Pharyngula

Tetrapods are older than we thought!

Some stunning fossil trackways have been discovered in Poland. The remarkable thing about them is that they’re very old, about 395 million years old, and they are clearly the tracks of tetrapods. Just to put that in perspective, Tiktaalik, probably the most famous specimen illustrating an early stage of the transition to land, is younger at 375 million years, but is more primitive in having less developed, more fin-like limbs. So what we’ve got is a set of footprints that tell us the actual age of the transition by vertebrates from water to land had to be much, much earlier than was expected, by tens of millions of years.

Here are the trackways. Note that what they show is distinct footprints from both the front and hind limbs, not drag marks, and all that that implies: these creatures had jointed limbs with knees and elbows and lifted them and swung them forward to plant in the mud. They were real walkers.

i-7c534308c9f00e73cf31353a1066764b-trackway1.jpegi-55a13b4b3a79452858e68be0cd01fa4f-trackway2.jpeg
Trackways. a, Muz. PGI 1728.II.16. (Geological Museum of the Polish Geological Institute). Trackway showing manus and pes prints in diagonal stride pattern, presumed direction of travel from bottom to top. A larger print (vertical hatching) may represent a swimming animal moving from top to bottom. b, On the left is a generic Devonian tetrapod based on Ichthyostega and Acanthostega fitted to the trackway. On the right, Tiktaalik (with tail reconstructed from Panderichthys) is drawn to the same shoulder-hip length. Positions of pectoral fins show approximate maximum ‘stride length’. c, Muz. PGI 1728.II.15. Trackway showing alternating diagonal and parallel stride patterns. In a and c, photographs are on the left, interpretative drawings are on the right. Thin lines linking prints indicate stride pattern. Dotted outlines indicate indistinct margins and wavy lines show the edge of the displacement rim. Scale bars, 10 cm.

They were also big, approximately 2 meters long. What you see here is a detailed scan of one of the footprints of this beast; no fossils of the animal itself have been found, so it’s being compared to the feet of Ichthyostega and Acanthostega, two later tetrapods. There are definite similarities, with the biggest obvious difference being how much larger the newly-discovered animal is. Per Ahlberg makes an appearance in a video to talk about the size and significance of the mystery tetrapod.

i-7f4600fb6e10f03a446adb1e20fde705-foot.jpeg
Foot morphologies. a, Laser surface scan of Muz. PGI 1728.II.1, left pes. b, Complete articulated left hind limb skeleton of Ichthyostega, MGUH f.n. 1349, with reconstructed soft tissue outline. c, Left hind limb of Acanthostega, reconstructed soft tissue outline based on skeletal reconstruction in ref. 8. We note the large size of the print compared to the limbs of Ichthyostega and Acanthostega, and that the print appears to represent not just the foot but the whole limb as far as the knee. d, digit; fe, femur; ti, tibia; fi, fibula; fib, fibulare. Scale bars, 10 mm.

What’s it all mean? Well, there’s the obvious implication that if you want to find earlier examples of the tetrapod transition, you should look in rocks that are about 400 million years old or older. However, it’s a little more complicated than that, because the mix of existing fossils tells us that there were viable, long-lasting niches for a diversity of fish, fishapods, and tetrapods that temporally coexisted for a long period of time; the evolution of these animals was not about a constant linear churn, replacing the old model with the new model every year. Comparing them to cars, it’s like there was a prolonged window of time in which horse-drawn buggies, Stanley Steamers, Model Ts, Studebakers, Ford Mustangs, and the Honda Civic were all being manufactured simultaneously and were all competitive with each other in specific markets…and that window lasted for 50 million years. Paleontologists are simply sampling bits and pieces of the model line-up and trying to sort out the relationships and timing of their origin.

The other phenomenon here is a demonstration of the spottiness of the fossil record. The Polish animal has left us no direct fossil remains; the rocks where its footprints were found formed in an ancient tide flat or lagoon, which is not a good location for the preservation of bones. This suggests that tetrapods may have first evolved in these kinds of marine environments, and only later expanded their ranges to live in the vegetated margins of rivers, where the flow of sediments is much more conducive to burial and preservation of animal remains. That complicates the story, too; not only do we have diverse stages of the tetrapod transition happily living together in time, but there may be a bit of selective fossilization going on, that only preserves some of the more derived forms living in taphonomically favorable environments.


Niedzwiedzki G, Szrek P, Narkiewicz K, Narkiewicz M, Ahlberg PE (2010) Tetrapod trackways from the early Middle Devonian period of Poland. Nature 463(7277): 43-48.

Comments

  1. #1 The Science Pundit
    January 7, 2010

    I found this find quite interesting and exciting. However, it would be nice to have some bones.

  2. #2 Rev. BigDumbChimp
    January 7, 2010

    Wait, is that a human footprint I see in there?

    /ducks

  3. #3 Bill Dauphin, OM
    January 7, 2010

    Uh, oh! I suspect the creobots will attempt to use this discover of evidence for a tetrapod that’s both older and more highly developed than Tiktaalik to attempt to discredit the reliability of the fossil evidence and/or the chronolgy of evolutionary change. I know this is BS, but please lay out the counterarguments for the benefit of us nonscientists.

  4. #4 davem
    January 7, 2010

    The size of these beasts leaves me wondering why they were so big. You don’t get 2 metres long feeding off detritus, surely? Is there some large victim around, or an even bigger carnivore trying to eat it?

  5. #5 Nerd of Redhead, OM
    January 7, 2010

    Ah, real science and coffee. A good mid-morning break. Looks like those DNA switches developed earlier than expected.

  6. #6 Ray Moscow
    January 7, 2010

    Thanks, PZ. This is exciting stuff.

  7. #7 Zeno
    January 7, 2010

    [creationism mode on]

    Aha! Once again science is wrong and must be revised! Only the Bible never changes! (Please ignore the multiple editions and discrepancies between Catholic and Protestant versions.)

    [creationism mode off]

  8. #8 Glen Davidson
    January 7, 2010

    Tetrapods are older, but should we suppose that these are our ancestors, or merely a good possibility of being our ancestors?

    Glen D
    http://tinyurl.com/mxaa3p

  9. #10 Glen Davidson
    January 7, 2010

    Tetrapods are older, but should we suppose that these are our ancestors, or merely a good possibility of being our ancestors?

    Or technically, close relatives of our ancestors.

    Glen D
    http://tinyurl.com/mxaa3p

  10. #11 Islander
    January 7, 2010

    Saw this yesterday on Not Exactly Rocket Science. Neil Shubin is excited but skeptical, and said IIRC that Tiktaalik could have made tracks that appeared the same way in the type of sediment the tracks are in.

