Putative amphibian fossil shows “broken” bone; said to be first indication of terrestriality

May 26, 2015 • 10:00 am

Now this paper is way above my pay grade, as it involves all kinds of complicated scanning, computer, and mathematical analysis of a “fishapod” fossil. The conclusion, from a new paper in PLoS ONE by Peter Bishop et al., is that the fossil, Ossinodus sp., shows a callus on its radius (one of the two lower forelimb bones), and that this callosity may represent partial but not complete healing following a fracture (there are suggestions of an infection, and the beast may have died before healing was complete). The analysis suggests in turn that the animal fell nearly a meter onto its leg, breaking the bone. Because such falls presumably cant occur in water, the authors conclude that this creature fell on land.

From this they say that Ossinodus, which dates to 333 million years ago, was a tetrapod—a proto-amphibian living at least partly on land—and that this is the earliest tetrapod known, since before this the oldest indisputable tetrapods come from 327-331 mya. If this report and its conclusions are correct, then, this pushes the earliest invasion of the land back between 2 and 5 million years earlier than previously known.

Here’s the beast, which looks a lot like the famous Tiktaalik, itself likely a transitional form (or something related to one) between fish and amphibians Also shown is the callus on the foreleg and the caption from the paper:

Screen Shot 2015-05-26 at 7.49.54 AM

Through CT (computer tomographic) scanning, they were able to visualize through the callus to the bone, and suggest that there was a crack (fracture) indicating that the animal had fallen on its foreleg. Here’s a reconstruction of the fracture; the arrows indicate five different models of weight loading that the authors used to see what might have caused the fracture:

Screen Shot 2015-05-26 at 7.54.24 AM

After making these models, calculating the putative weight of the animal (10-25 kg), and making a few other assumptions, they conclude that this fracture must have resulted from a fall of between 0.85 to 1.1 meters, or about three feet. They say this could not have happened in the water, and, ruling out other explanations, argue that this shows pretty clearly that the animal was terrestrial. In their view, the invasion of land had begun. From the paper:

As all of the estimations and assumptions underpinning the above calculations have been made on the conservative side (e.g., the purposeful overestimation of body mass), we are confident that the force required for the radius to fracture in the manner it did was very large, and that a fall on land is entirely possible from a mechanical perspective. Consistent with a fall hypothesis, the mode of displacement along the fracture (Fig 7a) is very similar to that of proximal radius fractures in humans which result from a fall onto an outstretched arm, where the humerus impacts upon the radial head [33,34]. Since the force required for fracture in the FEA was a distributed load, this excludes the possibility that it resulted from a bite (predatory or otherwise), where an impacting tooth would produce a spatially concentrated load. We therefore conclude that the most plausible explanation for the fracture in the radius of Ossinodus was that the animal was living on land and sustained a fall.

And they claim, with confidence, that the species antedates the next oldest terrestrial vertebrate by several million years:

Ossinodus is the oldest biomechanically demonstrable, terrestrially adapted tetrapod, being at least two million years older than Casineria kiddi, and at least five million years older than the East Kirkton tetrapod assemblage. These small (generally less than 40 cm long), Scottish tetrapods have previously been widely regarded as the oldest known terrestriality adapted vertebrates.

The confidence here is stronger than the indication in the previous paragraph that a land fall is “entirely possible.” For “entirely possible” is not the same as “very probable”!

This is an intriguing finding, and the authors did a huge amount of work.  It’s possible that they are indeed correct, but I have some questions about the assurance of the authors’ conclusion. Some of these caveats may reflect my ignorance of the analysis, for it’s indeed complicated, and readers with biomechanical interests should read the paper, which is available free at the link below.

1. Was the creature terrestrial? We don’t know. The fossil was of course found in sedimentary rock, i.e., it was buried underwater. Ossinodus could have been semi-aquatic, like early amphibians must have been, but it could have been fully aquatic. There is no independent evidence, beyond the broken radius, that it was partly or fully terrestrial.

