When life gives you lemons, make lemonade.

When life sticks you on an isolated island surrounded by shark-infested waters, make utterly badass weapons out of shark teeth.

This is what the people of the Pacific Gilbert Islands have been doing for centuries. Sharks are a central part of their lives. Many social customs and taboos revolve around the finer points of shark-hunting. Young boys go through initiation rites where they kneel on a beach, looking towards a rising sun and slice their hairlines open with shark teeth, letting the blood run into their eyes until sunset. And with no metal around, they used shark teeth to adorn their weapons.

A shark is a fast, electric-sensing torpedo, whose business end holds two conveyor belts of regenerating steak knives. To further weaponise its weapons is practically the definition of being badass. Here’s how to do it: You drill a tiny hole in each tooth, and bind them in long rows to a piece of wood, using braided coconut fibres and human hair. Depending on the shape of the wood, you can make a sword. Or a dagger. Or a trident. Or a four-metre-long lance. And then, presumably, you hit people really hard with them.

View Images Shark-tooth sword (left) and a puffer-fish helmet (right). Because, obviously, how else would you defend yourself against someone running at you with a shark-tooth sword? Joshua Drew, Columbia University.

No one knows when the Gilbertese first fashioned these arms, but they were already doing so by the time the first Western sailors arrived on the islands in the late 18th century. Many of them ended up in museums and Chicago’s Field Museum of Natural History has a particularly rich collection. It includes 124 specimens, including swords, tridents and a lance that Joshua Drew from Columbia University describes as “2.5 interns tall”.

Drew saw this collection was more than just an amazing armoury. It was also a time capsule. Since every item was carefully tagged with date and place of collection, and since different shark species have distinctively shaped teeth, Drew could use the weapons to identify the sharks that swam round the Gilbert Islands in centuries past.

The teeth came from 8 different species. Tiger sharks feature heavily—their thick, cleaver-like teeth, which can punch through turtle shells, make for good cutting edges. Most of the weapons featured teeth from just one shark species, but several have a rare blue shark tooth in the penultimate position—possibly the signature of an artisan.

But the biggest surprise was that some of the teeth belonged to two species—the dusky and spottail sharks—which no longer exist near the Gilberts!

Back then, they were common enough that their teeth were among the most popular choices for weaponsmiths. Today, no one has seen them within several thousand kilometres of the islands. Even before scientists knew that they were there… they weren’t any more.

(Drew has also found teeth from a third missing species—the bignose shark—on a weapon held at the American Museum of Natural History.)

View Images Shark tooth trident, by Joshua Drew, Columbia University.

Could these teeth have been imported from neighbouring people? No, says Drew. The Gilbertese had a strong culture of shark-fishing and if they were already heavily catching sharks, why would they trade for teeth? Besides, there is no historical, linguistic or archaeological evidence that these people communicated with those who live in the areas where those missing sharks are now found.

Could it be that the three species still live near the Gilberts but that no one has seen them? Again, it’s unlikely. All three are quite common in the areas where they actually live, so it’s doubtful that biologists have simply missed them.

It’s not clear why the sharks disappeared. Humans may well have been responsible—people were hacking off shark fins in the Gilbert Islands as far back as 1910 and by the 1950s, around 3,000 kilograms of fins were being shipped from the islands ever year. For sharks, many of which grow and reproduce slowly, it doesn’t take long for finning operations to drive a population locally extinct.

Whatever the reason, the teeth are signs of what Drew describes as “shadow diversity”—fleeting ghosts of the vivid splendour that once existed in the same waters. “Today’s Gilbertese live in a fundamentally duller environment than their forefathers,” he says.

That’s an important reminder for conservationists. Both coral reefs and shark populations are under severe threat and scientists are working on ways of restoring them. But what state are we going to restore them to? “Nested within this story is a cautionary tale of… how what we see today is not necessary indicative of the past,” Drew writes. We must not succumb to a “cultural amnesia, where people forget how vibrant reefs really were. “

View Images Shark tooth weapon close-up. By Joshua Drew, Columbia University.

Note: I originally covered this research for Nature News when it was presented at the 2012 Ecological Society of America Annual Meeting in August, 2012. Here is the original piece.