The term “living fossil” is often used to describe organisms in peer-reviewed scientific papers, news articles, blogposts and museum labels. Organisms like coelacanths, nautiluses, tuataras, horseshoe crabs, tadpole shrimps and hagfish are rarely mentioned without reference to them being living fossils and relics.

Living fossil conjures up the idea of long-lived species or organisms out of time longing for the good old days of the Devonian and disdainful of whippersnappers with their colour vision and adaptations to human pollution. It suggest that these anachronistic organisms are hanging on in there and lonesome, rather than modern, flourishing organisms. However, for such a widely used descriptor, particularly in science communication, the meaning is more widely misunderstood than understood and that’s probably because there isn’t really one robust definition.

These anomalous forms may almost be called living fossils; they have endured to the present day ... Charles Darwin

It is most often used to describe organisms that are vaguely similar to extinct relatives that were more abundant and diverse and is sometimes used as shorthand to imply that there has been little evolutionary change. The handful of living nautilus species are the only living externally shelled cephalopods that were represented by thousands of species going back to the Cambrian period. Until the discovery of living individuals off the coast of South Africa in 1938, coelacanths were only known from the fossil record and thought to have gone extinct in the Cretaceous period. The two living species of the lesser known tuataras from New Zealand, are the only living representatives of their entire group of reptiles. Some animals just can’t catch a break. Crocodiles and snapping turtles are often called living fossils, more on the basis that they look “prehistoric” than any scientific grounds.

In some of the above examples, of course, the idea that these organisms are living fossils are artefacts of our bizarre taxonomic classifications and history of discovery more than a useful and robust concept. To the wider public reading a museum label, this complexity may not be obvious and from experience, this often leads to confusion between individual organisms which live for a long time.



The concept of living fossils is yet another testament to the impact of, sometimes-over-celebrated, Charles Darwin’s philosophy. He is attributed as coining the term in On the Origin of Species in 1859 in reference to sturgeons, platypuses and lungfish. These freshwater species were thought to be subject to less competitive pressure and as such are remnant members of long established groups insulated from pressures that cause change in other organisms. These animals, Darwin argued, “... may almost be called living fossils; they have endured to the present day ... ” (Darwin 1859).

The platypus Ornithorhynchus anatinus, one of the animals that led Darwin to coin the term living fossils. Photograph: Dave Watts/Alamy

Unfortunately, this notion, mentioned by Darwin only in passing has persisted since and it’s unsurprising that creationists often use the example of living fossils in their arguments against evolution. One of the best examples of this is the Atlas of Creation, a glossy volume with high production value, tens of thousands of which were sent out unsolicited to universities, museums and research centres. The bulk of the book is large images of fossil and their supposedly related modern organisms evidencing the lack of change in organisms, albeit at the level of “spider” and “herring” and one example comparing a crinoid fossil with a tube worm because, you know, they look the same.

Although the idea of living fossils flourished after Darwin introduces the idea, it was never formally defined and was used as a catch all for apparently any organism that has an interesting fossil record. In a paper on rates of evolution, palaeontologist Thomas Schopf reviewed the problem of “living fossils” in scientific publishing. The term was invariably used to describe living species that: persist over a long geologic time interval; were thought to be extinct and rediscovered; are morphologically or physiologically similar across living and fossil species; have “primitive” traits; have a smaller geographic range than ancestors or have a lower diversity today than relatives in the past (Schopf 1984). Under at least one of these definitions, our species could be considered a living fossil. Schopf then went on to describe how many of the classic living fossil organisms don’t actually fit many of the criteria concluding that “the claim that ‘living fossils’ are ancient species is completely arbitrary”. Unfortunately, the living fossil idea, perpetuated through rote repetition without examination is now an unshakeable meme, with the concession that living fossil is often put in quotation marks, indicating that it is perhaps meaningless.

Tadpole shrimp, often described as a living fossil as they have varied vary little in their morphology. Photograph: WWT/PA

Even in the light of new evidence, the concept of “living fossils” persist, so much so that there are papers centred around refuting or maintaining the idea. Living fossil poster child, coelacanths, were shown to not be as unchanging as previously thought through molecular analysis and palaeontological data (Casane and Laurneti 2013) although subsequent genome work shows that they are comparatively morphologically stable compared to other groups so are “almost living fossils” (Cavin and Guinot 2014). One species of tadpole shrimp, Triops cancriformis once thought to be the oldest living species that has remained unchanged for 200 million years, is probably not as long lived, a study of the group has shown that tadpole shrimps have undergone a series of radiations, while remaining morphologically similar (Mathers et al. 2013). In studies of relict species, species which are restricted to a smaller range than extinct relatives, although they can be useful for biological conservation assessment, their use can be overemphasised and misleading (Grandcolas et al. 2014). Evolutionary analysis of other classic living fossils such as cycads, nautiloids, horseshoe crabs and monoplacophorans indicate that there’s an empirically weak basis for the subjective notion of living fossils, molecular complexity exposes the limitations of information from fossil analysis alone (summarised in Mathers et al. 2013).

With so many issues around living fossils, this is a call to scientists, educators, documentary makers and journalists to retire the term. It doesn’t help the understanding of science and in general use, creates more mystification than is resolves and detracts from the organisms themselves. Instead, a proposed adoption of the more positive idea of evolutionary distinct as used by the Zoological Society London’s EDGE of existence project, which seeks to highlight and prioritise the protection of species on the basis of their evolutionary distinctiveness, millions of years in the making.

References:

Casane, D., and Laurenti, P. 2013. Why coelacanths are not “living fossils.” Bioessays 35, 332–338

Cavin, L., and Guinot, G. 2014. Coelacanths as “almost living fossils”. Frontiers in Ecology and Evolution. Vol. 2:49.

Darwin, C. 1859. On the Origin of the Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.

Grandcolas, P., Nattier, R., and Trewick, S. 2014. Relict species: a relict concept? Trends in Ecology & Evolution , Volume 29. 12:655 - 663.

Mathers, T.C., Hammond, R.L., Jenner, R.A., Hänfling, B., and Gómez, A. (2013) Multiple global radiations in tadpole shrimps challenge the concept of ‘living fossils’. PeerJ 1:e62 https://doi.org/10.7717/peerj.62

Schopf, T.J.M. 1984. Rates of Evolution and the Notion of “Living Fossils”. Annual Review of Earth and Planetary Sciences Vol. 12: 245-292.