The "living fossil" discussion and that "living fossil" eel you might have heard of

Ref: Video still/Jiro Sakaue, Southern Marine Laboratory, Palau.

I don't like the term "living fossil". Sure, when used well it can be eye-catching in a pedagogical way, but it's still sort of vague and problematic, and used badly it's outright confusing and may reinforce misconceptions about evolution. That's why when you see it used, you often see it between quotation marks followed by an explanation motivating why the organism in question is called a "living fossil" to begin with. Today we learn about the discovery of a really striking and interesting new species of eel from Palau, Protoanguilla palau, heralded as a "living fossil" in the title of the scientific publication made available today (see reference below) as well as in most media reports.

As you guessed by now, I think this is a bit problematic. It starts with the fact that different people often mean different things when calling something a "living fossil". Darwin himself is acknowledged with originating the term in On the Origin of Species, if only in a passing comment. He wrote:

All fresh-water basins, taken together, make a small area compared with that of the sea or of the land; and, consequently, the competition between fresh-water productions will have been less severe than elsewhere; new forms will have been more slowly formed, and old forms more slowly exterminated. And it is in fresh water that we find seven genera of Ganoid fishes, remnants of a once preponderant order: and in fresh water we find some of the most anomalous forms now known in the world, as the Ornithorhynchus and Lepidosiren, which, like fossils, connect to a certain extent orders now widely separated in the natural scale. These anomalous forms may almost be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having thus been exposed to less severe competition.

In this chapter Darwin is speaking to us about the different environmental conditions that are favorable to natural selection, or rather to the creation of new varieties through natural selection. Here he is trying to explain why some organisms might appear relatively unchanged by natural selection, using ganoid fishes, a now obsolete term describing gars and bichirs or reedfishes, the platypus and the South American lungfish, as examples of this. With "living fossil" he is vaguely referring to organisms that are the sole or almost sole survivors of a relatively old and mostly extinct lineage. This meaning is a bit different to the common use today which is either a species that appears practically unchanged from its ancestors found in the fossil record, like the gingko or the horseshoe crab, or a species that was only known from the fossil record until it was suddenly found to be very much still alive, like the coelacanth. None of these organisms fit comfortably into one coherent definition of what a "living fossil" might be without having to include several other groups that are not usually included.*

So as a term, "living fossil" is potentially misleading and awkwardly defined. But most of all it's just redundant. It's just not worth it. Some biologists see this as a reason to take the term lightly and use it casually, but I guess I just don't take many things lightly.

Discussions about "living fossils" aside, the finding of Protoanguilla palau is very interesting for several reasons. Firstly, I don't think we should underestimate the fantastic sense of wonder about our planet and about life that something like this might awake. The almost 18 cm long Protoanguilla type specimen was discovered in a cave at about 35 m of depth in the reef waters of Palau in the pacific ocean. This sort of mirrors the discovery of the coelacanth and makes you think about the kind of enigmatic species we have yet to find in the as yet unreached corners of the ocean.

Eels were one of the very first now living lineages of bony fish to emerge - it's one of the most basal. They first appear in the fossil record in the Cretaceous about 100 million years ago, but the evolution of the bony fishes as a whole probably goes back to the mid-Paleozoic, some 400-300 million years ago. Protoanguilla has an interesting combination of characters, sharing several with all other now living eels as well as with the fossil eels from the Cretaceous, some specifically only with the fossil eels. It also has yet another number of characters that are seemingly specific to it, and some that are characteristic of bony fish lineages that diverged before eels: notably, gill rakers - toothed protrusions of cartilage along the inner rim of the gills used to trap food particles - and the presence of less than 90 vertebrae. Most eels show an expansion of the vertebral column to include up to 200 vertebrae. In simple terms, it looks like an eel but it also looks really primitive. This pattern of old morphological characteristics paired with molecular phylogenetics analyses based on the mitochondrial genome place Protoanguilla at about 200 million years ago, very close to previous molecular estimations of the earliest divergence of eels and 100 million years before the first known eel fossils. The phylogenetic analyses also place it confidently within the eel lineage, so it's not a different kind of bony fish, and they show that it represents the most basal or oldest known lineage of eel. I checked the method descriptions and the results of the phylogenetic analyses in the supplementary data provided by the journal and it looks solid.

However, none of this makes Protoanguilla a "living fossil" in the same way the coelacanth or the gingko might be considered "living fossils". It doesn't conform to Darwin's original exemplification of some species as "living fossils" either. There are no "dead fossils" of Protoanguilla or similar eels as far back as 200 million years ago to begin with! Frankly, the use of the term in the title of the scientific publication is puzzling. It's the first described species in a new and basal group - why call it a "living fossil" when so much surrounding data is absent? The authors might have answered that question for me themselves in the fist sentence of the article:

Ever since Charles Darwin coined the term ‘living fossil’ in On the Origin of Species (...), organisms that have been called living fossils have received considerable attention.

They define "living fossils" as "extremely long-lived or geologically long-ranging taxa", probably based on the fact that several of the morphological characteristics of Protoanguilla seem to have appeared relatively early in evolution and have been kept since. But this definition would have to include several other groups of organisms that are usually not considered "living fossils" at all. They've strangely conflated "living fossil" with "conserved", which is a very useful and established term in evolutionary biology.

This discussion might be peripheral, but it's worth having and it comes up quite often in evolutionary biology. I really want to highlight how good the analyses in the paper are though, and how exciting the finding of Protoanguilla is for our understanding of early bony fish evolution.

For summarizing reports and videos of this marvelous animal, check out BBC News and Wired Science. Swedish newspaper Dagens Nyheter also reports - "Living fossil swims in the pacific ocean".

* Are birds "living fossils", for instance, having been the only dinosaur group out of a great number to survive extinction? I think most people would argue that they're not because they are very diverse and numerous, but there's no other reason not to include them. "Living fossil" seems to imply it should be a rare group of organisms, or ideally a single surviving species.

Johnson, G., Ida, H., Sakaue, J., Sado, T., Asahida, T., & Miya, M. (2011). A 'living fossil' eel (Anguilliformes: Protoanguillidae, fam. nov.) from an undersea cave in Palau Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.1289