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The Falsity of Living Fossils

New studies of tadpole shrimp and other organisms show that the term “living fossil” is inaccurate and misleading.

By | April 2, 2013

Triops longicaudatusWIKIPEDIA, STEVE JURVETSONToday’s tadpole shrimp, or notostracans, have a shield-shaped body, ending in a forked pair of filaments—a shape that makes them almost indistinguishable from their ancestors in the Triassic period some 265 million years ago. This outward constancy has earned them the description of “living fossils”—a term referring to species with no close living relatives, which seem to have gone unchanged for long spans of time.

But according to a genetic analysis of notostracans published today in PeerJ, these animals have by no means stopped evolving. Indeed, researchers are coming to realize that the term “living fossil” is a misnomer. One by one, the classic examples—horseshoe crabs, coelacanths, cycads, and more—have turned out to be very different from the fossils that they apparently resemble, either at a genetic level or through subtle physical changes. Their recognizable nature is a red herring—these creatures simply did not exist in their current form millions of years ago.

“I would favor retiring the term ‘living fossil’ altogether as it is generally misleading,” said Africa Gomez at the University of Hull who led the study.

In the latest study to disprove the concept of a living fossil, Thomas Mathers, Gomez’s student from the University of Hull, sequenced several genes from 270 tadpole shrimp, and built a family tree charting their evolutionary relationships. Although taxonomists recognize 11 species of tadpole shrimp, based on DNA evidence, Mathers suspects that there are actually 38 distinct species alive. These fall within two living genera—Triops and Lepidurus—which diverged from one another 184 million years ago, during the Jurassic period. The diversification of species within these genera arose around 73 million years ago.  “Our work shows that organisms with conservative body plans are constantly radiating, and presumably, adapting to novel conditions,” said Gomez.

“The article provided more proof, if more were needed, that the term "living fossil" is both poorly defined and misleading,” said Patrick Laurenti, an evolutionary biologist from the CNRS in France. “Calling a given species a living fossil suggests that it has crossed time without evolving—a hypothesis that is in sharp contrast with evolutionary genetics principles.”

Laurenti has found the same to be true in his own research on coelacanths—two species of fish that are perhaps the most cited example of living fossils. The entire group was thought to have been extinct since the Cretaceous, until South African fisherman caught a live one in 1939. It was perhaps inevitable that the coelacanth came to symbolize stagnant relicts of a bygone age, but Laurenti points out that their genomes have evolved at comparable rates to other species, and that modern coelacanths are actually physically distinct from prehistoric ones in terms of their skulls, spines, size, and more. And in 2011, researchers found that one of the living species is actually two distinct species, which diverged from each other 200,000 years ago.

The same message applies to other living fossils. Cycads, for example, are a type of tree that first appeared around 280 million years ago and dominated the land during the time of the dinosaurs. Today, just 300 species survive of the many more that once flourished. But Nathalie Nagalingum from Harvard University found that they are just 12 million years old. They aren’t ancient survivors of the Jurassic and Cretaceous, but rather the descendants of a more recent comeback, which happened after most of their ancient relatives had disappeared.

The tuatara—a single lizard-like species from New Zealand—has also been branded as a living fossil. But a recent study showed that it differs significantly from its Triassic and Jurassic relatives in the number of its teeth, subtle features of its skull, and probably in its diet. And a 2008 analysis of its genome revealed an even bigger surprise—tuatara DNA has changed faster than that of any other vertebrate, confirming that molecular and physical changes do not necessarily reflect one another.

 “The body plan [of so-called living fossils] is effective at what they do, although they might change in minor anatomical features such as their leg hairs or details of their jaws,” said Gomez. And though they may superficially resemble their extinct relatives, they may differ in traits that do not fossilize, such as songs, pheromones, lifespan, or sexual systems. The notostracans, for example, have evolved to become hermaphrodites several times over, something that wouldn’t necessarily be reflected in the paleontological record. 

Although Darwin himself coined the term in On the Origin of Species, Gomez said, “I think he would have been pleased if he had found out that living fossils evolve as every organism does and we no longer had to use the term.”

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Comments

Avatar of: JoeK

JoeK

Posts: 2

April 2, 2013

The concept of a living fossil perhaps should be retained in defference to the acknowledged introduction by Darwin.  I would not be to quick to reverse that intuition of his based on morphological insight.  There might be a modern turn to this terminology.  Of course no one would think that mutation had stopped in these visually 'living fossils' but how about their pattern regulation genes?  Perhaps they retain an archaic organization of the homeo-box genes?  It seems that such a possibilty might be possible and that some organism, by chance, may have retained the archaic gene patterns is not an impossibility. At least we can retain the concept of a 'living fossil' as a modern null hypothesis as we learn more about their gene organization in their chromosomes.

Avatar of: Sergio Meirelles

Sergio Meirelles

Posts: 2

April 2, 2013

I think that the term "living fossil" points to the interesting resemblance of living organisms to their probable ancestors, not considering all the expected changes in their genes and anatomical details. The retention of the anatomical design is itself interesting, nothing more than that. Besides, some of the "living fossiles" are the only remaining organisms with the combination of characters that ressemble their ancestors (e.g. we dont have triops-like organisms in other groups besides those that are accepted as closely related).

Avatar of: Ed M.

Ed M.

Posts: 44

April 4, 2013

The phrase is poignant and poetic

And you can't stop poetry with mere logic.

Plus, the coelacanth is close enough to fill the bill.

 

 

Avatar of: jocaroth

jocaroth

Posts: 1

April 18, 2013

 

The second paragraph of Ed Yong's article contradics itself: "One by one, the classic examples ... have turned out to be very different ... through subtle physical changes." (emphases mine).

And today's (17 April) paper in Nature, showing that coelocanths HAVE had very slow genome evolution, authoritatively contradicts a cited author's assertions. Most clades of the tetrapod sarcopterygians have undergone morphological change that makes coelocanths appear morphologically frozen in time. Same goes for tuataras: to assert that a change in number of teeth or "subtle features of its skull" place it too in the mainstream of fossil change is, quite simply, contradicted. Triops is another great example since it is "almost indistinguishable from their ancestors in the Triassic period some 265 million years ago".  Fossils are by definition morphocentric. If the morph shows little change over large periods of time, then it has changed little: contrary to Yong's assertions, these creatures DID exist in their current form millions of years ago, albeit perhaps with a genome that has evolved (but very slowly in at least one case - coelacanths). 

The death of "living fossils" has been greaty exaggerated. 

Avatar of: Michele Rubini

Michele Rubini

Posts: 1

April 18, 2013

The term 'living fossil' could be retained if referred only to morphological appearance. Darwin's intuition was supported by the evidence of strong conservation of body morphology in some organism, and it is still valid.

Today's technology allows to have a look at the genome of species, and it is not surprising to find out that in some organism genetic evolution has continued across ages, although retaining the same overall body morphology. After all, no species has ever been so perfect to privilege stopping its evolution.

Nowadays, the concept of 'living fossil' should be replaced by 'pseudo-fossil', a term that basically means that a cartain pattern of body appearence that was found in a fossil species is now found in a living species, although the underlying genome could be much different.

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