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A living fossil is a living species (or clade) that appears to be similar to another species otherwise known only from fossils, typically with no close living relatives.

A living fossil is considered as a successful organism, which has made its way through many major extinction events. Also, the morphology of living fossils resemble some species of organisms which we know only through their fossil remains.

  • What is the reason for a particular type of species to become a living fossil; is the engineering of this particular species extraordinary, in that it can survive any selection process encountered thus far?

  • Is there not enough selection pressure exerted on this species in order to force it to change morphologically?

  • Have these organisms modified themselves, so that currently their morphology seems to be similar to a fossil organism?

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  • $\begingroup$ wasn't every fossil once a living fossil? $\endgroup$ – aaaaa says reinstate Monica Mar 9 '15 at 10:27
  • $\begingroup$ @aandreev: The term "living fossil" describes an organism which is alive today and it has great resemblance with some ancient creature(which is luckily fossilized). Every fossil we found are remains of living organisms and not remains of fossil :) $\endgroup$ – Jayachandran Mar 9 '15 at 10:35
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    $\begingroup$ Of course the selection pressure changes the morphology. These are just two different ways of saying the same thing. $\endgroup$ – Rodrigo Mar 9 '15 at 14:21
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    $\begingroup$ I think living fossil is a fairly meaningless term, since there will be fossils of extinct species out there resembling many living species. Species which are the sole survivor of a clade were simply the most successful, I suppose. If you are wondering: why has a species remained the sole survivor of a clade for a long time without speciating again - perhaps the niche it occupies isn't very conducive to speciation, e.g. mass synchronised mating. Perhaps we just don't recognise the different species of e.g. horseshoe crab which exist, for what they are. $\endgroup$ – Teige Mar 9 '15 at 14:22
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    $\begingroup$ There's a paleontologist from the University of Chicago called David Raup who wrote a book on extinction: "The disturbing reality is that for none of the thousands of well-documented extinctions in the geologic past do we have a solid explanation of why the extinction occurred... We have many proposals in specific cases, of course: ... These are all plausible scenarios, but no matter how plausible, they cannot be shown to be true beyond reasonable doubt. Equally plausible alternative scenarios can be invented with ease". so it seems we may have to admit our ignorance in this $\endgroup$ – r2d2 Mar 11 '15 at 6:59
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One part of your question betrays a serious error:

  • Is there not enough selection pressure exerted on this species in order to force it to change morphologically?

Actually the reverse is true; constancy of form can only be maintained in the presence of continuous selective pressure. It's just that this is stabilising selection that acts to maintain the existing form rather than push the organism to new morphologies. In fact, most selection acts in this manner. This shouldn't surprise you: organisms are typically well adapted to their environments so changes are more likely to reduce fitness than increase fitness.

It's also worth noting that although living fossils show little morphological change they can continue to show change at the molecular level at rates as high as, or higher than, other organisms - e.g. (May et al 2007; Cao et al 2013).

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  • $\begingroup$ Good point about molecular vs. morphological evolution. $\endgroup$ – fileunderwater Mar 10 '15 at 8:58
  • $\begingroup$ @Jack Aidley: Consider an organism which is well adapted to its environment, if a change happens it makes the organism shifted to a reduced fitness state(as you mentioned). Then if a morphological alteration could bypass the fitness reduction and make it more fit. How is this statement invalid by your perception ? $\endgroup$ – Jayachandran Mar 10 '15 at 9:05
  • $\begingroup$ @Jay: I'm sorry but I don't understand what you're asking? $\endgroup$ – Jack Aidley Mar 10 '15 at 13:37
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TL;DR: Living fossils are the children of long-dead fossilised creatures that were good at being alike, other animals are the children that were good at being different.

The boring answer is that living fossils are around because nothing has happened to make them go extinct.

The more exciting answer is a tale of intrigue and lies stretching back hundreds of millions of years:

The lie that is the 'species'

The notion of a species is, of course, a lie. It's a useful lie, and one without which the equally useful field of taxonomy would be impossible, but a lie all the same. Every living thing is ultimately a unique individual unto itself. We group these individuals up and call them things, according to things that are more or less arbitrary, and we arrange these groups into bigger groups and fill books upon books with this method, because it makes it super easy to get a good overview of how life developed.

However, if we want to understand the details, we have to see past species, and look at the lineages.

The shark, our ancestor1

One such lineage that is of interest is that of the shark. Sharks are ancient: They are roughly as old as plants, and older than almost all land fauna. They are vicious, unrelenting killing machines, and all but perfectly suited for being this, with their sleek shape, their conveyor belts of teeth, and rough skin. At some point way back when, however, the lineage of sharks split in two: One branch that leads to fish, amphibians and eventually dinosaurs, squirrels and dolphins, and one that leads to sharks. The forebear to all these things were the early cartilaginous fish of the Cambrian ocean.

To answer the question as posed, we have to compare the success factors of the two lineages.

Why sharks are still around as living fossils

The real answer here is that, in a very real sense, they aren't. Modern sharks are no more alike their long-dead forebears than squirrels are. However, the biological niche inhabited by sharks (apex predator) is a successful one, and sharks, already existing in this niche, have had successful children, made more successful by being superficially and behaviourally alike to their parents, and so, the modern shark presents as a living fossil.

Why shark contemporaries are not still around as living fossils

The real answer here is that, in a very real sense, they are. Morphological differences aside, squirrels are no more different from their long-dead forebears than sharks. Rather, crowded niches have made generation upon generation of increasingly squirrely animals more successful by being superficially and behaviourally different from their parents, and so, modern squirrels have arisen.


Edit: Footnotes:

  1. The shark is not in fact our ancestor, rather Cambrian early jawed fish are. The image of a shark is however far more evocative and easier to talk about.
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  • $\begingroup$ "The shark, our ancestor" can you provide some reference for your claim that fishes are evolved from sharks. Also there is no reference for any of your claims, so how can I accept your answer ? $\endgroup$ – Jayachandran Mar 10 '15 at 8:06
  • $\begingroup$ @Jay Sure, let me get right on that. $\endgroup$ – Williham Totland Mar 10 '15 at 8:14
  • $\begingroup$ @Jay And there I've specified more exactly our relationship with sharks. $\endgroup$ – Williham Totland Mar 10 '15 at 8:38
  • $\begingroup$ " the lineage of sharks split in two: One branch that leads to fish, amphibians and eventually dinosaurs, squirrels and dolphins, and one that leads to sharks". Here do you meant the shark instead of "Cambrian early jawed fish". $\endgroup$ – Jayachandran Mar 10 '15 at 9:19
  • $\begingroup$ @Jay The statement "The lineage of X split in two: One branch that leads to Y and one branch that leads to Z." is equally valid for literally any known living things X, Y and Z, which is largely the point of the answer. The details of which creatures are descent from which really aren't important to this answer. Besides all of which, whether we are descent from sharks is really a subjective matter: If you follow the lineage of sharks backwards, parent-to-parent-to-parent you'll find that it's damn hard to place a hard line on where 'early jawed fish' end and 'sharks' begin. $\endgroup$ – Williham Totland Mar 10 '15 at 9:28

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