Dec 04 2008

Turtle on the Half-Shell

I love it, on many levels, when dramatic new transitional fossils are discovered. Tiktaalik is a recent favorite – an important transition from fish to tetrapods. Ambulocetus (the walking whale) is also a beautiful transitional animal – it’s about as close as you can get to a half whale/half terrestrial mammal.

And, of course, my all time favorite is the classic archaeopteryx – bridging two major groups, dinosaurs and birds. Archeopteryx has teeth, clawed wings, and a bony tail, but also has feathers and could clearly fly. It lacks the keeled breast bone of modern birds for attaching powerful flight muscles, however, and also lacks a triossial canal, necessary to perform a wing flip maneuver for taking off from a standing start. This thing was half bird/half theropod dinosaur.

Now a new transitional fossil species has been discovered: Odontochelys semitestacea (the half-shelled turtle with teeth), which is 220 million years old.

The Evolution of Turtles

One of the ways in which I find these key transitional fossils so fascinating is that they tell us something about the particular pathway that the evolution of a certain group of creatures took.  Piecing together the evolutionary past is like putting together a jigsaw puzzle, except you have no picture of the final product to guide you. You have the satisfaction of clicking a new piece into place, and you also get a small clue as to what the final picture will look like.

Turtles are interesting because up until now all turtle ancestor fossils have had their shells completely intact. There were no fossils documenting how the turtle shell evolved (“how” meaning what phylogenetic path, not by what mechanism).  From a developmental perspective it looks as if the turtle shell forms out of the backbone and ribs. So we would expect to find turtle ancestors with partially expanded vertebra and/or ribs, but we could not be sure. Also, we did not know if turtles evolved on land, in the sea, if their top shell or bottom shell evolved first, and what purpose it served.

The turtle shell is a dramatic evolutionary adaption, and yet it appeared fully formed in the fossil record, so paleontologists could only speculate about its origins.

A report is about to be published in the journal Nature by authors Chun Li of the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences in Beijing, Xiao-Chun Wu of the Canadian Museum of Nature in Ottawa, and  Olivier Rieppel, PhD, chairman of The Field Museum’s department of geology. The report will present Odontochelys.

This turtle ancestor has a partially extended backbone – a partial top shell or carapace. It also has a complete plastron protecting its belly (the underside of the shell that modern turtles have).

This means that the bottom shell evolved first. This suggests that the creature was water-dwelling and needed protection for its vulnerable belly. On land such creatures would have their belly close to the ground and so it would not need protection. Once the lower shell was in place, a defensive strategy would favor upper protection as well. Once this process started, the pressure for more and more complete protection seems obvious.

This fossil also kills a competing theory – that turtle shells evolved from dermal plates, which is not uncommon among reptiles (such as the hard plates on many dinosaurs).

Incidentally – this species also had teeth, which is a primitive feature in turtles who lost their teeth in favor of a beak, much like birds.

Transitional Species

This term causes some confusion, which is exploited by creationists (who are themselves the most confused). In a very real sense all species are transitional. All species connect to the evolutionary web of life, and so represent a connection between neighboring branches. All species also have the potential to be the ancestors to future descendant species, and so are a transitional link from their past to their future.

But all species are also simply adapted to their current existence. They are not in the process of becoming anything – because evolution is blind to the future. The reptile ancestors of Odontochelys did not evolve into Odontochelys on their way to becoming modern turtles – they simply evolved into Odontochelys by adapting to their immediate needs. We can only see that Odontochelys was transitional from its ancestors to modern turtled by looking back at what happened.

This may seem a trivial point, but creationists perpetuate the myth that by “transitional” biologists mean some improbable monster, a chimera with mismatched parts that is in the process of becoming something but is not something itself. It is important to understand that transitional species are only transition in retrospect, and they were just as adapted to their environment as any creature.

Therefore the “half wings” of some bird ancestors were not really “half wings”, but were fully formed for something else, like display, or trapping insects. The “half shell” of Odontochelys is only a half shell because we know its descendants will evolve into modern turtles (or the descendants of one of its close relatives, as it may not be on the direct line to turtles), but it is fully formed for Odontochelys’s purposes.