  11. #12 Islander
    January 7, 2010

    For Shubin, the deal-breaker would be identifying the animals that made the trackways and establishing where they sit on the evolutionary tree. He says, “The skeletal anatomy, let alone evolutionary relationships, of a trackmaker is hard to interpret from a track or print.” For example, he says that a model of Tiktaalik’s skeleton would produce a print much like the one in the paper if it’s mushed into sand, and different consistencies or angles would produce an even closer match. He adds, “There is nothing in Tiktaalik’s described anatomy that suggests it didn’t have a stride.”

    The article from NERS.

  12. #13 Kathy Orlinsky
    January 7, 2010

    It’s interesting that some examples of evolution seem fairly straightforward, like whales moving into the seas, whereas others seem incredibly bushy and complex, like human evolution. Tetrapod evolution is starting to look a lot bushier, with this new animal being older and yet less primitive than Tiktaalik.

    I wonder if there really are more and less meandering evolutionary paths, or if it’s just that we haven’t found that many whale fossils.

    And of course, I don’t think evolution actually has a ‘path’.

  13. #14 mmelliott01
    January 7, 2010

    I’m curious about the brief moment of parallel limb movement in illustration c.

    How is it possible to tell that it isn’t just a continuation of the alternating limb movement with a small blip — like having to step over an obstacle or a slip or trip?

  14. #15 lose_the_woo
    January 7, 2010

    That complicates the story…

    I can hear them already:

    Complexity, complexity, …complexity, complexity..

  15. #16 cactusren
    January 7, 2010

    The Science Pundit–welcome to the world of paleontology. So many questions could be answered if we only had more bones…

    davem–while 2 meters is much larger than Tiktaalik, its not an unusual size for a vertebrate in the Devonian. There were some huge fish around then–the best known is probably Dunkleosteus, which, IIRC, is estimated to be at least 5 meters long. And early tetrapods were likely somewhat amphibious–the trackmaker in this article could have been eating fish rather than detritus. There were also some pretty big arthropods in the Devonian that could have been a food source (though I’m not sure if those were around by 395 mya).

    My main question about these tracks is how well constrained that date of 395 mya is. So I’m off to read the article and see what they have to say about it…

  16. #17 cervantes
    January 7, 2010

    Make peace with your inner fish . . .

  17. #18 PZ Myers
    January 7, 2010

    I know Shubin is also working up a description of Tiktaalik’s hindlimbs that is also going to be very interesting…it’s all going to make for a wonderfully complex story that will have the creationists crying.

  18. #19 SteveM
    January 7, 2010

    The size of these beasts leaves me wondering why they were so big. You don’t get 2 metres long feeding off detritus, surely? Is there some large victim around, or an even bigger carnivore trying to eat it?

    The largest animals around both on land and sea are not predators and do not have larger predators preying on them. Slow and steady eating of small things in huge quantities can produce very large creatures.

  19. #20 Rorschach
    January 7, 2010

    So does this mean there were local differences of the time of transition to land( like between Alaska and Poland) of 20 million yrs, if this tetrapod is less simple then Tiktaalik?

  20. #21 SEF
    January 7, 2010

    … and the three transitional tetrapods went “trip-trap, trip-trap, trip-trap” over the evolutionary bridge, under which the creationist troll was lurking.

    “How dare you mess up my nicely simplistic, ignorance-based religion,” roared the troll. “Get off my land! I’ll close my eyes, sing la-la-la to myself and wish you out of existence – see if I don’t! You’re not true transitional tetrapods at all but goats on fire.”

  21. #22 amphiox
    January 7, 2010

    re #13:

    Some recent fossil finds strongly suggest that whale evolution was bushy.

    We used to think that horse evolution was pretty straightforward, but we now know that it was in fact bushy.

    We used to think hominid evolution was pretty straightforward and linear too, but we now know that it was in fact bushy.

    There are other examples, too.

    At this stage of the game, with the evidence we currently have, I would think that “bushiness”, or adaptive radiation, should be the null hypothesis for all evolutionary lineages (I am almost tempted to consider this pattern tatamount to an evolutionary law), and any pattern we see that seems linear, we should suspect to be an artifact of an incomplete fossil record, unless we have some very compelling evidence to the contrary.

  22. #23 Insightful Ape
    January 7, 2010

    I must say I am disturbed.
    Unless the tracks were made by Tiktaalik or a close relative, this means that Tiktaalik was an evolutionary dead end. The transition happened long before Tiktaalik. And by whom? A lot of uncertainty.
    I must add that as much as I respect Neil Shubin, he has a stake in the results. The problem will need more data to be resolved. New models by themselves will not do.
    Oh well. Every chapter in science has its birth pangs.

  23. #24 Rachel Bronwyn
    January 7, 2010

    What an adorable little creature they’ve illustrated. I’d like to keep one as a pet.

  24. #25 davem
    January 7, 2010

    @cactusren: Sure, but for any evolutionary pressure to apply, the beasty must have had a better food source in the shallows, or an even bigger beasty trying to eat him in the depths. In the latter case, you don’t need to go all the way on land to escape, just into the shallows. To my mind, if you have a 2 metre creature walking on land, it needs a pressing reason to do so – a one metre food source that got there first, or a 3 metre carnivore that can also get into the shallows…

  25. #26 SEF
    January 7, 2010

    @ Rachel Bronwyn #24:

    You can has newts!

    (Or perhaps not if they’re all protected species where you live and you don’t have a licence.)

  26. #27 David Marjanovi?
    January 7, 2010

    So does this mean there were local differences of the time of transition to land( like between Alaska and Poland) of 20 million yrs, if this tetrapod is less simple then Tiktaalik?

    Whut?

    Such transitions aren’t periods that happen worldwide. They are events that happen to one population. That’s evolution we’re talking about here. All limbed vertebrates share a common ancestor and have inherited their limbs from it.

    Besides, there was no ocean between Poland and northeastern Canada (?not alaska?) at that time.

    And “less simple” isn’t the term I’d use. Fin rays are wonderfully complex. :-)

  27. #28 kalibhakta
    January 7, 2010

    Matthew Cobb also has a reeeeally fascinating post over at Why Evolution Is True about this discovery (+video!).

  28. #29 Chuk
    January 7, 2010

    Interesting. I always like to see these science type posts.

  29. #30 Becca Stareyes
    January 7, 2010

    Insightful Ape, I’d hazard a guess that a lot of fossils are ‘evolutionary dead ends’, or cousins of the ancestors of modern critters, rather than the direct ancestors. And not so much a dead end as in the wrong place at the wrong time.

