Protoanguilla palau is a species of eel, the only species in the genus Protoanguilla (lit., "first eel") which is in turn the only genus in its family, Protoanguillidae. The Palauan primitive cave eel (Protanguilla palau) has an evolutionary history that dates back some 200 million years. Individuals were found swimming in March 2010 in a deep underwater cave in a fringing reef off the coast of Palau. The body is very small and slender, about 18 cm long. They have a second premaxilla and under ninety vertebrae, features previously found only in fossilized eels. Their full set of gill rakers in their branchial arches have never previously been found in an eel but are common in bony fish. It is so different from all other living eels that scientists estimate that it must have diverged from the others around 200 million years ago, during the Mesozoic Era. It thus has not only its own species, but its own genus and family as well, and has been referred to by scientists as a "living fossil".