Weapons from the Gilbert Islands contain the teeth of shark species no longer found in the area. Photo by J. DREW/COLUMBIA UNIV.

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“This is shadow biodiversity,” said Drew, presenting his results at the 2012 Ecological Society of America Annual Meeting in Portland, Oregon, last week. “Three sharks disappeared from these reefs before we even knew that they existed there.”

Drew analysed 124 shark-tooth weapons housed in the collection of the Field Museum of Natural History in Chicago, Illinois. The artefacts included swords, tridents and a 4-metre-long lance, dating back over 120 years.

All are built in the same way: the islanders drill hole in the teeth and lash them to buttresses of wood with cords made from coconut leaves.