    Not a biologist, though.

  30. #31 cdgwyn
    January 7, 2010

    This suggests that tetrapods may have first evolved in these kinds of marine environments, and only later expanded their ranges to live in the vegetated margins of rivers,

    I reflexively assume that this is _not_ the case because – as far as I know – all extant amphibians lay their eggs in freshwater. If tetrapods had first come ashore from marine environments I would expect that there would still be a number of amphibians that lay their eggs in saltwater.

    Does anyone have a likely-seeming hypothesis as to why I might be mistaken? (The ‘none of the saltwater amphibians survived’ is certainly plausible, but it doesn’t seem ‘highly-likely’ to me…..)

  31. #32 David Marjanovi?
    January 7, 2010

    I am almost tempted to consider this pattern tatamount to an evolutionary law

    It follows inevitably from the fact that evolution lacks foresight. Diversification is impossible to prevent.

    What an adorable little creature they’ve illustrated. I’d like to keep one as a pet.

    Read up on its size again. And then note that the figure comes from assuming very long limbs, which may be wrong.

    or a 3 metre carnivore that can also get into the shallows…

    There is no limit to rhizodontid size. 5 m? 6 m? perhaps 7? though the biggest known ones are Early Carboniferous in age, in other words, much younger.

  32. #33 David Marjanovi?
    January 7, 2010

    I’d hazard a guess that a lot of fossils are ‘evolutionary dead ends’, or cousins of the ancestors of modern critters, rather than the direct ancestors.

    Almost all of them are.

    If tetrapods had first come ashore from marine environments I would expect that there would still be a number of amphibians that lay their eggs in saltwater.

    Why? All living amphibians form a closely-knit clade of freshwater specialists. There were plenty of marine temnospondyls in the Carboniferous through Triassic, and many other groups of limbed vertebrates occurred in at least brackish sites, too.

    Incidentally, there are several frog species that can spend most of their lives in saltwater. The most famous one is the crab-eating frog, Fejervarya cancrivora. But these are highly nested within the frog family tree, which means they’re most likely reversals; and they can’t metamorphose in saltwater.

  33. #34 Rachel Bronwyn
    January 7, 2010

    @ SEF #26

    We have these little cuties roaming free where I come from.

    Semi-regularily kids would arrive in class, crying, because they’d accidentally squished a frog or salamander with their bicycle on their way to school. It was awful. I grew up in buttfuck nowhere though.

  34. #35 Tulse
    January 7, 2010

    And once again, this kind of finding demonstrates the differences between science and religion — not only do these have the potential to overturn accepted wisdom, but scientists are excited about this prospect. In religion, neither would be true.

  35. #36 recovering catholic
    January 7, 2010

    I just jump up and down with excitement when I read stuff like this! So incredibly interesting! What a fascinating unfolding story evolution is. How anyone can choose willful ingnorance instead is completely beyond me. And Neil Shubin is hot. (Is it OK to say that?)

  36. #37 Sili
    January 7, 2010

    What’s with all those little toesies? Is this an inbread, backwater tetrapod?

  37. #38 JBlilie
    January 7, 2010

    I encourage everyone to watch the episode of Planet Earth that includes the cave angelfish. It shows sequences of this fish literally walking upstream and uphill on rocks, using pectoral and ventral fins (one of each on each side!). It shows you everything you need to know about how early tetrapod motion got going.

    To me these were stunnig sequences and made me go “holy shit! there’s tetrapods, right there!”

    Every creationist should be shown those sequences.

  38. #39 SEF
    January 7, 2010

    @ Sili #37:

    What’s with all those little toesies?

    It took a while for tetrapods to settle on 5 digits (and then to lose some of those in different derived lineages). For a while, 8 was quite a common number of digits to have. (I think that discussion should probably be on a thread here from a few years ago too – unless it was the original pharyngula website.)

  39. #40 recovering catholic
    January 7, 2010

    @Rachel Bronwyn–

    That’s so sweet! Not that you ran over herps, but that you cried when you did. I hope the kids where you grew up still cry when they do. Kids just seem to be so jaded today.

  40. #41 Robert MacDonald
    January 7, 2010

    “The Polish animal has left us no direct fossil remains…”

    No, but it’s left us the Polish language.

  41. #42 Josh
    January 7, 2010

    Uh, oh! I suspect the creobots will attempt to use this discover of evidence for a tetrapod that’s both older and more highly developed than Tiktaalik to attempt to discredit the reliability of the fossil evidence and/or the chronolgy of evolutionary change. I know this is BS, but please lay out the counterarguments for the benefit of us nonscientists.

    Evolution produces kind of “bushy” results. As PZ wrote:

    Comparing them to cars, it’s like there was a prolonged window of time in which horse-drawn buggies, Stanley Steamers, Model Ts, Studebakers, Ford Mustangs, and the Honda Civic were all being manufactured simultaneously and were all competitive with each other in specific markets?and that window lasted for 50 million years.

    Then you need to combine this “bushy” tendency with the fact that sedimentary environments get around to preserving fossils when and if they’re good and fucking ready. Those environments couldn’t give two shits if we have a clear picture of evolutionary history or not. Too often for our liking, the situation ends up with you finding, to use PZ’s analogy again, the one known specimen of a Stanley Steamer in rocks that are far younger than those that produce the one known Honda Civic specimen. At first glance, these situations look like something is wrong. Nothing’s wrong. Phylogeny is usually pretty congruent with stratigraphy, but not always.

    There has been a similar not-issue with early birds and non-avian theropods. Another nice nail into the coffin of this not-issue was recently hammered in:

    Hu, D., Hou, L., Zhang, L. and Xu, X. (2009) “A pre-Archaeopteryx troodontid theropod from China with long feathers on the metatarsus.” Nature, 461, 1 October 2009: 640-643.

    The paper wasn’t originally behind the firewall. It is now but you can read the abstract here:

    http://www.nature.com/nature/journal/v461/n7264/pdf/nature08322.pdf

    There was some discussion of it back in October in the endless thread (Thread 9 from outer space chapter).

  42. #43 cactusren
    January 7, 2010

    Insightful Ape @23

    I must say I am disturbed.
    Unless the tracks were made by Tiktaalik or a close relative, this means that Tiktaalik was an evolutionary dead end. The transition happened long before Tiktaalik.

