An orange-bellied frog with a brown back, covered in tiny spots that resemble a starry sky, has been discovered in a mountain range in India, surprising researchers who said its ancestors branched off on the evolutionary tree from other members of the same frog family tens of millions of years ago.

The frog, which is about 2cm to 3cm long, has been named Astrobatrachus kurichiyana, although some might prefer its more rock-star sobriquet: “starry dwarf frog.”

Dr Alex Pyron, a co-author of the study into the frog at George Washington University, said: “Astrobatrachus is from the Greek for star frog, and so we named it after the spots that sort of look like stars, and kurichiyana is the name of the local peoples in this area where it was found.”

Writing in the journal PeerJ, the team of researchers from the US and India said they first came across the creatures in 2010 while exploring a hill range called Kurichiyarmala in India’s Western Ghats mountain range.

The researchers explained they were working at night to survey amphibians and reptiles when they spotted the frogs on the forest floor and in adjacent grassland, adding that they were generally found lurking beneath leaf litter.

“Because individuals were secretive and difficult to spot, sampling involved an intensive search of the forest floor,” the authors wrote. “Individuals were found to be shy of torchlight and, upon disturbance, made quick hopping movements to hide.”

The team said the frog’s appearance was unusual. “When [the others] first saw it, they immediately knew that it was something unusual that hadn’t been seen before,” Pyron said.

Through a genetic analysis, the team said they worked out the starry dwarf frog was not only a new species, but the sole member of a whole new subfamily of frogs within the Nyctibatrachidae family.

“It may have had other relatives in the past that have since died out, but this is the sole living representative,” Pyron said.

The team said it was an example of an “ancient lineage”, with the last common ancestor of the starry dwarf frog and its closest living species thought to have lived somewhere in the region of 57m–76m years ago.

“It fills in a gap in our knowledge of what the ancient history of frogs in India looked like,” said Pyron, adding that it increased the period of time for which ancestors of the Nyctibatrachidae frog family have been in the area.

The team said other frogs in the area also have an ancient lineage, and the biodiversity in the Western Ghats is down to a combination of factors, including the broad diversity of species present when India broke away from other parts of the ancient supercontinent of Gondwana, as well as the wide range of habitats provided by mountain ranges.

“Having an ancient tropical mountain range like the Western Ghats is relatively rare, there are only a few places like that, so they tend to harbour very ancient, very diverse and very rich assemblages of plants and animals,” Pyron said.