Some of the "hottest" fossils of the last couple of decades could be considered "missing links" - Ardepithecus, Tiktaalik, some of those Chinese bird fossils. I guess my question would be, why do you think this concept is outdated? About 70% of Americans think that creation, rather than evolution, is responsible for the diversity of life, and so it seems to me that even what seems rather basic to a scientist might be a very useful tool for a high school teacher.

I think fossils remain the most direct evidence of evolutionary process. An alternative since last decades is DNA-based phylogeny: say more similar species have less differences in neutral DNA sequences, than more dissimilar species (usually it is like this). Then, given a concept of gene drift, one expects that the neutral nucleotide substitutions aggregate with time, and only plausible explanation of the pattern is that two species are more similar to each other than to a third species because they had more recent common ancestor. However, this evidence is more complicated and one should simultaneously explain how and why DNA differences should aggregate with time. Thus, fossils remain more illustrative and obvious. One should also consider that radioactive methods of fossil dating provide more and more exact results, and if fossil of an intermediate form (say, Homo habilis is intermediate between H. sapiens and Australopithecus sp.) is also proved to be fossilized in intermediate time (later than Australopithecus but of course earlier than modern human) - this is very direct and illustrative evidence, easily understandable for students, according to my experience. One should remember that when first "missing links" were discovered, methods of geological dating were indirect and not based on the radioactive methods, so there is a principal progress since that time.

It is clear as day that fossils are important as evidence for evolutionary transitions but - as noted by Gustavo - it is tricky to define or explain to others what 'transitional fossils'/ 'missing links' are from a modern scientific perspective on evolution.

Of cause one could try to tell a story how these terms evolved:

In earlier times when the fossil record was not yet well known (let alone to the general public), scientists were mainly looking at extant groups when they thought about evolution. Given the observation that of two allegedly related extant groups one often appears to be more primitive than the other and thus was interpreted as ancestral, people asked for missing links, i.e. some organism that was anatomically intermediate between the more primitive and the more derived extant group. And thus scientists searched for and found some fossil (e.g. Archaeopteryx) and extant examples (e.g. Platypus) for such transitional organisms and proclaimed them as "missing links".

However, when the fossil record became better known and the knowledge about phylogenetic relationships increased it became clear that the original idea of an intermediate between two extant groups (e.g. reptiles and mammals) was a misconception because both groups - also the seemingly primitive one - were in fact distinct from the common ancestral group and product of a long (post-split) evolutionary process, i.e. the central idea of 'missing links'/'transitional fossils' and how they should look like was found to be wrong in the first place.

Because claiming to have found a 'missing link' was such a nice media buzz, scientists kept the term and redefined it as a piece of fossil which is particularly informative concerning a certain morphological transition deep in time (e.g. Tiktaalik, Darwinius, Jehol birds). Otherwise the term 'transitional fossil' (newly discovered transitional fossil = (no longer) 'missing link') is empty because almost all fossil and recent species (except the terminal ones) can be considered as 'transitional' between others.

Or is there another/better definition in agreement with recent phylogenetic concepts?

This book explains why the 'stasis' is common among sexually-reproducing organisms:

http://www.frozenevolution.com/

and a link to the author's page w/more useful material:

http://web.natur.cuni.cz/flegr/

HTH, best regards, Peter.

@ Gustavo: Thanks for the ref - I will have a look how Wägele discusses transitional fossils etc. in relation to stem-group/ crown group concepts. Is 'being the product of evolutionary stasis' the only criterion for 'living fossils' or are there others (rare, low diverse group; conservation of a special/important feature)?

@ Andrew: Situation is somewhat different here with most of the teachers (evolution is generally accepted as a fact which is to some degree a secularization effect of GDR real socialism). However, topics/ content of evolutionary biology highschool teaching is often some tens of years behind (as usual).

Is e.g. Archaeopteryx still as important as it was 30 years ago? I daresay no, because there is now a large and overwhelming record for early birds and their ancestors and we could do without that particular 'missing link'. Perhaps we generally don't need a special category 'missing links' or 'transitional fossils' any longer...

@ David: Being a palaeontologist I am somewhat aware of the various possibilities how fossils (still) contribute to evolutionary biology. But can I leave out concepts such as 'missing link' and 'living fossil' and still be succesful in illustrating the importance of fossils to children and non-specialists?