    Presumably, all of these “fishapods” are closely related. And there’s no real reason to assume the animal that made the transition to land first is the ancestor of all tetrapods. That lineage could have died off or been outcompeted by Tiktaalik and its descendents. This is all arm waving, of course, since I have no evidence for this–I’m simply trying to illustrate that this evidence doesn’t preclude Tiktaalik from being our ancestor. That being said, given the spottiness of the fossil record, its fairly unlikely that Tiktaalik is our direct ancestor, anyway; its just one member of a group from which tetrapods evolved. Why should it be disturbing that Tiktaalik is our great-great-great-(ad naseum)-aunt rather than our great-great-great-(ad naseum)-grandmother?

  43. #44 Kel, OM
    January 7, 2010

    Creationists misusing this science in 3…2…1…

  44. #45 Kel, OM
    January 7, 2010

    I went to a talk last year (by one of the scientists who sequenced the Platypus genome) where there was a basic tree of life up and on there it showed 410 million years between our divergence with amphibians. I was sitting in the lecture hall thinking “how could this be? The fishapods aren’t in the fossil record until about 30 million years later”. I meant to ask her about it, but was a bit too much of a coward to speak up and ask what probably was a Dunnin-Kruger moment on my part.

  45. #46 Josh
    January 7, 2010

    Creationists misusing this science in 3…2…1…

    It doesn’t help when the goddamn news media are writing headlines like this:

    http://news.yahoo.com/s/livescience/20100106/sc_livescience/fourleggedcreaturesfootprintsforceevolutionrethink

  46. #47 linux7master
    January 7, 2010

    Hello everyone-
    Let me explain this a little further.
    First, a note to the wise:
    Creationists are going to LOVE this; at least the smart ones will, anyway. I really hope PZ explains this further in a post, because across the Creationisttubes we are going to see:
    “LOL, TIKTAALIK PWNED!!!” in a few days.

    First of all, this certainly doesn’t discredit the elegant Tetrapod transition we see today.

    In fact, we *expect* this kind of bush evolution. We see it strongly in the transitional evolution of birds, for example.

    But first, please note that the find is not conclusive in any way. It isn’t even an actual fossil, and, as Shubin points out in his article, the authors jumped to conclusions in associating the tracks with later tetrapods, instead of the earlier guys like Tiktaalik and co.

    Before, we saw something sort of like:
    –A bunch of lobe-finned fish- 400-390 mya

    **THIS FIND(7 hindlimb digits, I need to get forelimb info!)– 390 mya**

    (fin)Panderichthys- 380 mya
    (fin-ish)Tiktaalik- 375 mya
    (8 digits)Acanthostega- 365 mya
    (7 digits)Ichthyostega- 362 mya
    (6 digits)Tulerpeton- 360 mya
    (5.5 digits)Pederpes- 354 mya
    (5 digits) Casineria- 340 mya

    As you can see, this seemed to be an excellent transitional series before the new find. However, it is almost certain that the tetrapoda actually developed in a evolutionary bush, as the old “transitional series” idea has not been used since the “Victorian Evolutionists”, and is certainly rejected in favor of bushier evolution by modern Evolutionary Biologists.

    But, you might say, it sure did seem like the Victorians were right before this new find. However, one must understand that the fossil record is only a small statistical sample of the species living at the time, and one biased towards specific environments at that.

    Thus, we can safely assume that 390 mya, while tetrapods almost certainly existed, they were not as successful as non-tetrapods, as we do not find many tetrapods from that time. Over time, Tetrapods, and eventually Tetrapods with fewer digits, became more and more successful, and thus more and more likely to be in the fossil record, *WHICH IS WHY WE FIND THIS PSEUDO-VICTORIAN-EVOLUTIONARY TREND*!

    Here’s an example:
    Today, a polydactylic mutation causes 6 fingered cats. However, note that an extremely low number of cats are ever going to appear in the fossil record. Thus, almost certainly all fossilized cats from 2010 will be 5-fingered.
    However, if cats are selected for that 6-fingered mutation, and in 5 million years almost all cats have 6-fingers, then statistically almost all fossilized cats from that time will have 6 fingers.

    If this find had been 10 million years ago, of a rare 6 fingered cat’s tracks in 1800, 5 million years before most 6 fingered cats appear in the fossil record, Creationists and Victorian Evolutionists would be concerned about some precious evolutionary transition being broken. However, it would be completely understandable to modern Evolutionary Biologists.

    *Note that the vast, vast majority of Creationists only understand Victorian Evolution, and are clueless about modern Evolutionary Biology. Hence they blabble about missing links, evolutionary racism, evolutionary ladders, a lack of pre-Cambrian fossils(There hasn’t been a substantial “lack” for years), Haekel’s embryos, and some kind of “controversy within the scientific community”. As, of course, all of these are Victorian Evolutionary ideas that have since been rejected.

    Creationists have effectively deluded themselves into believing that they are in the 19th century, fighting 19th century scientists. They are quite literally over a century behind modern Science.

  47. #48 llewelly
    January 7, 2010

    Insightful Ape | January 7, 2010 1:22 PM:

    I must say I am disturbed.
    Unless the tracks were made by Tiktaalik or a close relative, this means that Tiktaalik was an evolutionary dead end.

    There’s no reason to be disturbed by the idea that Tiktaalik was an evolutionary dead end. Ornithischians and Sauropods are both evolutionary dead-ends. But that does not make them any less amazing and enthralling, and it doesn’t change the fact that other dinosaurs (birds descend from theropods) are still alive today. The overwhelming majority of species are extinct – which means that most creatures are “evolutionary dead-ends” to one extent or another. If you allow yourself to be disturbed by this, you’ll soon be a very disturbed individual.

  48. #49 linux7master
    January 7, 2010

    Whoops, grammarz mistake!
    “If this find had been 10 million years ago”
    I meant to say:
    If this find had been 10 million years in the future from now. So, it would be like we’d be looking back in time at now.

  49. #50 bybelknap
    January 7, 2010

    amphiox @22. Your bushiness point makes sense to me in that one of the drivers for selection is the variation in populations. There would be variation of certain traits for a particular body part being selected (limb length etc) and variation of which parts are being selected (tails vs limbs etc) and which direction the selection pressures are pushing or pulling the traits – thicker scales retaining more internal moisture as an environment dries vs thinning scales in a wetter environment. So with all of the available organ systems and subsystems and all of the selection pressures on those systems pushing and pulling at once depending on local conditions it is fairly easy to see that we would expect bushy and not linear “paths” from ancestors to extant species.

    I hope that makes sense. It does in my head. I just may not be expressing it very well.

