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Great white sharks may have sped up Megalodon extinction

Megalodons (Otodus megalodon) were megatooth sharks that swam the world’s oceans between 23 and 3.6 million years ago. They reached enormous proportions, some measuring as much as 20 meters, and their giant teeth are found regularly in fossil deposits. In comparison, the largest great white sharks today reach a total length of only six meters. There has been much discussion of the reasons for this gigantism, as well as for the eventual extinction of the giant shark species. Competition for food has been proposed as a possible contributory factor.

In a study published today in the journal Nature Communications, scientists have used a rather novel way in which to assess the diet of extinct megalodons. Diet is usually determined using the ratios of stable isotopes of nitrogen, or carbon, in the tooth collagen or dentine of an animal. These ratios are affected by the relative amounts of meat and plant matter in the animal’s diet. However, in the case of fossilized animals, the organic proteins have decomposed and mineralized, and may not be suitable for analysis any more. 

In the current study the researchers analyzed the ratios of stable zinc isotopes (64Zn / 66Zn) in the enamel of fossil and modern shark teeth from around the world, in order to determine the trophic level at which the animal fed. Previous research has shown that carnivores have a lower ratio of the two zinc isotopes, herbivores have the highest, and omnivores are somewhere in the middle. Zinc stable isotope analysis of tooth enamel tissue, the highly mineralized part of teeth, is comparable to nitrogen isotope analysis of tooth collagen, but is a less established method.

“On the timescales we investigate, collagen is not preserved, and traditional nitrogen isotope analysis is therefore not possible,” explains lead author Jeremy McCormack, a researcher at the Max Planck Institute for Evolutionary Anthropology

“Here, we demonstrate, for the first time, that diet-related zinc isotope signatures are preserved in the highly mineralized enameloid crown of fossil shark teeth,” added Professor Thomas Tütken of the Johannes Gutenberg University’s Institute of Geosciences.

Using this new method, the researchers compared the tooth zinc isotope signature of multiple extinct Early Miocene (20.4 to 16.0 million years ago) and Early Pliocene (5.3 to 3.6 million years ago) species with those of modern sharks. 

“We noticed a coherence of zinc isotope signals in fossil and modern analogue taxa, which boosts our confidence in the method and suggests that there may be minimal differences in zinc isotope values at the base of marine food webs, a confounding factor for nitrogen isotope studies,” explained Sora Kim, a professor at the University of California Merced.

The researchers also analyzed the zinc isotope ratios in megalodon teeth from the Early Pliocene and those in the teeth of earlier megatooth sharks, Otodus chubutensis, from the Early Miocene. They compared the results with those from modern great white sharks in order to understand the position of these various species in their respective food webs.

“Our results show, that both megalodon and its ancestor were indeed apex predators, feeding high up their respective food chains,” said Professor Michael Griffiths of William Paterson University. “But what was truly remarkable is that zinc isotope values from Early Pliocene shark teeth from North Carolina, suggest largely overlapping trophic levels of early great white sharks with the much larger megalodon.”

“These results likely imply at least some overlap in prey hunted by both shark species,” noted Professor Kenshu Shimada at DePaul University, Chicago. “While additional research is needed, our results appear to support the possibility for dietary competition of megalodon with Early Pliocene great white sharks.”

New isotope methods such as zinc can potentially provide a unique window into the past, as illustrated by this current study. “Our research illustrates the feasibility of using zinc isotopes to investigate the diet and trophic ecology of extinct animals over millions of years, a method that can also be applied to other groups of fossil animals including our own ancestors,” concluded McCormack.

By Alison Bosman, Staff Writer

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