Why did the largest shark ever die out about 3.6 million years ago? A study now provides new evidence that the smaller but potentially more adaptable white shark snatched food from its giant relative: comparisons of zinc isotope signatures in the fossil teeth of both shark species suggest they hunted similar prey and thus were food competitors. Combined with other factors, competitive pressure may have led to the deaths of the monstrous giants, the researchers say.
Giant teeth found in many places around the world are evidence of a giant shark that roamed the oceans about 3.6 million years ago. The triangular biters are sometimes larger than a human hand, suggesting that the megalodon (Otodus megalodon) reached a length of almost 20 meters. The great white shark (Carcharodon carcharias), which existed in parallel with it at the time, almost resembled a dwarf about 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, it remained unclear to what extent the two predatory fish actually pursued similar prey.
On the trail of diet
An international team of researchers has now gained insight into this issue using a new detection method. It examined the signatures of zinc isotopes (Zn) in fossil shark teeth. As the researchers explain, it is natural that the ratio between 66Zn and 64Zn reflects the so-called trophic level of the sharks – the place they have occupied in the food chain. In the ocean, 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, these marine mammals are predators. As part of their study, researchers have now investigated the extent to which zinc isotope signatures make it possible to draw conclusions about sharks’ diets.
The team analyzed the relationship between stable zinc isotopes in modern and fossil shark teeth from around the world, including those of megalodon and modern and fossil 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 the values are apparently not falsified by fossilization processes either. “The zinc isotope signals are coherent in fossils and their corresponding modern species. It strengthens our confidence in the method of analysis,” says co-author Sora Kim of the University of California at Merced. Her colleague Thomas Tütken of Johannes Gutenberg University of 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, scientists were also able to determine the zinc isotope ratio between megalodon teeth from the early Pliocene (5.3 to 3.6 million years ago) and from white sharks that lived then and now, to get indications of what interactions may occur. Garden. been between them. “Remarkably, the zinc isotope values for early Pliocene shark teeth from North Carolina suggest that the trophic levels of early white sharks and the much larger megalodon largely overlapped,” says co-author Michael Griffiths of 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 mix of different marine mammals. This indication of competition for food resources thus also supports the suspicion that the white shark played a crucial role in the extinction of the megalodon: it is possible that the smaller predator eventually had the fin in front of the competition, which could have contributed to its giant relatives death. “But this issue should now be investigated further,” Shimada stresses.
Finally, first author Jeremy McCormack of 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 has previously yielded conclusions about diets. “The protein collagen contained in bones and teeth, which are required for these assays, is difficult to preserve in the long term, so conventional nitrogen isotope analysis is often not possible,” McCormack says. “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