  50. #51 Kel, OM
    January 7, 2010

    It doesn’t help when the goddamn news media are writing headlines like this:

    http://news.yahoo.com/s/livescience/20100106/sc_livescience/fourleggedcreaturesfootprintsforceevolutionrethink

    ewwww.

    Though the article itself seemed okay. But getting a creationist to look past the headline? Maybe if we numbered paragraphs they might be more willing ;)

  51. #52 bybelknap
    January 7, 2010

    crap, preview fail. unclosed italics tag. sorry.

  52. #53 MadScientist
    January 7, 2010

    Dang, Zeno beat me to it at #7.

  53. #54 Tulse
    January 7, 2010

    I’m not clear how pushing back the evolution of tetrapods several million years will help most creationists. Our account of tetrapod evolution may have been incorrect, but incorrect in a way that be of little comfort to them, especially those of a young-Earth inclination, since arguing for the error would entail accepting evidence of an old earth.

  54. #55 Josh
    January 7, 2010

    I’m not clear how pushing back the evolution of tetrapods several million years will help most creationists.

    It won’t help them. They’ll just grab onto headlines that say “Forces evolution rethink” and similar and be annoying.

  55. #56 David Marjanovi?
    January 7, 2010

    on there it showed 410 million years between our divergence with amphibians

    Are you sure it wasn’t our (amphibians included) divergence with the lungfishes? That date would pretty much fit.

    (5.5 digits)Pederpes

    Unknown number of digits due to incomplete preservation.

    However, one preserved digit is so much smaller than the other one that it reminded the authors of the hand of Tulerpeton.

  56. #57 cactusren
    January 7, 2010

    Tulse–your right about it being illogical, but creationists don’t seem to care about that, do they? They simply take opportunities like this to say “Ha! You scientists were wrong! You keep changing your story.” They fail to understand that making a few edits to a manuscript doesn’t change it from non-fiction to fiction.

  57. #58 Sven DiMilo
    January 7, 2010

    it showed 410 million years between our divergence with amphibians. I was sitting in the lecture hall thinking “how could this be?

    Time Tree gives a range of (referenced) estimates for the Lissamphibia-Amniota split here (link to pdf) from 414-331 mya, settling on an approximate consensus value of 360.

  58. #59 Kel, OM
    January 7, 2010

    Are you sure it wasn’t our (amphibians included) divergence with the lungfishes? That date would pretty much fit.

    No, I distinctly remember it being amphibians. It was one of those cases where I looked at it a few times to make sure. It was a talk about sex, genes and dumb design – it had the diagram up to show the different chromosomes that Amphibians, reptiles, birds, monotremes, and other mammals used for sex.

    I’m not clear how pushing back the evolution of tetrapods several million years will help most creationists.

    It’s more that “See? Tiktaalik isn’t a transitional fossil. Heck, I’ve even had a creationist quote on my blog a paper that said Tiktaalik didn’t lead to us therefore not transitional. Now that the entire branch may be an evolutionary dead end? Prime for misconceptions about transitional fossils to start flying!

  59. #60 linux7master
    January 7, 2010

    @David #56:
    I suppose you are correct, but the number of digits seems fairly easy to deduce approximately from the fossil, see:
    http://www.nature.com/nature/journal/v418/n6893/fig_tab/nature00824_F3.html#figure-title

  60. #61 Sven DiMilo
    January 7, 2010

    I just noticed (by watching the video, actually) that the trackways shown in PZ’s top figure were made by a much smaller animal than made the footprint in the second figure.

  61. #62 windy
    January 7, 2010

    [creationism mode on]
    Aha! Once again science is wrong and must be revised!

    No, actually this is PROOF that evolution=Nazism! As soon as tetrapods got the chance, they marched into Poland!!

  62. #63 Adam Rutherford
    January 7, 2010

    Hi, Adam from Nature here. We made a little film about the discovery. It’s here on our YouTube Channel:

    http://www.youtube.com/user/NatureVideoChannel

    Cheers, bone diggers

  63. #64 David Marjanovi?
    January 7, 2010

    Time Tree gives a range of (referenced) estimates for the Lissamphibia-Amniota split here (link to pdf) from 414-331 mya, settling on an approximate consensus value of 360.

    Don’t bother with miscalibrated molecular dates. :-)

    the number of digits seems fairly easy to deduce approximately from the fossil, see:

    The foot has 5 preserved toes, and may have had more. The hand has 2 preserved fingers, and clearly had more.

    No, actually this is PROOF that evolution=Nazism! As soon as tetrapods got the chance, they marched into Poland!!

    :-D :-D :-D :-D :-D :-D :-D :-D :-D :-D

    Week saved. Was about time.

  64. #65 amphiox
    January 7, 2010

    re #50:

    Yes, and all the so-called “major” transitions (the sexy ones the lay public is most interested in) involve the colonization of a new, comparatively empty ecospace/new mode of living – whether it is bilaterian multicellularity in the Cambrian (or whatever it was that triggered the diversification then), tetrapod colonization of land, dinosaurs taking to the air (well a little less empty in this case since the pterosaurs were still around, but presumably the early birds must have found some niches free from competition from pterosaurs who at the time were much better adapted for flight), or whales returning to the sea.

    As David M stated in #32, diversification is not only not prevented, but positively encouraged in such circumstances.

    Indeed, I would say that the discovery and confirmation of any truly linear, unbranched evolutionary lineage would constitute positive evidence in favor of OEC, since it would literally take the continuous intervention of a deity to prevent diversification.

    (Though I suspect that current creos would not welcome such a discovery, since the linear clade would also identify the creator’s chosen lineage, and we already know humans are branched. . . .)

  65. #66 Quarknugget
    January 7, 2010

    So I guess if there was something 2m long walking around by 395 mya, then not only are tetrapods with advanced, jointed limbs much older than we thought, but there must be much older fossils (>400 mya) out there that represent something closer to the common ancestor between Tiktaalik and this new creature, having presumably had features more similar to those of the former. Looks like a new expedition to find the “real” first walker is in order!

    Oh and Glenn D, WTF? (the links?)

  66. #67 cdgwyn
    January 7, 2010

    David Marjanovi? (#33) quotes my post at #31
    If tetrapods had first come ashore from marine environments I would expect that there would still be a number of amphibians that lay their eggs in saltwater.

    And then replies:
    Why? All living amphibians form a closely-knit clade of freshwater specialists.

    That is precisely why I am inclined to assume that tetrapods came ashore in fresh water. If there were successful ‘saltwater amphibians’ way back when I would expect that there would still be some amphibians that deposit their eggs in saltwater, and ‘always’ have.

