Why did the largest shark of all time die out about 3.6 million ago? A study now provides new evidence that the smaller but potentially more adaptable great white shark snatched food from its giant relative: comparisons of zinc isotope signatures in the fossil teeth of both shark species suggest that they hunted similar prey and were thus food competitors. Combined with other factors, competitive pressures may have led to the demise of the monstrous behemoths, the researchers say.
Huge teeth found at many sites around the world are evidence of a gigantic shark that roamed the oceans around 3.6 million years ago. The triangular biters are sometimes larger than a human hand and suggest that megalodon (Otodus megalodon) reached a length of almost 20 meters. The great white shark (Carcharodon carcharias), which existed parallel to it at the time, almost looked like a dwarf at around six meters away. But he has survived to this day – but his monstrous cousin has not. It has already been suggested that the great white shark may even have played a role in the extinction of the megalodon. Because he could have represented a critical food competitor. However, the extent to which the two predatory fish were actually pursuing similar prey remained unclear.
On the trail of diet
An international team of researchers has now gained insight into this question using a new detection method. It examined the signatures of zinc isotopes (Zn) in fossil shark teeth. As the scientists explain, it stands to reason that the ratio of 66Zn to 64Zn reflects the so-called trophic level of the sharks – the place they have taken in the food chain. In the sea, it starts with the tiny algae that are eaten by small animals like krill. These crustaceans, in turn, serve as food for species that form the next higher trophic level. For example, because baleen whales eat krill directly, they occupy a lower position than toothed whales or harbor seals, which prey on larger fish. At the highest trophic level are the predators of these marine mammals. As part of their study, the researchers have now explored the extent to which the zinc isotope signatures allow conclusions to be drawn about the diet of sharks.
The team analyzed the ratio of stable zinc isotopes in modern and fossil shark teeth from around the world, including those of megalodon and modern and fossil great white sharks. As the researchers report, the basic potential of the method was initially activated: the trophic level of the various shark species was actually reflected in the signatures and apparently the values are not falsified by fossilization processes either. “The zinc isotope signals are coherent in fossil and their corresponding modern species. This strengthens our confidence in the analysis method,” says co-author Sora Kim from the University of California at Merced. Her colleague Thomas Tütken from the Johannes Gutenberg University in Mainz says: “For the first time, we have been able to draw conclusions about the diet of these animals based on zinc isotope signatures in the highly mineralized enamel crown of fossil shark teeth”.
Reference to food competition
In this way, the researchers were also able to determine the zinc isotope ratios of megalodon teeth from the early Pliocene (5.3 to 3.6 million years ago) and from great white sharks living then and now in order to get indications of what interactions there might have been between them . “The remarkable thing is that the zinc isotope values of early Pliocene shark teeth from North Carolina suggest that the trophic levels of early great white sharks and the much larger megalodon largely overlapped,” says co-author Michael Griffiths of the William Paterson University at Wayne. His colleague Kenshu Shimada from DePaul University in Chicago adds: “Our results indicate at least some overlap in the prey hunted by both shark species”.
In addition to fish, the sharks probably ate a similar mixture of different marine mammals. This indication of competition for food resources thus also underpins the suspicion that the great white shark played a critical role in the extinction of the megalodon: it is possible that the smaller predator ultimately had the fin in front in the competitive struggle, which could have contributed to the demise of its giant relative. “However, this topic should now be further researched,” emphasizes Shimada.
Finally, first author Jeremy McCormack from the Max Planck Institute for Evolutionary Anthropology in Leipzig emphasizes the far-reaching potential of the new detection method. Because it now represents an important addition to the nitrogen isotope analyzes of dental collagen, which have previously provided conclusions about diets. “The protein collagen contained in bones and teeth, which is required for these analyses, is difficult to preserve in the long term, so that conventional nitrogen isotope analysis is often not possible,” says McCormack. “Our research shows that zinc isotopes can be used to reconstruct the diet and trophic ecology of extinct animals over millions of years. This method is applicable to many groups, including our own ancestors,” says the scientist.
Source: Max Planck Institute for Evolutionary Anthropology, Article: Nature Communications, doi: 10.1038/s41467-022-30528-9