Giant toothed shark was once one of the largest carnivores on earth. It lived in the ocean from 23million to 3.6 million years ago, with a length of up to 20 meters. The cause of its extinction has been controversial. Now, scientists have found that although the total length of the largest great white shark is only 6 meters, its competition for food resources may lead to the extinction of the great toothed shark. Relevant achievements were published in nature communication on May 31. The trophic level of animals marks their position in the ecosystem, and diet is very important for understanding the lifestyle and ecological characteristics of certain animals. Zinc will be mixed in the enamel formation process, which can be used as a substitute index to understand the diet of animals and infer their nutritional level in the ecosystem. The researchers analyzed the zinc stable isotope ratios in the teeth of modern and fossil sharks from all over the world, including the teeth of giant toothed sharks and modern great white sharks. This new method allows scientists to investigate the nutritional level of animals and understand the position of animal food in the food chain. "We demonstrate for the first time that dietary zinc isotope characteristics are preserved in the highly mineralized enamel crown of shark tooth fossils." Thomas tutken, Professor of the Institute of Geosciences of Johannes Gutenberg University in Germany, said. Using the new method, the research team compared the tooth zinc isotope characteristics of several extinct species in the early Miocene (20.4 million to 16million years ago) and early Pliocene (5.3 million to 3.6 million years ago) with those of modern sharks. They have established a database of zinc isotope values of shark teeth, covering 20 existing shark species (including aquariums and wild individuals) and 13 fossil species, including megatoothed sharks. "Our results show that megatoothed sharks and their ancestors are indeed top predators and are at a high position in their respective food chains." Michael Griffiths, Professor of William Patterson University in the United States, said, "it is worth noting that the zinc isotope values from the teeth of Early Pliocene sharks in North Carolina show that the nutritional level of early great white sharks overlaps with that of larger giant toothed sharks to a large extent." The researchers compared the zinc isotope values of megatoothed shark and great white shark, and found that when they coexisted in the early Pliocene, their nutritional levels overlapped, and they may need to compete for the same food resources, such as marine mammals such as whales. Kenshu Shimada, a professor at DePaul University in Chicago, said, "although further research is needed, the results seem to support the possibility of diet competition between megatoothed sharks and great white sharks in the early Pliocene." (Xinhua News Agency)