    There were plenty of marine temnospondyls in the Carboniferous through Triassic, and many other groups of limbed vertebrates occurred in at least brackish sites, too.

    Did they deposit their eggs in saltwater or brackish water? Is their any evidence either way? Or are we left with assuming based on modern amphibians or the presumed/apparent situation that the fossils were found in?

    Incidentally, there are several frog species that can spend most of their lives in saltwater. The most famous one is the crab-eating frog, Fejervarya cancrivora. But these are highly nested within the frog family tree, which means they’re most likely reversals; and they can’t metamorphose in saltwater.

    My reflexive suspicion is that temnospondyls, even the marine forms, deposited their eggs in freshwater – like the ‘crab-eating frog’. It may well be that the situation was vastly different and that most tetrapods went directly from saltwater to land via the many beaches, shallow lagoons, and saltwater marshes of the world…..but since those environments have remained plentiful and we do not see any amphibians who deposit their eggs in saltwater, or even brackish water, I suspect that ‘the tetrapod advance’ took place in/adjacent to freshwater. Of course, if someone has evidence or a persuasive argument to the contrary I would be happy to hear it!

  67. #68 MosesZD
    January 7, 2010

    Great article. Thank you.

  68. #69 Kel, OM
    January 7, 2010

    No, actually this is PROOF that evolution=Nazism! As soon as tetrapods got the chance, they marched into Poland!!

    Hilarious!

  69. #70 kindcrow
    January 7, 2010

    To: Rachel Bronwyn

    Yup, that is one cute illustration. I’m going to embroider it!

  70. #71 kindcrow
    January 7, 2010

    Thanks so much for sharing the video, Adam Rutherford (#63)!

  71. #72 Exterus
    January 7, 2010

    Per Ahlberg posts over at the Talk Rational forums and he’s kind enough to send his paper to you if you ask nicely in a PM.

    http://talkrational.org/showthread.php?t=21577

  72. #73 ShadowWalkyr
    January 8, 2010

    When I was a kid, they taught us about the so-called “Invasion of Land,” when sea-beasties transited onto dry land and started evolving for that environment (notice no mention made about when plants made the transition; plants don’t invade, I guess).

    Thinking back on that (and looking at the comments here), it seems that most everybody is assuming that the Invasion only happened once. Has that been reliably established?

    It seems to me that the easiest answer is that Tiktaalik “Invaded” land some millions of years after something else did (and did in a similar fashion).

    I am by no means a paleontologist, but it seems to me that in the far distant past, when organisms were generally more generalized, such occurrences were likely and may have been (at least on an evolutionary timescale) commonplace.

  73. #74 Rachel Bronwyn
    January 8, 2010

    @kindcrow #70

    Make sure to show off the final product! I may need to comission you for some tetrapod-themed embroidery of my own.

    @recovering catholic #40

    I think kids who grow up in the middle of nowhere, surrounded by wild critters, have a better appreciation for them city kids whose only exposure to them is in tanks.

    That said, I grew up in an area where hunting wasn’t popular. I’m glad; cougars and bears in the backyard were awesome.

  74. #75 Rorschach
    January 8, 2010

    It seems to me that the easiest answer is that Tiktaalik “Invaded” land some millions of years after something else did (and did in a similar fashion).

    What I got from David M’s answer upthread was that this evolutionary change happened to populations, therefore the diversity in locations and times.

  75. #76 Knockgoats
    January 8, 2010

    The key point at issue seems to be: how much can you tell about limb anatomy from a trackway? If the authors are right that this is the track of a full-footed tetrapod, then Tiktaalik and similar are definitely off the line of modern tetrapod ancestry (the maker of the tracks is only probably without modern descendants, like any fossil), and the theory that they evolved in freshwater environments would be weaker.

    It seems to me that the easiest answer is that Tiktaalik “Invaded” land some millions of years after something else did (and did in a similar fashion). – ShadowWalkyr

    Possible; but perhaps more likely Tikaalik-like beasts evolved somewhere, spread and diversified, before the track-maker evolved from one of them, while another later gave rise to Tiktaalik.

    No, actually this is PROOF that evolution=Nazism! As soon as tetrapods got the chance, they marched into Poland!! – windy

    If windy didn’t already have a Molly, that would surely ensure one!

  76. #77 Knockgoats
    January 8, 2010

    Further to:
    “perhaps more likely Tikaalik-like beasts evolved somewhere, spread and diversified, before the track-maker evolved from one of them, while another later gave rise to Tiktaalik.”,
    there might have been a part of the world particularly favourable to diversification and the evolution of novel features in “semi-tetrapods”, from which repeated “pulses” of anatomical innovation spread; this would explain the up-to-now neatly chronological transition sequence in the fossils found previously – but the latter might also just be chance.

  77. #78 paleos04
    January 8, 2010

    Trace fossils FTW!!

  78. #79 John Pieret
    January 8, 2010

    The creationist bafflegab has already begun, led by the ever ridiculous Casey Luskin:

    http://www.evolutionnews.org/2010/01/tiktaalik_blown_out_of_the_wat.html

  79. #80 Tulse
    January 8, 2010

    The creationist bafflegab has already begun, led by the ever ridiculous Casey Luskin

    …who of course completely avoids the issue that moving tetrapod evolution back a few million years is no help to ID, nor does it undermine the prediction that a fossil like Tikaalik would be found.

    It’s like a witch doctor saying “See, you admit you were wrong about this case of gastroenteritis being caused by a rotavirus — you now think it was caused by an adenovirus! Therefore, it was really caused by demons!”

  80. #81 Andreas Johansson
    January 8, 2010

    cdgwyn wrote:

    That is precisely why I am inclined to assume that tetrapods came ashore in fresh water. If there were successful ‘saltwater amphibians’ way back when I would expect that there would still be some amphibians that deposit their eggs in saltwater, and ‘always’ have.

    There’s no reason to expect that – all but two early tetrapod lineages are extinct, and both of those (amniotes and lissamphibians, AKA amphibians in the restricted, monophyletic, sense) are pretty derived. It would be utterly unsurprising for an ancestral marine breeding habit to be entirely lost under such circumstances.

  81. #82 cactusren
    January 8, 2010

    cdgwyn @67:

    That is precisely why I am inclined to assume that tetrapods came ashore in fresh water. If there were successful ‘saltwater amphibians’ way back when I would expect that there would still be some amphibians that deposit their eggs in saltwater, and ‘always’ have.

    But modern amphibians (i.e. lissamphians), are a monophyletic group that arose much after the initial “invasion of land” (in the Mississippian, IIRC). That means they arose from a particular common ancestor that lived much later than the first tetrapods. Given that all lissamphibians lay eggs in fresh water, it is logical to assume that their common ancestor did, as well. That does not mean that the ancestors or relatives of that common ancestor necessarily shared that trait.