Evidence for Evolution

Of course, transitional species are powerful evidence for evolution, which is why creationists spend so much time and effort trying to (unsuccessfully) knock them down. There are always details they point to in order to attempt to cast doubt on the evidence – the species may not be in the direct line to the putative descendant, the time-line may not be obvious, and the quality of the specimen is never perfect.

But their big strategy is to simply deny that transitional species are transitional. They fall back on the unfalsifiable claim that God could have made a reptile with only the bottom half of a turtle shell – that doesn’t prove turtles evolved from this creature.

What they miss is that science is about making predictions, and the better any theory makes predictions the more confirmed it becomes. If all life on earth evolved, from a common ancestor, then all life shares a common descent. Therefore if you take any two branches on the “tree of life,” sometime in the past there must have been a common ancestor. Or, there must be a transition between any child group and its parent group.

Whales evolved from terrestrial mammals. Evolution predicts we should find specimens from a time when whale ancestors were part way between their terrestrial ancestors and modern aquatic whales. This prediction was validated by Ambulocetus and other specimens.

Birds as a group are most closely related to reptiles, which are a more ancient group. Therefore it is likely that we will find specimens from a group of reptiles that were bird-like but had not yet evolved all the characteristic of modern birds. This is exactly what we find.

We should find walking-fish who were ancestors to modern tetrapods – done.

And we should find turtle-like reptiles with only part of a modern turtle shell. Now we can check off that box as well.

Creation does not require that such species should exist at an appropriate time in the past. It does not predict them (actually, it predicts nothing, because it’s not a scientific theory). Common descent through evolution requires, and therefore predicts, and such specimens exist – and we are finding them at a steady rate, each one further validation of evolution.

Exploiting Legitimate Scientific Controversy

One of the tactics of creationist is to misinterpret legitimate discussion about how evolution unfolded as if it casts doubt on whether evolution happened.  This new find is a single fossil filling in a sparse area of the fossil evolutionary tree, so of course there are multiple ways to interpret its significance.

In the same issue of Nature Robert R. Reisz & Jason J. Head wrote an interesting comment:

Although this evolutionary scenario is plausible, we are particularly excited by an alternative interpretation and its evolutionary consequences. We interpret the condition seen in Odontochelys differently — that a carapace was present, but some of its dermal components were not ossified.

Very interesting. This is, of course, science at its best – considering all possibilities, presenting alternative interpretations of the data. This debate will likely continue until it is resolved by new specimens. Of course, both interpretations are evolutionary, and either way this specimen is an important primitive turtle – the most primitive yet discovered.

And, of course, the creationists are already exploiting this internal debate as if it casts down on evolution itself.

What creationists don’t do, of course, is make predictions. What fossil discoveries would invalidate creation? The answer is, none, because creation is not a falsifiable scientific theory. Evolution, on the other hand, would have crumbled if we kept finding fossils out of any possible evolutionary temporal sequence or morphological pattern. Horses in the Cambrian fauna would do it.

Conclusion

Odontochelys is the oldest, and most primitive turtle. It fits nicely into an evolutionary pattern – throwing one more piece of evidence onto the mountain that is already there.  The details and controversies will be worked out when further evidence comes to light. And I predict that it will.

The same thing happened with the evolution of whales – the first specimen was dismissed as an anomaly, but then a slew of whale ancestors were found fleshing out the whole sequence from land to sea.

There was a time when archaeopteryx was the only transitional species from reptiles to birds, and creationists claimed it was just some weird bird. Now, an entire group of feathered dinosaurs has been discovered, showing a variety of primitive, derived, and transitional features.

Answersingenesis has this to say about the fossil evidence for turtles:

The biblical account of Creation in Genesis —animals created to reproduce after their kinds—would mean that turtles should be instantly recognizable as turtles, with the shell and other unique features fully formed from the start, and no series of ‘pre-turtle ancestors’ should be found. It is obvious that the fossil record of turtles gives powerful support to biblical Creation, and stands opposed to the idea of evolution.

