Three new species of minuscule salamanders have been discovered ... and they're already headed for extinction.

The long-tailed minute salamander (Thorius longicaudus), the pine-dwelling minute salamander (Thorius pinicola) and the heroic minute salamander (Thorius tlaxiacus) were described in a PeerJ study published earlier this week.

Each of the newly identified species is smaller than a coin – in fact, they're so small that "many of their bones are extremely reduced and difficult to see", as Harvard biologist and co-author of the study Dr James Hanken explained to National Geographic. Their skulls, for example, are only about four millimetres long. As a result, researchers had to use special x-ray scanning techniques in order to map and visualise their bone structure. 

Thorius longicaudus, one of the newly described species of minute salamander. Image: Mario García-París
Thorius pinicola, or pine-dwelling minute salamander. Image: Mario García-París
A juvenile Thorius narisovalis, another, already known, species of Thorius. Shown on a blade of grass for scale. Image: James Hanken

The tiny salamanders were found in the state of Oaxaca in southern Mexico, which is one of the few locations where their particular genus of amphibians – Thorius – can be found. They now join 26 other species under the Thorius umbrella, which includes the tiniest known tailed tetrapods in existence.

Thorius salamanders are known for their strange proportions and sometimes bizarre anatomical configurations. Their tongues, for example, can shoot out up to half their total body length to catch insects, and their sex organs are comparatively oversized. "[T]he gonads occupy a relatively large proportion of the total body volume," Hanken explained in a Gizmodo interview. "In some females, much more than half the volume of the trunk is occupied by yolky eggs inside the oviducts; the rest of the visceral organs are pushed aside."

And the unusual features don't stop there. Unlike most amphibians, these salamanders breed on land rather than in water, and when their eggs hatch, young Thorius salamanders emerge as miniature versions of the adults. This means skipping the tadpole stage that most other amphibians go through after hatching. 

Unfortunately, the Thorius genus stands out for other – much more troubling – reasons, too: it's possibly the world's most endangered genus of amphibians, with many of its living species at serious risk of extinction within the next 50 years. Once abundant, these tiny critters have been rapidly dying off in the past three decades or so, which makes finding living specimens in the wild very difficult. Of the nearly 30 species now recognised, almost all are regarded as Endangered or Critically Endangered by IUCN. And there's no evidence to suggest that these three newly discovered species will buck the trend.

Researchers are also concerned that the exact reasons for the rapid die-off remain unclear. Although some common culprits can be pointed to, such as habitat destruction and shrinking breeding ranges, the salamanders have been disappearing even in places where they should persist. Other possible offenders include pollution, climate change and disease. For Hanken, the situation should serve as a wake-up call for everyone, not just scientists.

"In some respects amphibians are the canary in the coal mine," he warned in an interview with the New York Times. "The same environmental insults that are hammering them are hammering other species, including humans … It's not just salamander freaks who should be concerned."


Top header image: Thorius salamander species (Parra-Olea et al., 2016)