A team led by Jeremy McCormack at the Max Planck Institute for Evolutionary Anthropology has found evidence that competition with great white sharks may have contributed to the extinction of the megalodon, the largest shark that ever lived.
Living in oceans all over the world 23 to 3.6 million years ago, Otodus megalodon and other megatooth sharks grew to lengths of up to 20 m (65 ft) and weighed up to 103 tonnes, making them arguably the most powerful predators in history. By contrast, the famous great white shark only reaches a length of 6 m (20 ft) and weighs in at around 2,268 kg (5,000 lb), but recent studies of zinc isotopes in the teeth of modern and fossil great whites and fossil megalodons have provided insights into the diet of the ancient sharks that show they all went after very similar prey.
Isotope analysis is a very powerful tool in understanding diet because differences in the ratios of isotopes of one element or another can trace the sources of the foods an animal has consumed, the nature of its diet, and where it sits in the food chain. This not only allows scientists to deduce facts about the animal's lifestyle, but also helps to shed light on mysteries like what part diet and prey competition played in the gigantism and extinction of the megalodon.
Usually, researchers want to look at the isotopes found in organic matter, especially the isotopes of nitrogen in proteins found in bones. The problem is that sharks are part of the ancient order of cartilaginous fish that split off the evolutionary path from bony fish hundreds of millions of years ago. As a result, sharks have very little in the way of bony matter in their bodies.
Case in point is the megalodon. Though it has become well known in the last half century and has starred in a number of movies (with more to come), the only evidence for their existence comes from teeth and a few vertebrae fragments. Worse, these fossils are millions of years old, so any organic matter has long been replaced by minerals or has degraded so far as to be useless for analysis.
To learn more about the megalodon, the Max Planck team gathered isotope samples from modern and fossil shark teeth from various places around the globe. Their focus was on the enameloid, which is the highly mineralized part of the teeth and acts as a substitute for the collagen found in the dentine of living teeth for the purpose of sampling.
Looking at shark teeth from the Early Miocene (20.4 to 16.0 million years ago) and Early Pliocene (5.3 to 3.6 million years ago) and then comparing them to modern sharks, the team found that the modern and ancient zinc isotope measurements were nearly identical, suggesting that they had a high level of reliability.
They also found that, like the great white, the megalodon was at the top of the food chain and ate the same prey as the great white.
"Our results show that both megalodon and its ancestor were indeed apex predators, feeding high up their respective food chains," said Michael Griffiths, professor at the 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."
In other words, the great whites may have been in competition for food with the megalodons, which could have contributed to the latter's decline in the Early Pliocene. Exactly what the nature of this competition took is unclear. It also raises the question of how intense this competition was because the two species may have fed on the same grounds at different seasons. However, the study shows that the zinc technique is a promising tool.
"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," said McCormack.
Source: Max Planck Institute