2. Does the pathology unequivocally demonstrate a fall-induced fracture? I’m sure there are other explanations for this dysplasia, or even the crack (if it was indeed a fracture that occurred during life), including developmental anomalies. Can we rule out all other causes (including bites) with enough confidence to conclude this was a fall on land? Is there any evidence that fish break bones in the same way?

3. The sample size is small. We have a single individual with a single broken forelimb bone. Is that sufficient to lead to such sweeping conclusions? It would be nice to either have samples with other broken bones, or some other indication of terrestriality.

In sum, we have an intriguing finding here, and the authors may well be correct, but I’m not sufficiently convinced by the data—and the complicated analysis—to conclude that we have an animal that was unequivocally terrestrial or amphibious. Were I the authors, I would hedge my bets a bit more than they did when claiming they’d found a “biomechanically demonstrable” terrestrial tetrapod.

I also wonder why they published the article in PLoS ONE, a journal which has a less rigorous review process than journals which vet things not just for the soundness of the analysis, but for the novelty of the findings. If I had unequivocal evidence of the earliest land animal, as the authors say they do, I would have sent it to Nature or Science.

As I said, some of my questioning may reflect my naivité about the methods and analyses, but the earliest known tetrapod really deserves publication in a more visible journal.

h/t: Steve

__________

Bishop, P. J.,   Walmsley C. W., Phillips M. J., Quayle M. R. , Boisvert C. A., and McHenry C. R.. 2015. Oldest pathology in a tetrapod gone illuminates the origin of terrestrial vertebrates. PLoS ONE 10(5): e0125723. doi:10.1371/journal.pone.0125723

57 thoughts on “Putative amphibian fossil shows “broken” bone; said to be first indication of terrestriality

  1. My guess is they did send it to Nature or Science, and it was probably rejected for the reasons you suggest (as paleontology is not my field I can’t comment on whether that is overly persnickety or not), or the reviewers, if it was sent out for review, asked for years’ worth of extra data, and so they then sent it to PLoS One. Not claiming inside information; I’ve just seen this play out in my own field many times over, and while PLoS One is a respectable venue (unlike many OA venues, PLoS One is not predatory and the papers are peer reviewed), it does tend to be filled with papers making grandiose claim.

      1. What about a a fall into shallow water? Couldn’t the “fishapod” have been swept by currents over some sort of waterfall, landing in shallows? Fleeing a predator might have caused “leaping” before looking. Possibilities are endless. I might be able to accept that the fracture could only be caused by a fall, how and where the fall took place has been lost to father time. Very interesting though.

        1. In an actively in-filling sedimentary basin, there isn’t (generally) sufficient relief to produce waterfalls. To get sediment deposited you’re by definition in an environment where there is insufficient turbulence in the water to suspend the sediment load, let alone to actually erode material strong enough to form a metre-scale step in the landscape.

          1. So the damage happens at the waterfalls and the body is carried downstream where eventually it comes to rest in a spot with less turbulence and sedimentation… some kind of pool or floodplain.

            Unrealistic?

            1. Not impossible. But I’m trying to get better information on the actual environmental details. If the palaeoslope was steep (headwaters to base level in a few tens of kilometres) then it’s not unreasonable, but if the slope is over hundreds of km … much more iffy.
              The interbedding with silts implies a reasonably mature sediment. So fairly distal from the erosion areas.

    1. Palaeopathology isn’t exactly new, just not very well known. Other recent studies are analyses of puncture wounds in ceratopsian “frills” (think Triceratops for the best-known genus) which strongly suggest that they’re the result of ceratopsian-on-ceratopsian action. Possibly sexual competition (think bull-on-bull), but that is a lot harder to prove than to propose.
      Closer to home, healed bone scars in Neanderthals and/or Cro Magnon show that the practise of trepanning was carried out, and was not always rapidly lethal.
      But it’s a sweet find.

  2. Very interesting. Though it seems plausible to me that a fracture caused by a fall on land and a fracture caused by collision / impact in the water during a violent encounter with a predator could be indistinguishable. For just one example, dolphins use ramming and tail swatting to stun or otherwise inflict damage on prey and perceived threats.