@ Peter: But why is it important for the discussion of living fossils/ transitional fossils/ missing links how evolutionary stasis came about?

Do we asume that stasis in morphology is accompanied by stasis in things that do not fossilize eg, physiology and behaviour?

@ Gustavo; Yes, altogether there is an explanation (though no rigous definition) what 'living fossils' are and why they are relevant for conservation biology.

Perhaps interesting: Article of Casane & Laurenti (2013, in press) about coelacanths and why concepts, such as 'living fossils', make not much sense from a tree-thinking perspective on evolution.

@ Athol: The answer should be no: Stasis in (skeletal) morphology could also be the consequence of a lack of skeletal correlates to changes in soft body anatomy, physiology and behaviour.

(Actually, there are some indications for physiology and behaviour of fossil organisms, namely histology, chemical and isotope composition of hard parts as well as trace fossils.)

Certain remaining quests about stasis could be worth an own thread.

Article Why coelacanths are not ‘living fossils’: A review of molecu...

@ David: Being a palaeontologist I am somewhat aware of the various possibilities how fossils (still) contribute to evolutionary biology. But can I leave out concepts such as 'missing link' and 'living fossil' and still be succesful in illustrating the importance of fossils to children and non-specialists?

hm, good question. I am not palaeonthologist... Perhaps by this reason I do not exactly understand what "missing link" is. If you have a modern species, and a fossil that you think could be an ancestor of this species, you can (1) substantiate this opinion by similarity of homologous morphological structure, or (2) suggest that your evidence is not strong enough and can be strengthened if you find another fossil, dated with a later geological time and with morphology intermediate between your first fossil and respective current species. This second fossil you are looking for to support or reject your hypothesis is a missing link, or? If yes, why you should leave this concept out?

"Living fossil" is different, and I do not really understand how it helps to understand the evolution. It just says that at least some animals/plants, which lived many millions of years ago, did not change much. Then, it could be used as an argument against evolution :) One can hypothesize that all extant species are living fossils, which fossilized forms from early geological time just were not discovered yet. Consequently, the rows of gradually changing in time morphological structures are best evidences of evolution, if DNA is left out, but presence of "living fossils" or evolutionary stasis in general is rather a counter-argument :)

The problem about 'missing links' and 'transitional fossils' : You give a public audience the (wrong) expectation that there should be something morphologically intermediate between two related living animal groups like reptiles and mammals.

Extant reptiles are percieved as more primitive than mammals and thus - according to the logic of missing links - the ancestors of mammals should be extant-reptile-like or intermediate between extant reptiles and mammals. But in reality they are morphologically different:

Common ancestor of (extant) reptiles and mammals

= common features of reptiles and mammals

+ plesiomorph features of reptiles reduced in mammals

+ plesiomorph features of mammals reduced in reptiles

+ ancestral features reduced in both, reptiles and mammals

- convergent features of reptiles and mammals

A fossil organism which was a likely ancestor of two related modern groups is often not intermediate between them. But by claiming you have found a 'missing link' you give the impression that it should be like that.

OK, I think now I understand better. You suggest 'missing links' and 'intermediate morphologies' are the same. Of course your arguments completely apply to such situation. Moreover, if you accept cladistic phylosophy, there is no place for intermediate forms. I understand 'missing links' in much more narrow sense. If you have a chain of fossils linking Eogippus and modern horse, or Australopithecus to modern human, and within this chain a fossil with intermediate morphology is also fossilized in intermediate time, then it is a very good and most direct evidence of macroevolution, I think. And I think that palaeonthology is a good tool to provide evidence for evolution per se, but not so good to provide evidence of divergence. Divergence is easier demonstrated by DNA data, and (albeit indirectly) by absence of clear genetic boundaries among closely related forms.

