Coelacanths are not living fossils

ResearchBlogging.org

The term ‘living fossil’ is often misleadingly used in the popular press to describe species which have, supposedly, stopped evolving. Commonly cited examples include horseshoe crabs, Ginkgo trees, hagfish and, perhaps the most famous of all, the coelacanths, a group of lobe finned fish with a very long evolutionary history of which two species still survive in the deep waters of the West Indian Ocean.

A modern day coelacanth (Latimeria chalumnae)

A modern-day coelacanth (Latimeria chalumnae)

Coelacanths have long been known from the fossil record with the oldest specimen dating back to the Devonian period, some 400 million years ago. They were however thought to have gone extinct, along with many other animals, in the end Cretaceous mass extinction event. That all changed one day in 1938 when a South African museum curator named Marjorie Courtenay-Latimer discovered a coelacanth amongst the catch of a local fisherman. The discovery was a sensation, a fish that had been thought to have been extinct had been rediscovered 65 million years later, it was not extinct! It was alive! It was amazing!

That’s how the story goes at least, and ever since it’s discovery journalists have talked about the fish that has been “left behind by evolution”. But is this really true? Can a species really exist for a span of time so great that it will have seen ice ages come and go, mountain ranges form and the great super-continent of Gondwana break apart, and through all this not change at all? Over recent years a mountain of evidence has been steadily growing showing that this is in fact not the case, coelacanths, like any other species, are constantly evolving to adapt to changing conditions.

A comparison of the living coelacanth (Latimeria) with some of it's extinct relative. The morphological differences are striking

A comparison of the living coelacanths (genus Latimeria) with some of its extinct relatives. The morphological differences are striking. Image from Casane and Laurenti.

It is sometimes claimed that there is a low rate of change in coelacanth DNA and that this leads slow evolution. However, this idea is now being challenged by systematic studies of the coelacanth genome which do not detect slow rates of genetic change. In one study forty-four genes were analysed and no dramatic decrease in the rate of change compared to other species was detected. Furthermore, there is no known reason why coelacanths should have slowly evolving genomes. Their environment in the deep ocean, while relatively stable, is not particularly unusual and is inhabited by other species which are not considered living fossils. Another factor that may lead to a slow rate of evolution is a slow generation time, however, the reproductive rates of coelacanths are not thought to be particularly long. Finally, coelacanth populations are small, and small population size is known to increase the rate of genetic change within a species. We might therefore expect these species to be evolving rapidly, not standing still.

Probably the most widely held belief about coelacanths is that, even if they are genetically different, they look exactly the same now as they did millions of years ago. This belief is mistaken. No fossils are known for either species of surviving coelacanth or even for members of its genus, Latimeria. This suggests that the scientists responsible for classifying the fossil and living species consider the morphological differences so great the they should be placed in widely separated groups. In fact, there are significant differences in the body shape and structure of modern and extinct coelacanth species. These include changes in the number of vertebral arches and substantial differences in skull morphology. The swim bladder of coelacanths has also changed from being filled with oil in the extinct genus Macropoma, to being ossified in modern species, suggesting that the two groups lived in very different environments. Lastly, there are substantial differences in size, with modern coelacanths being three and a half times larger than their closest extinct relative (one and a half vs half a metre).

Comparison of the skeleton of modern and extinct coelacanths. A) Latimeria chalumnae (a modern species), B) Macropoma lewesiensis (extinct), C) L. chalumnae skull D) M. lewesiensis skull, E) Pectoral fins of L. chalumnae (above) and Shoshonia actopteryx (another extinct relative) (below). Image from Casane and Laurenti.

Comparison of the skeleton of modern and extinct coelacanths. A) Latimeria chalumnae (a modern species), B) Macropoma lewesiensis (extinct), C) L. chalumnae skull D) M. lewesiensis skull, E) Pectoral fins of L. chalumnae (above) and Shoshonia actopteryx (another extinct relative) (below). Image from Casane and Laurenti.

The view that coelacanths are ancient prehistoric fish which have stopped evolving has been around for a very long time. However, the evidence is now in and it shows that it is time to put this mistaken idea to bed.

