A newly discovered eel¹ that inhabits an undersea cave in the Pacific Ocean has been dubbed² a "living fossil" because of its primitive³ features.

一种最近在太平洋海底洞穴被发现的鳝鱼因其原始的特征而被冠以“活化石”称号。

’living fossil‘ Protoanguilla palau
It is so distinct, scientists created a new taxonomic（分类的） family to describe its relationship to other eels⁴.

The US-Palauan-Japanese team say the eel's features suggest it has a long and independent evolutionary⁵ history stretching back 200m years.

Details appear in the journal Proceedings⁶ 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 affinities⁷（密切关系，类同） . But genetic⁸ analysis confirmed that the fish was a "true" eel - albeit⁹（虽然，即使） 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, bestowing¹⁰ 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 Marine¹¹ 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 dinosaurs¹² were beginning their domination of the planet.

The researchers say the Protoanguilla lineage must have once been more widely distributed, because the undersea ridge¹³ where its cave home is located is between 60 and 70 million years old.