With a mouth you could easily step through and serrated teeth as big as your hand, megalodon was the biggest shark the world's seen.
But around 4 million years ago, this formidable apex predator died out. What was behind its downfall?
A clue may be in a study published in Nature Communications today, which suggests other sharks — namely great whites — also feasted on megalodon's preferred prey around the time it disappeared.
This competition for prey did not mean great whites solely caused megalodon's extinction, said study author and University of Frankfurt geoscientist Jeremy McCormack, but was probably one of many pressures that eventually condemned megalodon to a fossil relic.
And ultimately, megalodon's truly mega dimensions didn't help matters either, according to Erich Fitzgerald, senior curator of vertebrate palaeontology at Museums Victoria, who was not involved in the study.
"If you're larger, you're more on an ecological knife-edge — all apex predators are," Dr Fitzgerald said.
'Big tooth' by name …
Megalodon (Otodus megalodon or Carcharocles megalodon) had a pretty good run. Fossils showed it lived from around 20 million until 3.6 million years ago.
Being a shark, megalodon had a cartilage skeleton, which tends to decay rather than fossilise.
But what did survive the aeons were its enormous teeth.
Palaeontologists can infer a fair bit about the colossal carnivore from these ancient chompers. For instance, they're sharp and pointy, perfect for tearing flesh.
But the chemistry of megalodon's teeth — and other shark teeth — can tell us more specifically what was on the menu at the time.
Unlike us, who only have two sets of teeth for our whole life, sharks constantly lose and grow whole new rows of teeth.
The hard enamel layers of these growing teeth incorporate chemical elements, such as zinc, from the shark's food.
So when those teeth fossilise, some chemical elements stay locked away, and can be analysed today as a proxy for their diet.
Which was what Dr McCormack and his team did using a relatively new technique called zinc isotope ratio analysis.
Using a dentist's drill, they shaved off small pieces of enamel from the teeth.
Then, using lab equipment that can identify molecules in a sample, they looked for two types of zinc — one ever-so-slightly heavier than the other — and how much of each was present in the teeth.
This gives the zinc isotope ratio. And a diet rich in fish and squid would show up as a slightly different zinc isotope ratio compared to a diet mostly made up of, say, seals, Dr McCormack said.
The researchers first analysed teeth from 20 living shark species to see if they could, indeed, match zinc isotope ratios with diet.
It worked. Next, they used the same technique on teeth from 13 extinct shark species, including megalodon.
Around the time megalodon died out, they and great whites were eating the same kind of food — large, warm-blooded animals, probably whales.
This matches fossilised whale bones bearing megalodon bite marks, and sometimes the snapped-off tips of megalodon teeth still embedded in them.
So, what finished megalodon off?
University of New England palaeontologist Nicolas Campione, who was not involved in the study, said while there was undoubtedly overlap in megalodon and great white prey 3.6 million years ago, this competition alone wasn't the primary cause of the mega shark's demise.
There was more going on in the world at the time.
"The heyday of megalodon was actually a pretty nice time. Temperatures were pretty equable," Dr Campione said.
As Earth eased further into an ice age, ice sheets grew in both hemispheres, changing ocean currents and lowering sea levels.
This meant shallow coastal areas — where megalodon's prey liked to live — receded too, Dr Fitzgerald said.
"What you suddenly do is you wipe out richly productive, shallow coastal seas, and all that potential area of habitat [for small whales]."
This time coincides with lots of whale species, particularly smaller ones, dying out.
"Potentially, you're knocking out or at least decreasing the diversity and abundance of a pretty significant packet of prey for these predators," Dr Fitzgerald said.
So why did megalodon die out, and not great whites?
In essence, size mattered.
To sustain a body three times the length of a great white, a megalodon needed more food — a lot more, Dr Campione said.
We don't know how many babies megalodon had or how often, Dr Campione added, "but often the larger you are, the harder it is for you to maintain a stable population.
"You need to reproduce more, of course, but more individuals require more resources.
"All of those factors play in a lot more into long-term extinction processes rather than competition itself."
And in the end, it was all too much for megalodon.
Reconstructing ancient time
Dr McCormack concedes there are gaps in the shark fossil record story, but using the zinc isotope ratio analysis technique on more teeth should fill in some of those blanks.
"Certainly down the road, if we continue this type of research with more [tooth] samples to fill gaps in time, we'll hopefully have a stronger case, in terms of reconstructing the [history] of extinct sharks.
Dr Fitzgerald, for one, is keen to see more zinc isotope results.
"I now want to see what this [technique] can do with marine mammals, specifically whale and seal evolution, because when you put all that together, it's an even more fascinating story."