      1. Oh yes, thanks for clarifying that. Didn’t mean to imply it could have been an actual dolphin encounter. Just referencing the behavior.

    1. Yes, it could have been whacked by something, or gotten its leg stuck and twisted. This could happen while submerged. Or it could be a callous from infection or bone cancer.

      1. I was thinking similarly.
        Its leg could have got stuck and twisted in rocks on a shore with hefty wave motion.

        It could have been deposited in a crack by a wave.
        Or it could have been washed up on to a high rock and fell or jumped or was washed back down.
        Wave action around rocks can be brutal.

        1. Wave action around rocks can be brutal.

          What is the interpreted environment of deposition?
          The organism is reported to be found in the Ducabrook Formation (http://www.tandfonline.com/doi/abs/10.1671/0272-4634%282007%2927%5B850%3ANDOOPA%5D2.0.CO%3B2) which is described as consisting of ” Feldspatholithic sandstone, mudstone, siltstone (commonly tuffaceous), minor algal and oolitic limestone.” (http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=5739)
          That’s an accumulative suite of sediments, not an erosional one, so you’re not likely to have a rocky coastline nearby.
          There’s a presentation mentioned on academia.edu which sounds very relevant, but it hasn’t actually been published. https://www.academia.edu/1805896/Mississippian_fish_assemblage_from_the_Ducabrook_Formation_central_Queensland_palaeoenvironment_and_taphonomy I’ve mailed the author to see if she’ll upload it.

  3. Two comments:

    I have no broken bones…so future generations may infer correctly that I spent a lot more time in water than most humans :).

    Second, I see, too, in the deluge of scientific publications, the case where a really cool paper gets published in an off-the-hype-track publication. Weird.

  4. If the whole animal reconstruction is based on this or other specimens, it seems likely that this species was at least semi-terrestrial due to the design of the legs. The legs are pretty good tetrapod legs, even better than those of tetrapods like Ichthyostega or Acanthostega.
    There are the fossil footprints of an early tetrapod that is older than Tiktaalik and older than this leg bone. Those were considered to be 397 million years old(!)

  5. Can we rule out all other causes (including bites) with enough confidence to conclude this was a fall on land?

    That was my initial thought too. My second thought, however, was that I bet paleontologists have become pretty good at recognizing the signs of bite marks, and could probably tell from other marks on the bone if some predator had grabbed the limb and chomped on it hard enough to break it. So without any real knowledge of the subject at all, I’m probably willing to accept that it isn’t from a bite.

    Still, I find it hard to believe we can rule out all underwater breakages. What, its impossible a rock fell on it? The critter got a limb stuck in a clam or something? It chomped down on a big fish and the thing thrashed around strongly enough to cause a hairline fracture? I could believe any of those are a possibility.

    1. Yes. Even if we take as settled that the break was definitely caused by an impact as per . . .

      “Consistent with a fall hypothesis, the mode of displacement along the fracture (Fig 7a) is very similar to that of proximal radius fractures in humans which result from a fall onto an outstretched arm, where the humerus impacts upon the radial head [33,34]”

      . . ., I can still envision bite scenarios by something with a big enough mouth and strong enough bite that seem likely to cause the same forces on the same vectors necessary to cause that kind of break. Not to mention other plausible underwater scenarios like I mentioned in my earlier comment.

      But I agree with Mark at #6 also. If the reconstruction is fairly accurate, it seems very likely to me that this creature was capable of spending some of its time on land.

    2. What, its impossible a rock fell on it?

      Go to a natural sedimentary basin – say the middle of the Mississippi delta.
      Where are the rocks?

  6. I think the impact of this paper is reduced by the fact that authors have already published on Ossinodus, and so the only new bit is the structural analysis of the fractured bone. Unfortunately, the fossil in question was recovered from a unit that leaves the true environmental preferences of Ossinodus in question. The fossil remains come from a gravel and sandy layer up to 24 cm thick that contains a variety of disarticulated vertebrate remains swept into a tidal channel. The fauna include a wide range of fossil fishes typical of the time. Alas, I’m not expert enough to know whether they are all characteristically fresh water, but at least one is (Xenacanthidids were fresh-water sharks. Regardless, it is likely that the remains lived in one or more settings on a delta adjacent to the tidal channel.