                                                                         

For a comprehensive review of the evidence showing that coelacanths are not living fossils see: -

Casane D, & Laurenti P (2013). Why coelacanths are not ‘living fossils': a review of molecular and morphological data. BioEssays : news and reviews in molecular, cellular and developmental biology, 35 (4), 332-8 PMID: 23382020

For the study analysing forty-four coelacanth genes see: -

Takezaki N, Figueroa F, Zaleska-Rutczynska Z, Takahata N, & Klein J (2004). The phylogenetic relationship of tetrapod, coelacanth, and lungfish revealed by the sequences of forty-four nuclear genes. Molecular biology and evolution, 21 (8), 1512-24 PMID: 15128875

For a contrasting study claiming slow molecular evolution in these species see: -

Amemiya CT, Powers TP, Prohaska SJ, Grimwood J, Schmutz J, Dickson M, Miyake T, Schoenborn MA, Myers RM, Ruddle FH, & Stadler PF (2010). Complete HOX cluster characterization of the coelacanth provides further evidence for slow evolution of its genome. Proceedings of the National Academy of Sciences of the United States of America, 107 (8), 3622-7 PMID: 20139301

About these ads

5 responses to “Coelacanths are not living fossils

  1. Pingback: The Carnival of Evolution: Eclectic September edition | EvoAnth·

  2. Hey there! I’m currently working on a blog post about this fish (specifically, trying to write a refutation of its use in a creationist pamphlet).

    While I see what you mean about clear differences between the living species and the fossils, the living species does still have a type of scales (type of scale…singular?) that is only ever found in fossils. At any rate, it does seem, in appearance, far more ancient than pretty much anything else.

    I’m fairly new to this area of evolution writing (grew up a creationist myself), so I’m still learning…is there anything out there on why it appears to have evolved so (seemingly) little compared to other species?

  3. Hi Eric, I agree that coelacanths do look more ancient than most other fish but I think the reason for it is that they are so rare that we hardly ever see them and are not used to their appearance. If you look back at the fossil record for the coelacanth family you will see that they were once quite a diverse group with many different body forms that were found in both salt and fresh water. For some reason (I’m really not sure what) most of this group went extinct several million years ago leaving only two species alive today. Although the surviving species do retain a coelacanth body plan they are not the same as the ancient species from which they descend.

    If you look at the ray finned fish that we are most familiar with they also look quite similar to the fossil forms, possibly because there are only a limited number of ways to adapt to swimming in water. I have on my desk a 45 million year old fossil Knightia which I suspect only a trained paleontologist could tell apart from a more modern related fish such as a sardine. If you imagine a scenario in which the coelacanth line survived and diversified while only two species of ray finned fish survived we might today be asking why sardines or salmon looked so ancient.

    One more thing to remember is that fossils only (usually) preserve the hard parts of an organism, we have no idea how the internal organs or biochemistry of coelacanths have evolved over the years.

    I hope that helps a bit let me know if it doesn’t.
    I’d also be really interested to see your pamphlet when it’s done, I was raised a creationist too, it’s so nice to be free isn’t it?

  4. Thanks for your reply, Sam. I’ve done a lot more reading since writing to you and found PZ Myers had a bit to say about this topic. He quoted a BioEssays article entitled…almost exactly the same thing as your blog post. I doubt that is a coincidence, though it might be convergent evolution. ;-)

    I also realized (like I said, I’m new to this), that Coelacanth is the Order…an Order is a long way from a species. It would be like saying a Cheetah is a living fossil b/c it looks very similar to a few fossils of Carnivora.

    But also, like you pointed out, the fish in question HAVE changed over time. They exhibit some unusual traits (like the ancient form of scales), but all that proves is that they’ve been around for a long time and that that form of scales has served them well.

    PZ also quoted from a BioEssays article that said if you bought into the “living fossil” line of thinking, then 4 out of 5 Papers would find some type of Slow Mutation rate, but of those who did not buy the “living fossil” cliche, 7 out of 7 did NOT find abnormal mutation rates.

    The pamphlet I’m refuting can be found here: http://www.huffingtonpost.com/paul-raushenbush/christians-lgbt-rights_b_4794753.html?ncid=edlinkusaolp00000009

    It is from the 1960s, but it is the main one our church has used for this topic (well, there are Some Fishy Stories; A Theory for the Birds, and A Whale of a Tale). The coelacanth is the last fish in this first book, and then I’m going on to the other books–it is a joint effort with a couple friends in Proving (through evidence) that everything our church taught us was wrong. It started out as a way to prove WHETHER it was wrong or not, but the Whether didn’t last long.

    Here is our blog: http://armstrongdelusion.com/ and my series has been called Fish Fry…yes, it is nice to be free!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s