Charles Darwin’s theory of evolution was formed during his trip to the Galapagos Islands. It was there that he noticed that species tended to adapt to their environment and that the animals on one island would sometimes have very different features from related animals on the next island.

If isolation causes genetic change, that may help to explain researchers’ difficulty in identifying the family tree of the catfish Kryptoglanis shajii. The tiny fish, about as long as a human pinkie finger, is known to exist only in the Western Ghats mountain region of Kerala, India. It lives primarily underground, only occasionally turning up in springs, wells and flooded rice paddies.

Recently two different groups of researchers took a closer look at the fish. One team was led by Raif Britz of the Natural History Museum of London, the other by John Lundberg, emeritus curator of Ichthyology at the Academy of Natural Sciences of Drexel University.

Lundberg’s team used, among other things, digital radiography and high-definition CAT scans. The closer they looked, the more mysterious the fish became.

“The characteristics of this animal are just so different that we have a hard time fitting it into the family tree of catfishes,” said Lundberg in a statement.

The Kryptoglanis is less than ten centimeters long and, like many subterranean fish, is missing several bony elements. There were also changes in bone structure that Ludenberg believes are unique among catfish. The animal has a jutting lower jaw, like a bulldog’s which gives it a compressed front end. The jaw also contains four rows of sharp, conical teeth.

“In dogs that was the result of selective breeding. In Kryptoglanis, we don’t know yet what in their natural evolution would have led to this modified shape,” Lundberg said.

The researchers believe, based on its jaw, that the animal is a carnivore, most likely feeding on small invertebrates and insect larvae.

Neither Lundberg’s team nor Britz’s were able to determine which catfish the Kryptoglanis is most closely related to, even with DNA evidence. Britz’s team chose to give the animal its own taxonomic family within the order of catfishes.

However, given the animal’s isolation and habitat it might be difficult to find Kryptoglanis’ evolutionary tree. The catfish’s ancestors may be buried beneath the subterranean waterways where it lives.

The work of Lundberg and his colleagues was published in the 2014 issue of the Proceedings of the Academy of Natural Sciences of Philadelphia.