    The rocks around Pittsburgh (where I live) are one period younger than those hosting Ossinodus. Here in the Pennsylvanian the association of a tetrapod with lots of fish fossils suggests that they lived together. Thus, most tetrapods living before the end of the Pennsylvanian Period are considered to have lived primarily in lakes and streams on land. The tetrapods did not become primarily terrestrially well-adapted until the latest Pennsylvanian, when the bones became more robust and the fossils are no longer commonly found associated with fish remains.

    Alas, since Ossinodus and the fishes were largely disarticulated by the floods that brought in the gravels, it is possible they were all swept in when river cut through an old lake or they were swept in when the river cut through old lake and old flood plain deposits.

    It would be really great if the authors could find Ossinodus living in the delta deposits sitting immediately above the 24 cm tidal channel flood deposit. Finding this guy living on a flood plain would clinch their argument.

    1. Unfortunately, the fossil in question was recovered from a unit that leaves the true environmental preferences of Ossinodus in question.

      Do you ahve a PDF or link to those papers? (I don’t have institutional access to anything other than Jnl.Geol.Soc.Lond.)

  7. The orientation of trabeculae in the cancellous bone parallel to the shaft implies regular compressive loads, which is evidence for terrestriality independent of the fracture (which is most consistent with a transient high load in approximately the same direction). Fish fins don’t get compressed axially, probably ever, so this seems very reasonable.

    Ossinodus was described from incomplete, disarticulated cranial and postcranial remains of large and small specimens by Anne Warren and Sue Turner in 2004 (Palaeontology 47(1):151-184); they already inferred the possibility of terrestriality in large individuals from the hindlimb bones (e.g. femur with proportionally shorter adductor blade and more prominent trochanters than small specimens), and apparent close relationship to Whatcheeria and Pederpes which are younger (but known from articulated specimens) and considered to be capable of terrestrial locomotion.

    1. Some things to look for (I do not know what is seen in this particular species):
      1) Is the pelvic girdle attached to the vertebrae? This would be consistent with terrestrial adaptations.
      2) Are there paired ribs? Again, this is a terrestrial adaptation.

      1. Hmmm, I see the relevance of the pelvic girdle, but I’m not so sure of the relevance of the pairing of ribs. I don’t have access to my copy of Clack’s monograph on “Gaining ground” – buried in the preparations for moving – but I don’t recall that assertion being made.

  8. More and more fine paleo papers get into PLoS. Why? Free downloads to anybody. No paywall. Long papers w color pictures welcome. If you are a poor student in a third-world country, like Thailand or Lubbock TX, PLoS is a god-send.

    By my census, there are as many excellent illustrations of new bones in PLoS than in the two “Bigs” combined.

    1. Do I get any science geek cred for apparently being the first one to tell Robert that “endothermic dinosaurs rock!” And just for good measure…“welcome back Brontosaurus.”

      My initial love of science was astronomy and Dr. Bakker’s “big book” was one of my first introductions to vertebrate paleontology and evolution in general, and my well dog-eared copy of The Dinosaur Heresies has had a prominent place on my bookshelf for over 25 years now. Thank You.

  9. Sometimes I get frustrated that the past eons are so unclear. It is amazing how much we’ve learned about the past, but so much will always be shrouded in mystery. With that being said, I admire the assiduous nature of paleontologists.

  10. Very interesting; but waaaaay above my pay grade! Thanks for posting. I find all the stuff about fish-tetrapod and land-mammal –> cetacean/pinniped very interesting.

  11. Not sure if it’s actually correct, but I’ve gotten the impression that people working in paleontology are less interested in the impact of publishing in a glam magazine such as Science or Nature, and are more into distribution of their work through Open Access. I may be wrong, but I appreciate every effort of authors to publish In OA venues, rather than to hide their results behind a paywall.

