The spade-toothed beaked whale is so rare that nobody has seen one alive, but two whales that beached themselves in New Zealand in 2010 have now been identified as belonging to the species.
It was almost a missed opportunity, however, since conservation workers misidentified the carcasses as a much more common type of whale and buried them.
In a paper published yesterday in the journal Current Biology, researchers from New Zealand and the United States say of their discovery: "For the first time we have a description of the world's rarest and perhaps most enigmatic marine mammal."
Previously only three skull fragments of the species had been found: in New Zealand in 1872 and in the 1950s and the last one 26 years ago on an island off Chile. The males have broad, blade-like tusk teeth that give the species its name. Both males and females have beaks which make them resemble dolphins.
"This is pretty fantastic," said Ewan Fordyce, a geology professor at the University of Otago. "There would be few, if any, mammalian species in the world that would be rarer. And we know much more about pandas and other iconic, rare animals."
The beached whales, a 17ft (5.2m) adult and her 11ft male calf, were discovered on Opape Beach on the North Island on New Year's Eve in 2010. Conservation workers thought they were Gray's beaked whales and took tissue samples before burying them.