depending on the number of neutrons, they come in "light" or "heavy" forms called isotopes. Their special composition in certain materials can, in turn, depend on environmental factors at the time of their formation. The researchers explain that isotope signatures in tooth material can also provide information about the body temperatures at which they formed in the respective living being. "You can think of the isotopes in the minerals that make up teeth as a kind of thermometer that can be read for millions of years," says co-author Randy Flores of UCLA. "Because teeth form in the tissues of a living animal, we can measure the isotopic composition of the fossil teeth to estimate the temperature at which they formed, and this gives us an approximate body temperature of the animal when it was alive."
The team carried out the mass spectrometric studies of the isotopic signatures on megalodon teeth from different localities around the world. They also used fossil teeth from the same areas that came from other shark species as comparison material. Estimates of the average seawater temperatures at each of the locations where the teeth were collected served as further data basis. As the scientists report, their results have now confirmed the previous assumption that the "king of sharks" was also mesothermic: the body temperature of megalodon was significantly higher than that of sharks, which are considered cold-blooded or ectothermic. Specifically, it became apparent that he was able to maintain a body temperature that was about seven degrees Celsius warmer than the surrounding water.
Expensive heat
As the scientists explain, this trait may also have played a role in the development of the shark's enormous growth in size. It also seems clear that Megalodon benefited from being warm-blooded: it was probably able to move particularly quickly and also spread in comparatively cold waters around the world. Warm-bloodedness comes at a price, however, which the benefits have to outweigh. It is possible that the megalodon's tolerance range was critically small and the adaptation could have been fatal to the giant 3.6 million years ago, the scientists say. "Maintaining an energy level that allowed the megalodon's elevated body temperature would require enormous appetites that would not have been sustainable at a time when the balance of marine ecosystems has changed," Flores said.
Her colleague Robert makes a connection to today: "Studying the factors that led to the extinction of a very successful predatory shark like the megalodon can shed light on the vulnerability of large marine predators in modern marine ecosystems confronted with the effects of climate change are,” says the scientist. The team now also sees further potential in their approach: The researchers plan to use their method to investigate the body temperatures that they once produced in other extinct animal species. "Now that we have demonstrated endothermy in megalodon, the question arises to what extent it might have occurred in other large marine predators throughout Earth's history," said senior author Aradhna Tripati of UCLA.
Source: University of California – Los Angeles, William Paterson University, professional article: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2218153120