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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.”
In the news
A new species of horned gopher and the evolution of rodent horns
Early dinosaur eggs were soft-shelled
Strange fossil turns out to be the largest known soft-shelled egg … but who laid it?
Footprints in Korea come from a bipedal predatory croc-cousin
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.
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.
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.
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.”
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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If you enjoyed this topic and want more like it, check out these related episodes:
- Episode 46 – Cryptozoology
- Episode 2 – Crocodilians Past and Present
- Episode 56 – Evolution of Evolutionary Theory
- Episode 49 – Fake Fossils
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COMMON DESCENT 
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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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.
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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.
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Catching up and listening to the news on this one. That giant softshell egg seems like a slug (infertile ova). This makes sense if mosasaurs were live bearers because live bearing reptiles do lay slugs. The picture of the fossil actually looks like a dried up slug too.
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