In terms of "living fossils", no one expects a taxon to be identical to its similar morphotype of 480my, like Lunataspis and Limulus (or Limulus polyphemus and Limulus sp of the Jurrassic) having similar body morphology and likely very similar "marine niche roles" in both the ancient and modern ocean, some things do not change in ecological requirements over time. But it is also important to NOT give the false idea that evolution is "progress" or "goal directed" and that every modern species must be different from every extinct species. Clearly there are new niches and ecosystems that have opened over time, but some are the same, like the shoreline, shallow sea niche filled by the likes of at least 4 modern living species of the merostomates, horseshoe crabs. The key I think is to NOT think that everything has to change, a species changes, OR NOT, in response to ECOLOGICAL AND ENVIRONMENTAL SELECTION PRESSURES! that it is faced with over time. I think that is what Darwin meant when he coined the very useful term "living fossil", otherwise we would not still be arguing about it and trying to publish books and papers to "debunk" and devalue the concept. An idea does not have to be pristinely perfectly correct in every detail to have VALUE both in teaching and for stimulating research. In working with the public in teaching animal natural history and biology, If I talk about crown groups and phylogenies and stasis, they will tune out, if I raise the question: "Do all organisms physically appear to change over time? which ones changed the most and which have more similar morphological features and therefore reasonably similar "marine jobs" in the marine ecosystem, then not only do i pique the interest of those people who are not systematists ( I general get about a few seconds or minutes to make that connection), but it gets them thinking about what evolution is and they may even read more about it. It establishes interest and a real discussion that evolution is not some magickal event, but simply individual organisms and their genes/adaptations selected for or against by the world they live in, surviving or not, and passing on their genes. If something works, don't change it. The horseshoe crab suggests there is job security on Earth for some.

Dear Ingrid and Jon, the initial question was arguably not well posed because it consists of four individual questions:

Are "missing links"/ transitional fossils a concept still used in scientific debates?

Are "missing links"/ transitional fossils (nevertheless) useful for teaching science?

Are "living fossils" a concept still used in scientific debates?

Are living fossils (nevertheless) useful for teaching science?

"Living fossils" can be easily explained as descendants of a lineage with almost no change in appearance over long time. But is it reasonable to keep "living fossils" as an own category or is it better to point out that they simply represent one end of the spectrum from evolutionary stasis to fast evolution?

As a teacher you could mention the term without giving it too much importance: "Some groups evolve fast, some slowly, and if a member of an apparently slowly evolving group has survived until present it is sometimes called 'living fossil'."

How a "Missing links"/ "transitional fossils" are defined and can be distinguished was never properly explained at my school. And indeed why some species are special when almost all are in transition is hard to convey. Perhaps a try:

"At the top of the tree you can see several isolated tree limbs. Imagine you cannot see how they grow together further downwards when they form the trunk. This is how we see the tree of life today without fossils. Among the fossils (deeper branches) some are particularly helpful for our understanding of tree shape because they can tell us how/ in which order the tree limbs we see at the top have split. These deeper branches of the tree of life are sometimes called "missing links"/"transitional fossils".

Jon and Michael! your observations are very insightful, I will be thinking about them in preparation for my next round of "trying to inspire the public to think about fossils, evolutionary trees and adaptations of marine life"! thank you

Here is an interesting paper which examines, and firmly rejects, the idea that coelacanths are ' living fossils':

Casane, D. & Laurenti, P. 2013. Why coelacanths are not ‘living fossils’: A review of molecular and morphological data. BioEssays 35: 332–338

http://onlinelibrary.wiley.com/doi/10.1002/bies.201200145/abstract

As to the idea that some lineages are ' intermediate', 'transitional' or 'missing links', it is important to realise that all species are intermediate in the sense that they share a combination of primitive and derived character states. In fact, this is what makes phylogenetic analysis possible.

A taxon (e.g. chimpansees) which shares some plesiomorphic character states with one taxon (Gorilla) and some derived character states with another (humans), will no doubt be considered ' intermediate' by many non-phylogeneticists. However, this is misleading because it attempts to describe the Chimpansees relationships (i.e. their position in the tree, sister to humans) in terms of some of its characters (which like any other taxon are a mixture of primitive and derived character states).

So, I'd recommend that anyone teaching evolution clarifies the difference between taxa, relationships and characters. When these become confused, people will not fully understand evolution. The terms 'primitive taxon', 'basal taxon', 'intermediate taxon', 'transitional fossil' or 'missing links' are unhelpful and should be avoided.

Dear Jon, you can separate cladistic methods of optimizing phylogenetic trees from "tree-thinking" (as discussed in the paper mentioned by George Sangster and me in the discussion above). The idea that extant and fossil species can be put in a hierarchy of dichotomic relationships which reflects the exact order of divergences is arguably fruitful in itself and independent from the mathematical procedure through which a model of such a tree is gained.

