Since 2004 scientists have spotted just four Durrell's vontsiras (including this one, pictured recently in Madagascar). Two were captured briefly, while another two have only been photographed.
Fa and colleagues had been working around Lac Alaotra for years, "and we never, ever heard anyone say anything about this species of mammal. So it was very surprising to see it swimming in the lake," Fa said.
The Durrell's vontsira closely resembles the brown-tailed vontsira, found in Madagascar's eastern rain forests, about 30 miles (50 kilometers) away (see a Masdgascar map). But the teeth of the new species appear to be more robust, and there are some differences in its paws, which may be adaptions to living near water, Fa said.
"From its dentition, we think it might be living off mollusks and crustaceans," he added.
The new species was discovered near the sight of the last sighting, in 1989, of an Alaotra grebe. The water bird, never seen in any other country, was officially declared extinct in April.
"The Alaotra grebe was the last species anywhere in the world to be declared extinct," said Frank Hawkins of Conservation International, a co-author of the new study describing the Durrell's vontsira. "Within 50 yards [160 feet] of where [the grebe] was last seen, we discovered another creature that wasn't known to science beforehand."
(See related carnivore pictures: "Cameras 'Trap' Hairy-Nosed Otter, More Rarities.")