Eels first appear in the fossil record about 100 million years ago, in the Mid Cretaceous. These Cretaceous forms are primitive compared to modern forms, with incomplete fusion of the dorsal, caudal and anal fins, scales on their bodies and many of the bones lost or fused still present. However they are still clearly eels, with elongate bodies and the loss and fusion of some bones associated with the group, in particular the gill rakers and the pectoral fins.
Volume 279 of the Proceedings of the Royal Society B (Biological Sciences) contains a paper by a team lead by David Johnson of the Division of Fishes at the National Museum of Natural History at the Smithsonian Institution, describing the discovery of a remarkable living fossil eel from a submarine cave in a coral reef fringing Ngemelis Island, part of the Pacific Ocean Republic of Palau. This eel shows many features in common with Cretaceous eels, and has been named Protanguilla palau (the first/earliest eel from Palau).
Protanguila palau. Top, adult female named as the Holotype, 176 mm in length. All other pictures of juvenile specimen named as Paratype. Top centre, whole specimen, 65 mm in length, scale bar is 5 mm. Left centre, head in lateral view, scale bar is 5 mm. Right centre, head in ventral view, scale bar is 2 mm. Bottom left, close up of left gill opening in ventral view, scale bar is o.5 mm. Bottom centre, stained scales on lateral body-line, scale bar is 0.5 mm. Bottom right, close up of scales, scale bar is 0.5 mm. From Johnson et al. (2012).
Protoanguila palau shows many similarities to Cretaceous eels, it has an unfused palette, incompletely fused fins, and retains its scales and some bones lost in modern eels. In addition it shows some features lost in even Cretaceous eels, it retains its gill rakers, and whilst clearly an eel, lacks the elongate form found in all other eels. Gene sequencing of P. palau confirms that while it is more closely related to eels than any other fish, all other eels are more closely related to one-another than they are to P. palau.
Based upon this Johnson et al. suggest that P. palau is a living fossil, which branched off from other eels at some time around the boundary between the Triassic and the Jurassic, and has been living in isolation ever since. Whilst this seems fairly reasonable (the date is suspiciously precise, 'before the Mid Cretaceous' would have been more defensible), Ngemelis Island is a coraline limestone platform on top of an extinct submarine volcano - a feature that is unlikely to have remained unaltered since the Mesozoic. This implies that P. palau must have migrated here some time in the more recent past, and may well have relatives elsewhere in the West Pacific.