Concerning the epistemic vagueness of ancestors: You could explain to students that modern palaeontologists (often) don't seek after ancestors because they (often) have no means to determine whether or not an older species was the direct ancestor of some related younger species. Ignoring the question of direct ancestry these scientists focus on more promising issues, e.g. try to infer modes of character evolution as well as the historical circumstances and timing of morphological transitions from (time-calibrated) phylogenetic trees.

In this regard Tiktaalik is not important because it could be a tetrapod ancestor but because its study and inclusion in phylogenetic analyses modifies our views about phylogenetic tree shape as well as the order of morphological transitions in tetrapodomorphs and gives us an idea how and how not tetrapod ancestors looked like and functioned.

Hi Michael,

unfortunately I didn't have the time to read the previous answers but here is my opinion, I hope it helps: these concepts are very often misunderstood. You can use them of course with the idea that (1) they are outdated conepts and (2) they do help us in the sense that they present more ancestral characters, and this helps to verify hypotheses etc. This is the way that these concepts were introduced to us during the introduction to systematics course during my masters degree, toghether with the specification that we should never write these words and only use them in oral discussions if we wish to make a point.

You can take a look at this book: http://sfs.snv.jussieu.fr/resources/darlu_tassy.pdf

it is in french.

Thank you for the ref.

I think the whole issue would be worth an article, probably written in German/ for biology education purposes only, if it turns out the matter has been excessively treated by someone else. Particularly interesting: how the scientific perception of major evolutionary transititions changes with an increasing amount of fossil (and other) evidence and how scientific advances affect earlier scientific concepts.

I once heard a professor claim that the coelacanth had been evolving, just like all creatures - but only on the inside!

Involuntarily, I burst out laughing. Perhaps I was unjustified to do so. The 'intuitive' side of me assumed that if it was the same on the outside then surely, it was the same on the inside.

Thinking about the coelacanth made me think of life in an entirely different way. What the consesus view is that evolution is blind. Somehow, the image of evolution being driven by some blind molecular mechanism buried deep inside the cell just did not seem to chime which what my eyes could witness.

I imagined the humming bird and asked - Did evolution grow the humming bird's beak down into the deep receiving flower, or else, did the flower leap up and embrace the humming birds beak. The answer seemed then, and still does, obvious - they have slowly cleaved , each to the other, over the millennia.

For me, this vision seems to suggest that what drives evolution, is not a blind mechanism buried deep inside the cell, we should look to epigentics and the possibility that it is the ENVIRONMENT that drives evolution.

The coelacant's habitat is just below the rim of the continental shelf - a sparse, simple environemtn that has existed as an uninterrupted habitat for millions of years..

The coelacanth hasn't evolved because its environrnent hasn't evolved!

The reason that the coelacanth should still figure as a an educational example, regardless of the point of view of the educator, is because it is a useful way to illustrate two very different approaches to the understanding of the evolutionary process.

"The evolution of life is the evolution of catalysis".

Davia, C.J (June 2006), "Life, Catalysis and Excitable Media: A Dynamic Systems Approach to Metabolism and Cognition", in Tuszynski, J.A, The Emerging Physics of Consciousness (The Frontiers Collection), Springer, pp. 255–292, ISBN 978-3540238904

Considering 'transitional fossils' ('missing links') and/or 'living fossils' :

There is a nice book (annd other materials) on Frozen Evolution by Jaroslav Flegr

http://www.frozenevolution.com/ explaining both gradualism and punctuated equilibria

https://en.wikipedia.org/wiki/Punctuated_equilibria.

HTH, Peter.

Dear all,

First, many thanks to have cited our publication. I appreciate because It is very difficult to make the tree-thinking perspective spreading outside the evolutionists community, especially because generalist review like Nature or Current Biology LOVE to use the "living fossil" baloney to advertise theit publications.

In addition to my published arguments, I want to make clear that the idea that "The coelacanth hasn't evolved because its environrnent hasn't evolved!" is wrong.

Natural selection IS NOT the only force that trigger evolution. The genetic drift is the major source of changes in small population. As Latimeria population are very small, they may change very quickly.

Therefore, the low substitution rate detected in Latimeria proteins by Chris Amemiya team is either due to an unidentified Dna-repair (as it exists in paramecium) or to a bias in dataset.

