Scientists are baffled about why the copper-striped blue-tailed skink has become extinct from the Hawaiian Islands — and might have been gone for decades without much ado.

The skink, scientifically known as Emoia impar, was once common throughout Hawaii.

But the polished-skin lizard with its long, sky-blue tail was last seen in 1960s on the Na Pali coast of Kauai, and has now been declared extinct in Hawaii by the U.S. Geological Survey.

Repeated field surveys on Kauai, Oahu, Maui and Hawaii island from 1988 to 2008 have yielded no sightings or specimens, the USGS said Tuesday in a news release.

"Today we close the book on one more animal that is unlikely to ever be re-established in this fragile island home," said USGS Director Marcia McNutt.

McNutt said extinctions and endangered species have been associated with the Hawaiian Islands like no other landscape in the country, and that the long-term consequences are unknown.

According to the Bishop Museum website, Hawaii is the "endangered species capital of the world," with hundreds of species of plants and animals listed as endangered and more than 270 birds, plants, snails and insects extinct.

At one point in the late 1990s, two-thirds of all the plants and birds known to have gone extinct in the United States were from Hawaii, according to a University of Hawaii study.

Robert Fisher, a USGS biologist, said the skink was in the Hawaiian Islands before European contact more than 230 years ago and could have been in Hawaii prior to Polynesian contact or introduced during the Polynesian migration.

Other Pacific islands still have members of the same skink species, which are common and a little more than about two to three inches long.

"That’s what makes this extinction so intriguing," he said. "If an otherwise common animal can be completely extirpated from one island ecosystem but not others, then what does that tell us?"

Fisher said the skink started disappearing between 1900 and 1910, even before a similar skink from Australia made its way to Hawaii.

Fisher said he and other scientists are looking at whether the alien bigheaded ant, entering Hawaii and perhaps competing with lizards for similar food, might have been the cause of the precipitous decrease.

The same species of skink exists in Chuuk, Tonga, the Cook Islands and Samoa.

Fisher said Samoa has some invasive ants as well as native ants, which may explain the skink’s ability to continue to exist outside of Hawaii. "The ecosystem there evolved with ants, whereas the ecosystem in Hawaii did not," he said.

He said lizards play an important role in consuming insects and as a source of food for carnivorous birds.

Fisher and colleague Ivan Ineich of the French national museum of natural history in Paris announced their findings on the skink this month in the conservation journal Oryx, published by Fauna and Flora International.

Unlike that of Hawaiian bird species, the disappearance of the skink went unnoticed for decades because it was easily confused with an alien Australian species, they said.

Ineich said that without regular field surveys, the disappearances of smaller, secretive species can be overlooked.