A species of frog not seen for 60 years and declared extinct, recently rediscovered in Israel, has been reclassified as a "living fossil," scientists say. Israel's Hula painted frog -- the first amphibian to have been officially declared extinct by the International Union for Conservation of Nature -- had not been seen since the drying up of the Hula Lake at the end of the 1950s, but in 2011 one was found lurking in a patch of swampy undergrowth, Hebrew University of Jerusalem reported Tuesday. Scientists said they believe the frogs remained hidden in patches of the swampy habitat that remained as the lake dried, and 13 additional specimens have been found since 2011. Tests have revealed the frog belongs to a group of amphibians that died out 15,000 years ago and differs strongly from its only living relatives, the painted frogs of northern and western Africa. "We felt like we had a great finding when we first rediscovered it -- this [frog] was like an idol in Israel," Sarig Gafny, from the Ruppin Academic Center in Israel, told the BBC. "But then we found it was a living fossil: This was amazing." Although describing the frog as "a strikingly resilient" species, researchers said it was vital to ensure its future survival. Work to reflood parts of the Hula valley would help create habitat where it could thrive, they said.