A newly discovered eel that inhabits an undersea cave in the Pacific Ocean has been dubbed a "living fossil" because of its primitive features.
It is so distinct, scientists created a new taxonomic family to describe its relationship to other eels.
The US-Palauan-Japanese team say the eel's features suggest it has a long and independent evolutionary history stretching back 200m years.
Details appear in the journal Proceedings of the Royal Society B.
The animal used as the basis for the new study was an 18cm-long female, collected by one of the researchers during a dive at a 35m-deep cave in the Republic of Palau.
But the scientists also mention other examples of the new eel species in their research paper.
At first there was much discussion among the researchers about the animal's affinities. But genetic analysis confirmed that the fish was a "true" eel - albeit a primitive one.
"In some features it is more primitive than recent eels, and in others, even more primitive than the oldest known fossil eels, suggesting that it represents a 'living fossil' without a known fossil record," write the scientists.
In order to classify the new animal, the researchers had to create a new family, genus and species, bestowing on the animal the latin name Protoanguilla palau.
The team - including Masaki Miya from Chiba's Natural History Museum in Japan, Jiro Sakaue from the Southern Marine Laboratory in Palau and G David Johnson from the Smithsonian Institute in Washington DC - drew up a family tree of different eels, showing the relationships between them.
This allowed them to estimate when the ancestors of P. palau split away from other types of eel.
Their results suggest this new family has been evolving independently for the last 200m years, placing their origins in the early Mesozoic era, when dinosaurs were beginning their domination of the planet.
The researchers say the Protoanguilla lineage must have once been more widely distributed, because the undersea ridge where its cave home is located is between 60 and 70 million years old.
By Paul Rincon Science editor, BBC News website
http://www.bbc.co.uk/news/science-environment-14547942
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