Hmm…but then the pre-birds, pre-whales, pre-terretrial vertebrates, pre-mammals, etc. should have already invalidated creationism. I will hold them to their statement, however. When more turtle ancestors are discovered will they stick to their prediction that according to creation they should not exist? Of course not. They will do what they did for whales and birds – make ridiculous arguments about the tiny details and miss the obvious big picture.

I predict that now that we know where to look, we will find further primitive turtles and we will flesh out a branching bush of creatures filling in the morphological space between reptile ancestors and more modern turtles.

This scenario has played itself out numerous times now. Each time creationists scream and yelp about the gaps in the fossil record. But when those gaps are filled, they never acknowledge it or adjust their conclusions – they just move over to some other gap.

The cognitive dissonance must be deafening.

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21 responses so far

21 Responses to “Turtle on the Half-Shell”

  1. Jim Shaveron 04 Dec 2008 at 11:08 am

    Thanks, Dr. Novella. This is cool stuff!

    My observation is that you have followed a solid scientific process with regard to the creationist paradigm. You’ve hypothesized that creationists are stupid, unimaginative, and disingenuous. You’ve then made specific, testable predictions about their future behavior and arguments, based on your original hypothesis. Where necessary, you’ve modified your original hypothesis to better explain the growing warehouse of collected data and made further predictions based on the new model.

    At the rate at which data confirming your creationist predictions is compiling, I think your hypothesis is close to becoming a recognized scientific theory, or even a natural law. Ain’t science great! :D

  2. DevilsAdvocateon 04 Dec 2008 at 11:45 am

    The creationists must be giddy over this finding because filling one gap in the fossil records creates two gaps around it for a net gain of one new gap for every new finding.

  3. colluvialon 04 Dec 2008 at 1:40 pm

    After so many defeats when trying to sound sciencey, you’d think the creationists would just give it up. If I were a creationist, I think I would stick with only fun, magical explanations. Maybe something like: “God’s life-creating spell was so powerful that even the rocks tried to take the forms of creatures. Therefore, fossils have no meaning.”

  4. Traveleron 04 Dec 2008 at 3:06 pm

    Proof that The Fall resulted in defective, half-shelled turtles! Creationism wins again!!! I don’t know how paleontologists can keep supporting Darwinism when they keep digging up incontrovertible evidence of Genesis.

  5. DevilsAdvocateon 04 Dec 2008 at 3:43 pm

    Consider what would happen if the impossible were to occur, that science was able to fill in every single gap in the fossil record. How long do we suppose it would take creationists to confabulate a belief-serving explanation?

  6. pecon 04 Dec 2008 at 7:50 pm

    “Of course, transitional species are powerful evidence for evolution, which is why creationists spend so much time and effort trying to (unsuccessfully) knock them down.”

    There is no real scientific controversy over evolution, and creationists have no evidence or logic to support their dogmatic faith. I don’t know why you bother arguing with creationists — oh wait, I do know. It’s easy to win arguments with creationists, and to imply that Christian creationism is the same thing as Intelligent Design.

    I am sure you know that ID does not deny evolution. You might even know that there are several competing theories of evolution, none of which have been proven.

    Transitional forms support the theory evolution. Oh big deal. But they do not support the currently accepted mainstream theory of what causes evolution (random, goal-free, variations chosen by natural selection).

    I believe in evolution and I believe in natural selection, as does every rational scientific person. But I know that the currently accepted theory has no supporting evidence.

    Materialists have a hard time understanding this logic. How can you believe in evolution and natural selection, yet deny that evolution is caused by random variations and natural selection?

    Well just because two or more things are accepted as true does not mean there must be a simple causal relationship between them.

  7. Clinton Huxleyon 05 Dec 2008 at 4:17 am

    You can put lipstick on a pig – it’s still a pig. You can put a scientific veneer on creationism – it’s still creationism. To argue that ID and creationism are different beasts is merest sophistry.

  8. PaulGon 05 Dec 2008 at 5:53 am

    The constant, dynamic results of evolution are anything but random.