    In other words, saying that basal tetrapods must have laid their eggs in fresh water because that’s what modern amphibians do is like saying that basal tetrapods must have given live birth because that’s what mammals do.

  82. #83 cactusren
    January 8, 2010

    Agh, Andreas Johansson beat me to it. That’s what I get for not refreshing the page before posting my comment…

  83. #84 bsa
    January 8, 2010

    Wow, who knew evolution could be so complex? (Just checking to see if I could sign in with Typepad yet.)

  84. #85 David Marjanovi?
    January 8, 2010

    That is precisely why I am inclined to assume that tetrapods came ashore in fresh water. If there were successful ‘saltwater amphibians’ way back when I would expect that there would still be some amphibians that deposit their eggs in saltwater, and ‘always’ have.

    Except that, of all limbed vertebrates, all except the lissamphibians and the amniotes are extinct. That’s a large diversity that has bought the farm here. Worse yet, the amphibians and us are more closely related to each other than at least some, perhaps most, of that dead diversity. It isn’t defensible to declare the lissamphibians some sort of standard.

    That is the biggie: it is misleading to use “amphibian” for “every limbed vertebrate that is not an amniote”. You have been fooled by a paraphyletic taxon.

    “There were plenty of marine temnospondyls in the Carboniferous through Triassic, and many other groups of limbed vertebrates occurred in at least brackish sites, too.”

    Did they deposit their eggs in saltwater or brackish water? Is their any evidence either way? Or are we left with assuming based on modern amphibians or the presumed/apparent situation that the fossils were found in?

    No evidence either way ? but to base an inference about temnospondyls on modern amphibians would be circular logic, because it’s a matter of contention whether temnospondyls are amphibians in the first place, and to base an inference about, say, anthracosaurs on modern amphibians would be just wrong, because anthracosaurs aren’t amphibians.

    And if the lissamphibians are temnospondyls, that still won’t tell us much about those temnospondyls that are not lissamphibians! The restriction to freshwater could still be a lissamphibian specialty. Amniotes don’t lay their eggs in water at all, and while all surviving lungfishes live in freshwater, most Devonian ones were marine, as Tulerpeton was and as Latimeria is.

    Some of the sites that preserve very small larvae of certain temnospondyls could have been brackish and/or with seasonally changing salinity. A lot more research has yet to be done on this, however.

    It seems to me that the easiest answer is that Tiktaalik “Invaded” land some millions of years after something else did (and did in a similar fashion).

    Then what are its descendants?

    No, there’s no evidence Tiktaalik had any terrestrial descendants (?or in fact any known descendants). It itself probably wasn’t more terrestrial than a catfish or eel, BTW. I don’t think Tiktaalik went anywhere ? it’s one branch of the bush. Tree-thinking, people! :-)

    in the far distant past, when organisms were generally more generalized

    Wrong.

    What I got from David M’s answer upthread was that this evolutionary change happened to populations, therefore the diversity in locations and times.

    I tried to say something entirely different ? evolution (in general!) is something that happens to populations, not to the entire biosphere. Populations evolve, neither individuals nor planets do.

    There are no locations. Pangaea wasn’t quite established yet, but all continents were close enough together for these marine or euryhaline* animals to spread between all of them.

    The times are best explained the way PZ did: it’s not a ladder, it’s a bush; when one branch grows, that doesn’t mean any other must die out.

    * Able to support a wide range of salt contents in the water they live in. Opposite: stenohaline.

  85. #86 Andreas Johansson
    January 8, 2010

    How and why did Sb learn my Skype handle anyway? How do I make it stop showing it?

  86. #87 cdgwyn
    January 8, 2010

    Andreas Johansson [#81] points out that
    all but two early tetrapod lineages are extinct, and both of those (amniotes and lissamphibians, AKA amphibians in the restricted, monophyletic, sense) are pretty derived. It would be utterly unsurprising for an ancestral marine breeding habit to be entirely lost under such circumstances.

    And cactusren [#82] adds that:
    modern amphibians (i.e. lissamphians), are a monophyletic group that arose much after the initial “invasion of land” [snip] they arose from a particular common ancestor that lived much later than the first tetrapods. Given that all lissamphibians lay eggs in fresh water, it is logical to assume that their common ancestor did, as well. That does not mean that the ancestors or relatives of that common ancestor necessarily shared that trait.
    In other words, saying that basal tetrapods must have laid their eggs in fresh water because that’s what modern amphibians do is like saying that basal tetrapods must have given live birth because that’s what mammals do.

    These are relevant and informative points that also suggest that I have somehow not made myself clear. To my eyes the fact that all extant amphibians deposit their eggs in fresh water suggests a possibility that the first tetrapods also deposited their eggs in freshwater, and even that perhaps there may be something about the differences between freshwater and saltwater dwelling creatures that made transitioning to land from freshwater ‘more possible’ than from seawater to land. Notice the ‘suggests a possibility’ and ‘perhaps’ in that sentence.

    The number of beaches, salt marshes, and shallow seawater lagoons also strongly suggests a possibility that tetrapods came ashore from saltwater, and that several different lineages did so. It may also be that tetrapods came ashore in both fresh and seawater and that it simply happened to be that only ‘the descendents of the freshwater tetrapods’ survive in that same water/land ‘transitional’ niche.

    Clearly Andreas Johansson, cactusren, and others find the idea that the rather extensive number of beaches, salt marshes, etc. make the assumption that tetrapods (at least some of them) came ashore from saltwater a very plausible interpretation of the known evidence. There have always been a lot of beaches and salt marshes ? the idea is very very plausible.

    My point though is that I see the lack of an ‘amphibian’ (of any lineage) that deposits its eggs in saltwater as a reason to wonder if maybe there was something about dwelling in freshwater that enabled (made more likely) the transition to land from water. I do not suggest that “basal tetrapods must have laid their eggs in fresh water because that’s what modern amphibians do”, I am wondering if the reason that modern amphibians all lay their eggs in freshwater is because the basal tetrapods laid their eggs in freshwater. (If I was wondering if the ancestors of all mammals gave live birth the existence of platypuses would be persuasive evidence that perhaps this was not so.)

