In an undersea cavern off the coast of Palau, biologists have discovered an eel unlike any other.

Called Protoanguilla palau, its last common ancestor with any other living creature swam 200 million years ago. Not only is there nothing like it alive, there’s nothing like it in the fossil record. Protoanguilla means “first eel.”

“There hasn’t been anything comparable to this since the coelacanth was discovered,” said Smithsonian Institution ichthyologist Dave Johnson, referring to a fish thought to have gone extinct 70 million years ago and famously rediscovered in the 1930s.

“But coelacanths were known from fossils. In this case, there’s no representation in the fossil record of this form,” he said.

Described in an Aug. 16 Philosophical Transactions of the Royal Society B paper, P. palau has certain characteristics, such as a fringed gill collar, unseen in any living member of the eel order.

Other features, such as its jaw structure and relatively small number of vertebrae, are found only in fossils of the earliest eels, which were thought to have split from other fishes about 100 million years ago.

Still other traits are unique to P. palau. Its gill rakers resemble those found in bony fishes.

From anatomy alone, P. palau thus appeared ancient, but this could have been a recent, coincidental development. (Hagfish, for example, were thought to represent an ancient link between invertebrates and vertebrates, but turned out to be relatively modern creatures that lost their jaws.)

Final proof of primitiveness would come from mitochondrial DNA, which accumulates mutations at a steady rate and is used by evolutionary biologists as a molecular clock. P. palau’s clock started 200 million years ago. In a computer-generated family tree of all eels, it’s the sole occupant of an entire branch.

“Eels known from fossils go back to the Cretaceous,” said Johnson. “P. palau takes that back 100 million years earlier.” At the time, life was finally flourishing after the Permian-Triassic mass extinction. The age of dinosaurs hadn’t yet begun.

Many questions remain about P. palau. Its basic life history, the tricks that helped it survive so long — coelacanths, for example, have fantastically slow metabolisms adapted to low-nutrient environments — are unknown. Johnson and the other researchers have yet to see a baby.

Johnson does think it’s unlikely that the 115-foot-deep cavern off Palau, a Pacific island nation, is P. palau’s only abode. “That’s most likely a shallow-water example of some habitat that is more widespread in deeper water,” he said.

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