Living fossils don't exist...

… except in Hollywood movies.

Let’s make this clear from the start, I don’t like the term “living fossils” at all. It’s as if we decided that certain species are second class organisms that should have gone extinct a long time ago. Unfortunately for me, the term crops up in the popular scientific press all the time. Especially the BBC seems to have a particular affinity for the concept. Even worse, scientific literature is polluted with this muddy term now and again1. Why would that be a problem? Because the term makes absolutely no evolutionary or scientific sense at all.

Ben Stiller is the only human known to have encountered a real living fossil. Unfortunately for Mr Stiller, of all the ammonites, ground sloths and trilobites that populate the fossil record, he had the bad luck to walk into a T. Rex.

There are many reasons people refer to extant organisms as living fossils. They are all bullocks. Let’s go through the list.

1. Living fossils have been around for billions/millions of years and have changed little over time.

Even the morphologically most constant species will change and diverge over time. The horseshoe crab, the poster child of ‘living fossils’, can make this insightful.

It is true that the first recognizable horseshoe crabs make first appear in the fossil record around ~445 million years ago. But based on these fossilized shells alone, paleontologists can recognize dozens of different horseshoe crab species that lived in the past, in addition to the four different species of horseshoe crabs alive today. In fact, the extant Atlantic horseshoe crab doesn’t show up in the fossil record at all!

So the horseshoe crabs that live today did NOT exist millions of years ago. Furthermore, the only part of the horseshoe crab that fossilizes well is its shell, so we know almost nothing about the soft tissues of ancient horseshoe crabs. Any information about particular physiological organization of organs or genomic differences is lost forever. Maybe fossil horseshoe crabs maybe would be more recognizable as different species if we DID had access to that information. If anything, genetic drift will have made sure that the genetic composition of an ancestral horseshoe crab and an extant one are fully incompatible as millions of years passed. In other words, while it would make a cool experiment, trying to cross a ‘modern’ horseshoe crab with one of its ancestors will most likely fail.

The horseshoe crab, while sporting an impressive pedigree, is NOT a 'living fossil'.

Continuing with the completely arbitrary list of properties that define a living fossil (partly taken from this awful wikipedia page):

2. Living fossils… have all survived major extinction events

Newsflash: the ancestors of EVERY organism alive today have experienced ‘major extinction events’. That’s sort of why we’re here today.. This kind of reasoning can take completely ridiculous forms, as in the case of the Cypriot mouse2.

“All other endemic mammals of Mediterranean islands died out following the arrival of man, with the exception of two species of shrew. The new mouse of Cyprus is the only endemic rodent still alive, and as such can be considered as a living fossil,” said Dr. Cucchi.

The Cypriot mouse diverged approximately half a million years ago from its fellow mice and continued to evolve, adapted to the Cypriot island while genetically drifting away from its mainland nephews. So does the status of sole survivor of an extinction event make the Cypriot mouse a living fossil? Humans are the only surviving members of the genus Homo, but we don’t go about calling ourselves living fossils now do we?

The cypriot mouse. Cute, but not a living fossil.

3. A species which successfully radiates … has become too successful to be considered a “living fossil”

If you needed any reason to regard “living fossils” as complete bullcrap, this is it. “Successful radiation”?? How on earth can that be measured? For all we know, ‘living fossils’ may have ‘successfully’ radiated in the past, even if few species in that clade survive today. This definition also implies that species can lose their status of a living fossil if they divergein the future, making it a pretty worthless biological concept.

As an example. both Coelacanths and Elephantidae are genera that spanned dozens of species in the past. So subjectively, we could think of them both as ‘succesful’ groups of species. However, extinctions eliminated all but two species in both Elephantidae and Coelacantha. Yet, it are anoly the coelacanths who are viewed as ‘living fossils’ whereas elephants are apparently alive and kicking! If that’s not arbitrary, I don’t know what is.

The list of living fossils on wikipedia has no biological relevance at all. The troubles the wikipedians have in getting a clear definition of what constitutes a living fossil really says enough. To me it looks like a half baked attempt to provide some psuedoscientific justification for branding some species as ‘primitive’. Of course, the usual victims such as the highly derived platypuses and crocodiles feature on the list. Guess what: as Jonathan Eisen pointed out earlier, there are no ‘primitive’ species. Or ‘living fossils’ for that matter.

So please, spread the word, and whack people on the head with a fossilized trilobite if they ever use the word ‘living fossil’.

This post was partly inspired by this question asked on the very cool website ask a biologist.

1.Amemiya, C., Powers, T., Prohaska, S., Grimwood, J., Schmutz, J., Dickson, M., Miyake, T., Schoenborn, M., Myers, R., Ruddle, F., & Stadler, P. (2010). Complete HOX cluster characterization of the coelacanth provides further evidence for slow evolution of its genome Proceedings of the National Academy of Sciences, 107 (8), 3622-3627 DOI: 10.1073/pnas.0914312107

2. T. CUCCHI , A. ORTH, J.-C. AUFFRAY , S. RENAUD , L. FABRE, J. CATALAN, E. HADJISTERKOTIS, F. BONHOMME & J.-D. VIGNE (2006). A new endemic species of the subgenus Mus (Rodentia, Mammalia) on the Island of Cyprus Zootaxa

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24 comments to Living fossils don’t exist…

  • You’ve missed one of the more frequent uses of the term: A lineage that was previously thought to be extinct and known only from the fossil record is discovered with to still be alive. The coelacanth is a great example of this use of the term “living fossil.”

