Most primitive living eel

The world's most primitive living eel is the Palauan cave eel Protanguilla palau, native to deepwater caves fringing the tiny Pacific island country of Palau. Discovered as recently as 2010, and formally described and named a year later, it possesses several anatomical traits found only in ancient eel fossils 100 million years old, i.e., dating from the Cretaceous Period. Moreover, analysing its mitochondrial DNA has revealed that it last shared a common ancestor with other fishes 200 million years ago, thereby making it a bona fide living fossil. Consequently, this small (only 18 cm long) but very significant new species has been allocated a new taxonomic family, Protanguillidae, all to itself.

Its primitive features include a disproportionately large head, certain fused skull bones, a particular type of jawbone, and a relatively small number of vertebrae (less than 90). Also of interest is that because this is such an old species yet its cavern homeland off Palau only formed around 10,000 to 110,000 years ago, scientists are speculating that it may exist elsewhere, too, in other remote, little-explored locations. Incidentally, it was initially named Protoanguilla but when scientists realized that there was already a fossil eel with this same name, the new, living species was renamed Protanguilla in 2012.