Hoping you'll appologize my poor English

Patrick

This is a GREAT forum and everyone's ideas have merit. My perspective is that I think we have "over-thought" the use of "living fossil".

No one is saying the the horseshow crab or coelocanth have remained unchanged since their appearance in the fossil record. The point is that throughout geological history there has been alot of structural change in cladisitc lineages for some taxa but not for others. This is a valid and interesting subject to debate (as we are doing!) NOT TO BURY or ignore or stamp out the use of a phrase or idea. Correct errors, restate terminology and see what value there is in the idea. It is also VERY important that lay people do not think evolution is GOAL ORIENTED and MUST INVOLVE CHANGE. Gould observed the importance of long periods of STASIS in lineages, even if it is only morphological. Outshape shape does impact ecological function and behavior. Macro-observable change (molecular is a different argument) in evolution is in response to changes in environment or behavioral shifts. If there is no change in selection pressures there is no selection for peripheral phenotypes that might match a new niche better. It is also interesting to ask why some species represent lineages that have evidenced less structural design change like the horseshoe crabs. It is inherently interesting.

Ingrid Kaatz wrote :

"No one is saying the the horseshow crab or coelocanth have remained unchanged since their appearance in the fossil record"

Unfortunately, a series of genomic papers issued in Nature or Current Biology between 2010 to 2013 WERE actualy saying that coelacanths underwent werry few changes since the Devonian. That is pecisely why we wrote our review "Why Coelacanths are not Living Fossils". Unfortunately too, several recent papers still refer to Latimeria as a living fossil.

I am affraid that the living fossil concept is like the Phoenix : it rebirth from its ashes!

For whom should be interested by this question : a symposium intitled "Living Fossil, Myth or reality ?" will be organized at the next European Evo-Devo Society Meeting that will happen in Vienna (22-25 july) http://evodevo2014.univie.ac.at/

Previsionnal program see also: http://evodevo.eu/conferences/2014

Patrick: thanks for that link. Myth or reality? There is also something in between: A deliberately created (over-)simplification that is kept and nourished for reasons of publicity but from time to time erroneously taken for real by the scientists themselves.

@Patrick: thanks for the link. I wrote a short reply to the paper published in Gene 2003, criticising that concept:

http://www.sciencedirect.com/science/article/pii/S0378111904000800

Perhaps this is a foolish question, but don't the evidence of the Burgess Shale and the concept of Punctuated Equilibrium obviate the need for trying to establish "transitional forms"?

Sorry for the delay in responding!

@Michael : I am affraid that you are right. One striking evidence for the advertising-entertaining hypothesis is the fact that "Living Fossil" is a term that is used mainly in very high impact publications, not in pecialized ones.

@Peter :I must confess that I missed your response to the 2003 paper. Thanks for the link, I gonna read it carefully !

@Bill : You're right "transitional form" is a ill-defined concept because no fossil species could be identified as an "ancestor" in a phylogeny. During decades "transitional form" was mixed with the concept of mosaicism. For instance Archaeopteryx was called a transitional form between dinosaurs and birds because it has tooth AND feather. From a tree-thinking point of view, Archaeopteryx is not a link, it is a bird and birds are the last living dinosaurs. Every extant species is a mosaic of ancestral and derived traits, and think that extant chiken have feather (like birds) AND scale (like dinosaurs) but no one ever called them "tansitional forms" ;o)

I think that the ancient fossils of living organisms are one of the best evidences of evolutionary science in the emergence of life and the evolution of living organisms, so it cannot be dispensed with.

Hello all; The context of the original question was pitching a course to a particular audience - High School biology teachers. Where I live in California, not all biology teachers are well trained in evolutionary biology and a fair number do not have a degree in biology. That's the audience.

Allow me to focus on only one aspect. The term "transitional" has a teleological connotation. The word transitional implies directionality and/or goal directedness. There is no evidence that evolutionary processes display either. The English language makes it easy to speak teleologically, even when it isn't intended. Both students and teachers do it all the time and rarely recognize it. I see the error in the refereed litarature all the time...committed by professionals. If the course taught nothing else, that distinction would make it worthwhile! Hopefully, Jim Des Lauriers

every organism is transitional until they went extinct

also nothing is missing, it just not discovered yet