    Of all the various meanings of “living fossil,” I think this one, indicating rediscovery, has the most claim to legitimacy. I know some call these “Lazarus taxa,” but not many people know this term.

    • Thanks for your comment Zen!

      I agree that the definition of ‘living fossil’ that you give is one of the few coherent ones. The discovery of a presumed extinct species is a wonderful thing, as if a dead twig on the tree of life suddenly shows that it has persisted and has sprouted new leaves for millions of years. The story of the (re)discovered coelacanth as I read it in ‘The ancestor’s tale’ by Dawkins, is truly marvelous.

      However, even if the term is used in this way, it still carries a lot of extra baggage with it as I have tried to shown. The image that it conveys is one of evolutionary stasis, as if the species has been frozen in time. The first article in PNAS that I reference follows a similar line when discussing the coelacanths. Their main point is that coelacanths have slowly evolving genomes (Hox clusters), consistent with their image as ‘living fossils’. Their status rediscovery is thus pulled entirely out of context, as if that necessarily implies that they also show morphological and genomic persistence.

      As the headlines in the popscience press show, the term is rarely used in the way you propose. Therefore I think it’s better to abandon it altogether, and use the (less known and sexy, but better defined) “lazarus taxon” that you mentioned!

  • As well as ‘living fossils’ I do get irritated whenever people call bacteria ‘primitive’ organisms. My bacteria are *not* primitive, in fact due to the tiny generation time they have been evolving far faster than almost everything else, and are cool exciting modern bacteria well equipped for the challenges of modern life, who would sneer at their ancient ancestors (i.e the bacteria of 50 years ago) as being less adapted and less diverse.

    Some of them eat concrete for heavens sake. How is that primitive?

  • Your comment made my day, had to laugh out loud at the cool modern bacteria sneering at their ancestors! Bacteria really are the hip cosmopolitans of our world, picking up trends faster than youth cultures do!

  • Ditto what Lab Rat said for MY organisms =P

    “Living fossil” makes me twitch in all sorts of ways, some previously considered impossible. In fact, the effects of that phrase on me a reminiscent of those 19th century engravings of tetanus or whatever it was…*snap*

    The coelocanth usage is *almost* justified, but considering the vast majority of life doesn’t bother leaving much of a fossil record, such cases are not worth getting their own common-sounding term =P

  • I’ve just been reading Dawkins’ The Greatest Show on Earth. It uses the phrase ‘living fossil’ several times….

    Personally, I always found the phrase slightly grating, if for no other reason than it’s a bit of an oxymoron – fossilisation being a process that only happens to dead things.

    • I guess that it’s this apparent contradiction that makes the term so attractive for science journalists and popular science writers..

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  • jarno verhoofstad

    Uitstekende post Lucas: right on the spot! Though I have been struggling quite a while with the concept of evolutionary successful bodyplans conserved throughout the ages, it doesn’t say anything about the extant species necessarily being a living fossil.

    • Thanks! I guess nature hits a home run (e.g. the horseshoe crab) every now and then, where a specific morphology allows an organism to fill a niche so successfully that the ‘design’ doesn’t need to be improved! However, evolution trudges on beneath the carapace!

  • I understand why you don’t like the term, but your last post hints at a concept of a ‘durable design’ for which, to my knowledge, we have no term. Some invertebrates have many cryptic species and some ‘living fossils’ are truly impossible to distinguish morphologically from living species (e.g. Triops cancriformis). Maybe a new term has to be made to describe these organisms in which the body plan is essentially identical, which, has nothing to do with ‘evolution stopping’.

    • I know the term ‘morphological stasis‘ is sometimes used to describe the persistence of phenotypes in some species. Such a description is fair I guess, since it doesn’t suggest that evolution has somehow stopped, but it’s just the morphology that has remained similar over time. But I fear ‘stasis’ still still be interpreted as ‘primitive’ or backwards somehow, while I think it’s important to view organisms and species as dynamic systems, continuously in flux with their environment and their own genomes.

    • Brian Switek from Laelaps has written an excellent blogpost on the subject of evolutionary stasis here:

      The undirected pattern of evolution is not always striving upwards to outdo itself with more-perfect radiations of species. Instability at least partially fuels the twin engines of diversity and disparity, and it may be that the fragile nature of species is what has allowed so many to come to be.

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  • Raymond

    I know that living fossil do exist. I read somewhere that a living Coelocanth was caught off the coast of south africa and fossils of the coelocanth were found and dated back more than 360 million years ago. How do you explain that?
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