Episode 90 – “Living Fossils”

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You may be familiar with the concept of organisms alive today that are relics of the ancient past, holdovers from times long ago. But this idea, misleading and poorly defined as it often is, is the source of much debate among scientists and science communicators, often revolving around the specific term we discuss in this episode: “Living Fossils.”

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What is a “Living Fossil?”

Depending on the source, “living fossil” can mean different things. Typically, the term is used to describe modern organisms, often rare or unique species, whose fossil relatives show little physical differences in shape and features. In other words, these are organisms that seem strangely similar to their ancient cousins. This usage of the term started with Charles Darwin in On the Origin of Species.

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Some classic examples of so-called “living fossils” include tuataras (top left, Public domain), coelacanths (top right, BrokenSphere [CC BY-SA 3.0]), horseshoe crabs (bottom left, Chosovi [ CC BY-SA 2.5]), and ginkgo trees (bottom right, James Field [CC BY-SA 3.0]).

But there is no official definition of “living fossil,” and you’ll often see it used in varying ways, in the news, in books, in movies, and even in scientific discussions. There have been a few modern studies that have attempted to define the term in a more technical sense.

Lidgard and Love (2018) collected a list of criteria typically used to describe “living fossils,” which included prolonged persistence in the fossil record, slow evolutionary change, low diversity in modern groups, and retention of ancestral features, among other things.

Turner (2019) suggested a new phylogenetic concept of “living fossils” to describe organisms that: a) show little morphological change through deep time; b) contain few living species; and c) are unique in terms of their distant relation to other living species.

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There are also other terms you might hear which are similar to and often conflated with “living fossils,” including bradytelic evolution, relict species, and Lazarus taxon.

A Debated Term

“Living fossil” is a controversial term among scientists. Many criticize the term because of its lack of a clear definition, and because it can be downright misleading for the general public. Opponents of the term argue that it can give the impression that these organisms haven’t evolved since ancient times, which doesn’t fit with our understanding of evolution as a process that is always ongoing, even if it’s largely restricted to the kinds of things that don’t often fossilize, like soft tissues, genetic information, and behavior. Some have pointed out that it can even mislead scientists, that the perception of an organism as a “living fossil” can bias results of studies.

 

This kind of confusion can make it that much easier for misunderstandings to spread through schools or news media, and it’s not uncommon for anti-evolution groups to use the term (the idea of organisms that “don’t evolve”) to try to undermine evolutionary science.

Some have suggested we get rid of the term altogether, while others have recommended replacements like the term stabilomorph to refer to organisms whose stability over time has allowed them to persist through major extinctions and environmental change.

But the term has its defenders. Both of the above studies, Lidgard and Love (2018) and Turner (2019), were in support of the term while trying to provide a more precise definition.

800px-Velemlok_čínský_zoo_praha_1
Defenders of the term argue that “living fossil” can be important for bringing unusual evolutionary situations to light and presenting questions that need answering, even drawing attention to cases that need protection, such as the giant salamander. Organizations such as the Zoological Society London’s EDGE of Existence project aim to protect such animals. Image from Petr Hamerník [CC BY-SA 4.0]

Examples

The list of organisms that have been called “living fossils” is very long, but as you might expect, there’s plenty of disagreement over which – if any – actually deserve the title.

Even famous examples like coelacanths and horseshoe crabs are surrounded by debate as to whether they are “not living fossils,” “almost living fossils,” or simply “evolving in the slow lane.”

Beni-KabutoEbiAlbino1
Shield shrimp, or tadpole shrimp, scientifically known as Notostracans, are famous examples of “living fossils” due to having recognizable fossils all the way back in the late Carboniferous, 300 million years ago. But recent research shows that living species are more diverse and evolved more recently than we once thought. Pictured here is an albino specimen, from John Alan Elson [CC BY-SA 3.0]
Nautilus_Palau
The nautilus has a look that brings to mind the ancient shelled cephalopods of the past, with nautiloids recognizable as far back as 500 million years ago, there are actually features of their modern shells that seem to have evolved fairly recently. Image from Manuae [CC BY-SA 3.0]
450px-Cycas_circinalis
Cycads are gymnosperms that date back to the Permian, nearly 300 million years ago, and reached their highest diversity during the Jurassic and Cretaceous. But research shows that living cycads are the result of a re-diversification only about 12 million years ago, and not relicts from the distant past. Image from Raulbot [CC BY-SA 3.0]

If you’d like to read more about discussion of this term, take a look at these links:
Evolution in the Slow Lane (non-technical)
Rates of evolution and the notion of “living fossils” (technical)

 

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3 thoughts on “Episode 90 – “Living Fossils”

  1. yankee1635 July 3, 2020 / 10:32 am

    As a science teacher, I tend to find scientific nomenclature to be a problem for a lot of students, a problem which is not helped by various scientific disciplines using different definitions of similar terms, with a trivial example being the dissimilar meaning of “vector” to physicists and epidemiologists.

    My preference, as a non-paleontologist, non-biologist, would be to apply “living fossils” only to recently found organisms in orders or at least families that were first identified in the fossil record and believed to be extinct. I’d leave out species, as even with extant species, it’s sometimes difficult to reliably separate organisms into species without genetic analysis.

    I get to see horseshoe crabs (which I know are not crabs) pretty often (I live on the Connecticut shoreline). I never really thought of them as “living fossils,” just as really weird looking and even somewhat alien (that blue blood…).

    Ed C

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  2. llewelly October 24, 2020 / 7:32 am

    Recently I was using google books “ngrams” tool to graph uses of the phrase “living fossil” in comparison with other phrases. And I compared it to “plesiomorphic”, whose meaning is roughly similar to Darwin’s usage, and I got a surprise:

    https://books.google.com/ngrams/graph?content=living+fossil%2Cplesiomorphic&year_start=1850&year_end=2019

    Starting in about 1970, uses of “plesiomorphic” soared above uses of “living fossil” (a thing I never would have expected for a technical term compared to a popular phrase), peaked in about 1991, at about 8x the usage rate of “living fossil”

    Then uses of “plesiomorphic” fell as rapidly as they had risen, become nearly the same as “living fossil” by about 2017.

    And I’m wondering, how did “plesiomorphic” ever surpass “living fossil” in the first place, and then, what the heck happened in 1991?

    (There’s a whole discussion to be had about the biases resulting from the corpus of books (“try to scan everything!” is not as unbiased as it seems) scanned by the tool, but I’m not ready for that.)

    (There are some uses of “living fossil” which are counted by the tool, but in which “living” and “fossil” are separated by punctuation. This exaggerates the apparent uses of “living fossil”, unfortunately. However – removing that exaggeration, if I could figure out how, would still leave “plesiomorphic” with a huge expansion in uses from about 1970 to a peak in 1991, followed by a big fall, and it wouldn’t change the fact that there were at one time far more uses of “plesiomorphic” than of “living fossil” .)

    And on that note, if I haven’t already made too many episode requests, and episode that covered “plesiomorphic”, and related terms and concepts, might be a good thing to have.

    Like

    • commondescentpc October 24, 2020 / 6:52 pm

      An interesting finding! We don’t know much about the history of these terms, so we’re afraid we don’t have any stirring insights, but it’s a fascinating subject.

      Like

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