    Obviously modern amphibians being highly derived descendants of early tetrapods, the vast number of beaches, salt marshes, etc., and the passage of hundreds of millions of years obliterating much of those beaches and salt marshes strongly suggests that other explanations for why modern amphibians only deposit their eggs in freshwater are very plausible. But, does anyone know of any actual evidence that there were early tetrapods that deposited their eggs in seawater? The very plausible and reasonable arguments that they must have deposited their eggs in seawater may seem much more reasonable to many people ? but my conceding the persuasiveness and plausibility of an argument isn’t enough to prove it correct.

  87. #88 raven
    January 8, 2010

    I must say I am disturbed.
    Unless the tracks were made by Tiktaalik or a close relative, this means that Tiktaalik was an evolutionary dead end.

    More common than not. According to Jerry Coyne, 8 out of 10 species will end up extinct. One out of 10 will leave a descendant species and one out of ten will split.

    Since all life is related, if you go far enough back, some relative or another will still survive.

  88. #89 Sven DiMilo
    January 8, 2010

    I don’t see any way of deciding how these animals reproduced even if the adults were marine/semi-terrestrial. Maybe they were anadromous or estuarine.
    As a comparison, many are surprised to learn that extant marine teleost fishes are all derived from freshwater (or estuarine ancestry), as illustrated (for one thing) by the energy their eggs have to spend to stay hyposmotic in seawater. Fishapods could have done that, too. Who knows?

  89. #90 cactusren
    January 8, 2010

    cdgwyn,

    It’s quite possible that the earliest tetrapods did evolve from fresh water fishes. I was simply pointing out that the fact that the modern clade Lissamphibia lays eggs in freshwater is not evidence for this, since many of those early lineages are now extinct. The fact that lissamphibians lay their eggs in fresh water is evidence that their common ancestor laid eggs in fresh water–it does not, by itself, indicate that other early tetrapods laid their eggs in fresh water. If all sarcopterigians (lobe-finned fishes, from which the tetrapods evolved)dwelled in fresh water, we would be able to infer (through phylogenetic bracketing) that the common ancestor of both of these lineages (sarcopterigians and tetrapods) lived and laid eggs in fresh water. But as David Marjanovic has already pointed out, not all sarcopterigians live in fresh water.

    Since the modern taxa are ambiguous on the fresh vs. salt water point, this question will have to be answered through fossils. Tiktaalik appears to have lived in fresh water, and the prevailing hypotheses have been that the earliest tetrapods evolved from a freshwater fish. However, these tracks in a tidal flat argue for tetrapods that are descendant from a marine fish. So things remain a bit fuzzy.

  90. #91 Andreas Johansson
    January 8, 2010

    What cactusren said, pretty much. That amniotes and lissamphibians don’t breed in salt water is unsurprising if ancestral tetrapods did, but it’s equally unsurprising if they didn’t, so it doesn’t mean much either way.

    Do note that even if the original “conquest of land” was from saltwater, the common ancestor of living tetrapods may have spawned in freshwater. Indeed, every indication is AFAIK still that it was.

    (Re: my Skype query, it appears it’s actually the browser on this computer that’s inserting links to my account, to Sb would be innocent here.)

  91. #92 David Marjanovi?
    January 10, 2010

    But, does anyone know of any actual evidence that there were early tetrapods that deposited their eggs in seawater?

    There’s no evidence either way. There are no fossil eggs before the mid-late Permian at all, and those belong to amniotes.

    Probably sarcopterygians are by default euryhaline. Most of the Carboniferous fossil-bearing sites show some degree of marine influence (all the way to mangrove environments), and while Acanthostega lived in freshwater, Tulerpeton lived flat-out in the sea, with corals and all, IIRC 200 km from the nearest land.

    Stenohalinity really seems to be a lissamphibian autapomorphy. The evolution of terrestriality seems to have happened neither in a saltwater nor in a freshwater specialist.

    That amniotes and lissamphibians don’t breed in salt water

    Amniotes don’t count, because they don’t lay eggs in water at all…

    Many live-bearing amniotes do spend their entire lives in the sea, however, even though there are no amniotes that can’t drink freshwater for a living (that is, there are no saltwater specialists).

    Even we can drink water with 2 % salt for a living.

  92. #93 Sven DiMilo
    January 10, 2010

    Amniotes don’t count, because they don’t lay eggs in water at all…

    Never say never!
    http://www.jstor.org/pss/30164317
    http://www.uga.edu/srel/ecoviews/ecoview080706.htm

    Even we can drink water with 2 % salt for a living.

    Word (for reference, current seawater is about 3.5%)

  93. #94 David Marjanovi?
    January 10, 2010

    Okaaaaay… I’ll check out those links tomorrow (JSTOR access at the museum)…

    Anyway, here is the second paragraph of Nied?wiedzki et al. (2010):

    In parallel with this expansion of the morphological data set, the environmental, ecological and temporal contexts of the transition have been reassessed. It has become clear that many of the earliest tetrapods and elpistostegids derive from brackish to marginal marine deposits, and their wide geographical distribution also points to marine tolerance13,16,19. Temporally, the earliest record of tetrapod morphology has been pushed back from the late Famennian (about 360 million years ago) to the late Frasnian (about 375 million years ago)3,6,9,33. Known elpistostegids range from late Givetian to mid-Frasnian (approximately 386 to 380 million years ago), and the Frasnian Elpistostege and Tiktaalik appear more derived than the Givetian Panderichthys12,29, suggesting a good fit between stratigraphy and phylogeny, with tetrapods originating sometime during the mid?late Frasnian. Many recent publications argue that tetrapods evolved from and rapidly replaced the elpistostegids, probably in brackish to freshwater environments, in response to the modification of the terrestrial and water?s edge environment caused by the development of extensive tree-sized land vegetation21. However, a few data points have clashed with this consensus picture. Notably, the fragmentary genus Livoniana, although Givetian and thus contemporary with Panderichthys, is more derived than Tiktaalik, judging from its limited preserved anatomy12.

    Pretty much what I said. “More derived” is wrong for “closer to us”.

    Also, the lack of body and tail impressions in the trackways are interpreted as evidence that the animals were walking in shallow water, not on the shore. This explanation is also used for the isolated footprints that do not form tracks: “We interpret this type of isolated print, which is seen frequently at Zache?mie, as an aquatic print where a swimming tetrapod has used a single limb to kick against the substrate.” (p. 44)

  94. #95 David Marjanovi?
    January 11, 2010

    Ah, so, first, it’s a turtle, and turtles are well known to cheat about pretty much everything. Second, the embryo doesn’t develop as long as the egg is underwater.

  95. #96 luggom
    April 6, 2010

    Maybe a good creacionist joke :
    Hey, a bit more and the tetrapods are older than the first multicelular creatures,600 million year ago…