They're each smaller than a coin—and in big trouble.

Three salamander species recently identified from Oaxaca, Mexico, are likely headed for extinction, scientists say.

The newfound amphibians belong to what may be the most endangered genus of amphibians in the world: Thorius. Although once extremely abundant, all known Thorius species have declined precipitously over the last three decades, and living specimens are now difficult to find in the wild.

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For that reason, nearly all the specimens examined in this study are museum specimens that are more than 35 years old. (See "Smallest Frogs Found—Each Tinier Than an M&M.")

Not only are the salamanders rare, they're so small that "many of their bones are extremely reduced and difficult to see,” says study co-author James Hanken of Harvard University. For instance, their skulls are about four millimeters long.

“We used a special x-ray scan ... that allowed us to visualize the skeletons in three dimensions," says Hanken, whose findings were published today in the journal PeerJ.

With these techniques, Hanken and his colleagues named the pine-dwelling minute salamander (Thorius pinicola), the long-tailed minute salamander (Thorius longicaudus), and the heroic minute salamander (Thorius tlaxiacus).

Small and Similar

Thorius salamanders live only in southern Mexico, mostly in high-altitude forests. Unlike most amphibians, they breed on land and the young hatch out of the eggs as miniaturized versions of the adults, with no tadpole stage. They have very short limbs and long tails, and usually have a red stripe on their backs. (See "'Extinct' Amphibians Rediscovered After Nearly Half a Century.")

Although they look very similar at first glance, the animals do have slight differences: For instance, adult T. tlaxiacus, at about an inch long, are relatively large and have long, oval-shaped nostrils, unlike their relatives.

View Images The heroic minute salamander (Thorius tlaxiacus) is among the most endangered salamanders on Earth. Photograph by James Hanken

“This particular group of salamanders is difficult to work with because of the challenges involved with finding them as well as their size and external similarities,” says Roy McDiarmid, an amphibian expert at the National Museum of Natural History who was not involved in the new research.

“Understanding what is going on when you have critters this small is difficult, but these researchers did a very nice job distinguishing new species.”

Insidious Phenomenon

Scientists aren't sure why the Mexican salamanders are vanishing. (Read more about vanishing amphibians in National Geographic magazine.)

“When the entire forest is cut down, it’s not a surprise that the animals that made their home there also disappear,” says Hanken. “But in the case of these salamanders, in some instances the forests are still intact and yet there are no salamanders. It’s an insidious phenomenon.”

Climate change, pollution, and disease may all be culprits, in addition to an overall shrinking habitat, he says.

The findings remind Hanken and his colleagues of the vast number of species that remain to be discovered and formally described.

“Much of the world’s biodiversity remains unknown,” says Hanken. “There are hundreds of thousands, probably millions, of undescribed species alive today.”