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Shark-tooth weapons once used for warfare in the Central Pacific suggest that two extinct shark species used to populate the area, a new study says.

Joshua Drew from Columbia University in New York and colleagues from the Field Museumin Chicago scoured natural history museums for the spiky swords, as well as clubs, daggers, lances and spears. The teeth lashed to this sword with coconut fibers and human hair offer evidence of past ecosystems, before written records. The study was published today in PLOS ONE.

The unusual historical data would help evaluate the success of ecological conservation measures, they added. The researchers said that the indigenous artifacts often represented an "under-utilized source of data".

"Initially, we just wanted to catalog what shark species were there. We didn't suspect that two of them would be gone," said study co-author Joshua Drew, a biologist at New York's Columbia University and the Field Museum of Natural History in Chicago.

"Had we never done this work, nobody would have ever known that these things ever existed there. It had been erased from our collective memories that these sharks once plied these waters," said Drew. 

"I just wanted to do something cool and different," said Drew, now at Columbia University. "I just wanted to go down and look at really cool stuff. We were just going to see what was there."

The researchers compared some 100 teeth to examples in catalogs, photographs and in real jaws, then searched the historical and contemporary record of species found in the Gilberts. There was no mention of spot-tail (Carcharhinus sorrah) and dusky (Carcharhinus obscurus) sharks.