Text by David McGuire

Imagine witnessing a white spot moving quickly up from below. From a blur of white and gray, it turns into an open mouth, two black eyes, and large wing-like fins. In three seconds, the image transforms from a vague spot to a great white grin. Five rows (with two protruding rows) of around 300 triangular serrated teeth line a jaw of cartilage around one meter (three feet) wide in a mature great white shark sized around 5 to 6 meters (16.5 to 20 feet).

Now, imagine a shark three to four times larger, with a mouth 2.9 to 3.7 meters (9.5 to 12.1 feet) wide with teeth 18 centimeters (9.1 inches) long, and with distinct serrations.

Based on descriptions from the fossil record, megalodon (Greek for "big tooth") had six to seven rows of teeth. The front row of teeth numbered around 46, with 24 in the upper jaw and 22 in the lower, with a total of about 276 teeth.

This magnificent shark was the apex predator of the sea, until it went extinct about 2.6 million years ago.

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Why did it go extinct?

Scientists are uncertain as to what caused the megalodon to go extinct, but the demise of this shark occurred during a period of cooling and drying in many parts of the world at the end of the Pliocene—perhaps impacted its range and the availability of prey species. Some scientists have proposed that competition for the same food source may have been a contributing factor.

A 2019 study suggested the possibility that the evolution of the great white shark (Carcharodon carcharias) may have contributed to the extinction of megalodon.

Now, a new study published in the journal Nature Communications has added to the theory that great white sharks may have helped push their larger cousins forever into the fossilized records.

The truth is in the tooth

Teeth fossilize well

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Shark teeth are heavily mineralized and preserve well as fossils. Given that sharks can shed thousands of teeth over their lifetimes, they provide unique clues into the past.

The team led by researchers at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, measured zinc isotopes in fossilized teeth from both megalodon and white sharks.

Using teeth from more than a hundred sharks, drawing from species alive today and fossilized, the researchers confirmed that the zinc isotope values of sharks living today reflect their place in the ecosystem.

This work is based on a previous study showing zinc isotope ratios as indicators of diet and trophic levels in arctic marine mammals.

The zinc isotope levels in the teeth of present-day mammals correlate with where they are in the food chain. The higher up the food chain an animal is, the lower the zinc isotope values they show. For example, sharks that eat small fish have higher values of zinc than sharks that are higher in the food chain that eat whales.

Based on mineral analysis, the authors suggest that ancient great white sharks consumed the same kinds of prey that the much larger shark. If the great white was eating the same kinds of prey, then the smaller sharks may have competed with the megalodon for food. If so, they might have contributed to its eventual demise, alongside potential changes in other aspects of the ecosystem, like climate.

This evidence helps to support the theory that competition with the great white shark might have been one factor that removed the mighty megalodon forever from the high seas.

Cousins, not descendants

The similarity in shape and design of teeth between white sharks and megalodons has led to the perception that modern-day white sharks are the smaller counterparts that evolved from megalodons. Given the absence of a complete fossil record, scientists built the megalodon image into a great white shark on steroids. This image of a ferocious, ship-swallowing super-predator has intrigued many, and fueled many a Shark Week episode and films like The Meg.

The relationship between white sharks and megalodon was first formalized by the Swiss naturalist Louis Agassiz who first named the megalodon (Carcharodon megalodon) in 1843. Agassiz observed that great white shark teeth and the fossil megalodon teeth were both serrated, and placed megalodon into the same genus as white sharks, Carcharodon.

However, new data analyzing tooth morphology and root type confirmed that the megalodon is not ancestral to the great white shark. Hence, megalodon was placed into a new genus, Carcharocles. Further investigation and analysis of dentition led scientists to again reclassify megalodon into the genus Otodon, more closely related to the extinct O. obliquus and represents a separate lineage from the great white shark.

White sharks and makos are closer

Instead, it is believed that white sharks are more closely related to mako sharks, both in the family Laminidae. Fossils of a newfound species of shark, Carcharodon hubbelli, suggest the modern great white shark actually may have descended from broad-toothed mako sharks. 

Using computer-assisted imaging and measurement methods, Kevin Nyberg and Gregory Allan Wray of Duke University and Charles Ciampaglio of Wright State University examined the similarities and differences among great white, megalodon and extinct mako teeth. They determined that the extinct mako and great white teeth and roots were similar in shape and were clearly distinct from megalodon.

On the other hand, white sharks evolved from a more moderate-size, smooth-toothed relative of mako sharks.

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So much attention is given to great white sharks, much of it negative. As we discover the relationships of sharks over time and their role in marine ecosystems, we cannot overlook the current crisis facing many species of sharks.

Over one-third of shark species are threatened with extinction largely due to overfishing. If we do not act now, many modern-day sharks will follow the megalodon, and within a far shorter time span.