The event took place five years ago in Kerala, a mountainous tropical region in Southwest India.

Farm workers were digging a ditch in the middle of a cardamom plantation when one of them suddenly found himself face to face with a strange animal buried nearly two metres deep.

It was obviously a frog, but what a peculiar specimen! The batrachian was violet in colour with a bloated body, podgy legs and pointed snout...

Intrigued by this bizarre discovery, the farm worker handed the animal over to S. D. Biju, a research scientist at the Thiruvananthapuram Tropical Botanic Garden and Research Institute. But neither the biologist nor the other researchers at the Institute could identify it: the frog was like none other identified so far!

«It's not easy to identify a frog, admits Anne-Marie Ohler, a researcher at the Laboratory of Herpetology at the National Natural History Museum. Batrachians generally display a very high level of adaptation to their environment, independently of their membership of a given group. »« In other words, nothing is more like a tree frog than another tree frog, even if the two amphibians belong to very distant species. »

The frog discovered in Kerala presents many analogies with certain African burrowing species. Like the latter, it shows a very high degree of adaptation to underground life. It also shares characteristics with moles and shrews: small eyes, a tiny mouth, a pointed snout, feet adapted to burrowing and not swimming…

Despite all these points, detailed examination showed that the frog did not belong to the line of African burrowing frogs.

A first surprise: this Indian frog was a genuine living fossil! While 96% of the 4,800 species of contemporary frog belong to the Neobatrachia group ("modern frogs"), phylogenetic analysis showed that the Kerala frog split off from this group 180 million years ago, when dinosaurs walked the Earth.

So now the new frog was named* Nasikabatrachus sahyadrensis, from nasika, – 'nose' in Sanskrit – batrachus, – 'frog' in Greek – and Sahyadri, – one of the names for the Western Ghâts, the mountains where the animal was found.

Since it did not belong to any known group of frogs, S. D. Biju and Franky Bossuyt also decided to create a new family of batrachians for it: Nasikabatrachidae. It was the first time since 1926, that a discovery had led to the creation of a new family of frogs.

* "Nature" vol. 425, p. 711

DNA analysis brought a second surprise. Nasikabatrachus had close relatives… in the Seychelles! This came as a complete shock to the researchers, since insular frogs belonging to the Sooglossidae family (which currently includes four species) are radically different to Nasikabatrachus. They are tiny and live in torrents, so are not suited to a burrowing life.

So how can frogs separated by 3,000 kilometres of ocean be close cousins? Naturally, Nasikabatrachus did not swim across the Indian Ocean. The answer lies in plate tectonics.

According to the hypothesis put forward by the researchers, the common ancestors of these two families lived in Gondwana, one of the two super-continents that emerged from the ocean 200 million years ago. When it split about 160 million years ago, there were frogs on the shared India-Seychelles subcontinent. When this land also split 65 million years ago, the frogs cut off in the Seychelles evolved independently, resulting in the Sooglossidae family. Those that remained on the Indian subcontinent were the ancestors of Nasikabatrachidae.

There remains a detail that could be seen as contradicting this hypothesis: genetic analysis shows that the separation of the two families of frogs occurred 130 million years ago, i.e. well before the split between India and the Seychelles. « This only seems to be contradictory,explains Annemarie Ohler. . It simply shows that differing natural environments in the India-Seychelles bloc led to the separation of the two families of frog. The split between the two subcontinents simply cut them off permanently. »

Olivier Boulanger