    Just try and theorise a more cut-throat, relentless method of effectively routing out failures in favour of successes, than simply killing off unsuccessful variants and allowing successful ones to populate an environment. An environment, that helps shape the variants in the first place.

    Evolution is anything but random, it is a “red in tooth and claw” method of eliminating the weak in favour of the successful.

    Why is it, that whenever proponents of ID (that is, creationism in a lab’ coat), attempt to criticise the scientific consensus that is evolution (a “theory” in name only), they fall back on the fallacious notion that evolution is random?

  9. PaulGon 05 Dec 2008 at 7:22 am

    As an add-on to my earlier posting, after I logged out I remembered a paper I read a while ago and did a quick Google-search. It’s a rather neat study, published in the journal, Current Biology (Kiontke et al., 2007). The team were looking at variation in organs which change very little across related species. They looked at the vulva of 50 different species of nematode (roundworm), predicting that they would find little variation in vulva development.

    The team were (in their words) “astonished” at the amount of developmental variation they found. They then reasoned that “since [this variation] did not affect the final adult vulva” it should have evolved in a random (stochastic) manner.

    Looking at “more than 40 characteristics of vulva development, including cell fates, fate induction, cell competence, division patterns, morphogenesis, and related aspects of gonad development”, the team drew up a “phylogenetic tree”, illustrating evolutionary change.

    The results, instead of showing few changes, showed exactly the opposite. They found – to quote the abstract “an astonishing amount of variation and an even larger number of evolutionary changes, suggesting a high degree of homoplasy (convergences and reversals).”

    In addition, the evolutionary changes they observed were nearly all unidirectional in nature. Only two characteristics showed what they termed “unbiased evolution”.

    In their conclusions, they proposed that “developmental evolution is primarily governed by selection and/or selection-independent constraints, not stochastic (random) processes such as drift.”

    Now, I know that this is only one study and that we should not look at these things in isolation, but the point is, nematodes are well-studied and well-understood, they provide an excellent example of observable change over time as a result of environmental pressure – evolution. The nematode species, analysed for this study, reflect the findings of current research and what we would expect from predictions of a non-random system.

    Put into context, this is not an isolated paper, it reflects an analytical process which has come to an objective consensus. Science.

    Reference
    =========

    Kiontke, et al. 2007: Kiontke K, Barrière A, Kolotuev I, Podbilewicz B, Sommer R, Fitch DH, Félix MA. Trends, stasis, and drift in the evolution of nematode vulva development. Curr Biol. 2007 Nov 20;17(22):1925-37.

  10. Steven Novellaon 05 Dec 2008 at 8:34 am

    For the record, the Disco-tute (the primary institute of ID) writes numerous blog entries denying evolution and denying the transitional status of accepted transitional fossils, like Tiktaalik. Just wait – they will deny this one too.

    The notion that evolution happened but was guided by an inherent “intelligence” is held by some, but is a small minority.

    I challenge anyone to find an official statement by the DI that indicates they accept the fact of evolution (and not just “microevolution”, but the origin of all species through descent with modification).

  11. Clinton Huxleyon 05 Dec 2008 at 9:55 am

    @ PEC

    “There is no real scientific controversy over evolution, and creationists have no evidence or logic to support their dogmatic faith. I don’t know why you bother arguing with creationists — oh wait, I do know. It’s easy to win arguments with creationists, and to imply that Christian creationism is the same thing as Intelligent Design.”

    No, the reason why many fight so strenuously against these charlatans is to try to stop the creeping growth of their influence on the public, school boards and legislators.

    @ PEC
    “You might even know that there are several competing theories of evolution, none of which have been proven. ”

    No, the theory of evolution merely states that populations show descent with modification. There may be debate about precisely how that happens and the relative importance of different factors in evolution but there is only ONE theory.

    @PEC
    “Transitional forms support the theory evolution. Oh big deal. But they do not support the currently accepted mainstream theory of what causes evolution (random, goal-free, variations chosen by natural selection).”

    Descent with modification, transitional forms, extinctions, vestigial or clumsily cobbled together organs or body parts. All these support the currently accepted mainstream theory of evolution. I’d be interested to know precisely what evidence there is that contradicts it…..

    @PEC
    “I believe in evolution and I believe in natural selection, as does every rational scientific person. But I know that the currently accepted theory has no supporting evidence. ”

    Do you often believe in things for which you think there is no evidence?

  12. pecon 05 Dec 2008 at 11:01 am

    “Do you often believe in things for which you think there is no evidence?”

    THERE IS EVIDENCE FOR EVOLUTION. I don’t know how else to say this. THERE IS NO EVIDENCE FOR THE CURRENTLY ACCEPTED THEORY ABOUT WHAT CAUSES EVOLUTION.

    SN: “I challenge anyone to find an official statement by the DI that indicates they accept the fact of evolution”

    The major ID theorists all accept the fact of evolution. You are wrong.

  13. DevilsAdvocateon 05 Dec 2008 at 12:13 pm

    Pec describes herself to scientists as a ‘rational scientific person’ in the same way a drunk describes himself as sober to a policeman. And with the same results.

  14. Puppet_Masteron 05 Dec 2008 at 3:57 pm

    “Well just because two or more things are accepted as true does not mean there must be a simple causal relationship between them.”

    pec, when you roll a steel ball in a bowl it will eventually settle at the bottom of the bowl at it’s lowest energy state. It’s obvious that things want to be at their lowest energy state possible.

    The ball obviously fell to a lower energy state (fact). Presumably, animals evolve (fact). Earth has a gravitational field (cause). Environmental pressures arise (cause).

    It is just as appropriate to say that things falling only correlates a gravitational field than it is to say things evolving only correlates with selective pressures.

    Maybe Steve can help me out on this, but I’m pretty sure, logically, you can never 100% prove a causal relationship.

  15. Traveleron 06 Dec 2008 at 1:31 am

    Pec,

    Who are these major ID theorists accepting evolution of whom you speak?

  16. PaulGon 06 Dec 2008 at 1:27 pm

    SN: “I challenge anyone to find an official statement by the DI that indicates they accept the fact of evolution”

    pec: “The major ID theorists all accept the fact of evolution. You are wrong.”

    So, er, where are the “official statements” by the DI and/or these “major ID theorists” then?

    In the UK we have a technical term for the sort of statement provided by pec (above – choose any really). It’s not used by all scientists, but any that have had anything to do with discussing ID/creationism at any time have used it. The term is… “complete bollocks”.

  17. TheBlackCaton 06 Dec 2008 at 6:40 pm

    @ Pec: “THERE IS NO EVIDENCE FOR THE CURRENTLY ACCEPTED THEORY ABOUT WHAT CAUSES EVOLUTION.”

    What sort of evidence, if it existed, would you think of as supporting the currently accepted theory about what causes evolution? You are obviously not convinced by the existing evidence, so what evidence would you need to be convinced?

  18. wertyson 08 Dec 2008 at 2:21 am

    What does this finding do to the theory that it its ‘turtles all the way down?”….

  19. Clinton Huxleyon 08 Dec 2008 at 5:33 am

    I agree 100% with Pec. The evidence, as it stands, does not support the currently accepted theory of evolution. But then, I don’t believe that the geographical evidence as it stands supports “spherical Earth” theory, either. Furthermore, it’s time for my mashed banana and then the nurses are going to let me watch “The Worlds Most Tedious Pets” in a minute. Gibber.

  20. TheBlackCaton 08 Dec 2008 at 8:30 pm

    @ Pec: It’s been two days, I am still waiting for you to tell me what sort of evidence you would find convincing.

  21. [...] At Neurologica, Steven Novella describes a new palaeontological find, Odontochelys semitestacea (the…. O. semitestacea is a 220 million year old fossil from the turtle lineage, and suggests that the carapace (top of the shell) and plastron (bottom) may have evolved separately rather than at the same time. O. semitestacea has a well-developed plastron but only a hint of the formation of a carapace. Thus the shell in this specimen gives protection to the lower half of the body, perhaps particularly useful when the turtle is at the surface breathing. [...]

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