  12. This kind of detective work on fossils is really quite amazing and fun to read about. It would impress Sherlock Holmes.
    Ken Ham would laugh at this. God put fish in the sea and lizards on the land. All done.

    1. Plus, it’s “historical science” which is bad (‘cuz it refutes my Big Book of the Magic Sky Fairy’s Deeds), not “observational science”, which is good (‘cuz it give me lots of techie gewgaws to play with while I pretend I’m out saving the world).

      Never mind that that sort of “thinking” would lead us to deny the existence of Abraham Lincoln (were you there?), as well as denying the possibility of convicting any criminal for any crime unless he were to be caught in the act.

      1. Never mind even that much; it’s self-refuting.

        So, you say that the whole world was covered in a giant Flood? How do you know? Were you there?

        Oh, so you don’t know, but you want us to believe that the Bible was written by God who was there.

        Well, how do you know that the Bible was written by God? Were you there when he wrote it?

        No?

        Well, shut the fuck up, then.

        b&

  13. I’ll buy that the injury was incurred in a fall — it looks just like the “spiral fracture” of radius I incurred as a kid from a similar drop [but I doubt the creature fell off the back of its father’s favorite chair.’]

    Tiktaalik-like? Restoration looks much more advanced, but I haven’t found any open-to-view representations of known skeletal parts, beyond a heavily-restored cranial roof.

    This is dated firmly after Romer’s Gap, where, apparently, much of the fishapod–>tetrapod action occurred.
    So we know that the God of the Gap was Romer.

  14. I body surfed as a kid. Can disagree with the authors that you can’t sustain a “fall” injury in the water. I was pounded against the bottom more times than I can count. Maybe this animal was just dashed against a rock.

    1. I mentioned that too, and I have been pounded against a rock face.
      It is surprising how absolutely helpless you are too.

      1. Having been pounded on rocks more than a few times too, I’m wondering how it’s surprising after the second occasion.
        But for a depositional environment, you’re going to have a muddy/ sandy bottom, not a rocky one.
        Oh, the mental hoops I’ve had to jump through to explain cm-scale gravel in mudstone successions.

  15. Alas, poor Ossinodus. After getting the lead role in the “scottish play” it unfortuneately uttered the word “Macbeth” and fell through the ropes bordering the stage, and ironically broke a leg. Later, a stage hand
    was overheard to say “I told you the ropes were too loose, Lautrec!”, to which a second stagehand replied “Don’t blame me! I told you to make it Titus, Andronicus!”

    Roger Bacon subsequently rolled over in his grave.

  16. A couple of things from this amateur (that has yet to digest the paper)…

    On the coelacanth–Tiktaalik–Hili continuum of investment in adaptations to terrestrial life, it seems that the claim that Ossinodus was past the Tiktaalik stage is not unwarranted.

    On the skeptical side, in the coelacanth–Tiktaalik–Hili continuum of terrestrial evolution, is there evidence that bone structure (density, tensile strength, etc.) changed in response to the greater load-bearing requirements of the landlubbing life. If there was such change, over what timescales, and was this accounted for in the analysis?

    1. I’m sure the answer to your first question is, yes. Long ago adaptation to being out of water drove thicker, stronger bones. By the time Hili’s ancestors evolved terrestrial accents like fur, claws, pointed ears and a tail, gravitational resistance would place it’s greatest demands on Felis catus with their tendency to become overweight under domestication.

  17. Great to see Dr. Bakker drop in; has he been here before?
    The part of this report that brings me up short is the claim that the fall was 0.98 m, plus or minus 0.13 m. Hard to believe that much precision can be determined, and it isn’t even relevant. Why couldn’t a 3-4 m fall off a cliff (or waterfall) be possible?

  18. I might have missed it…but how closely related would this fishapod have been to our own ancestors? Is it perhaps even a reasonable candidate for a great-great…great-grandparent?

    b&

    1. There’s no way of knowing whether it was an actual ancestor. The real tree is probably a lot bushier than we imagine from the relatively few fossils we have of fishponds and tetrapods. Some of Ossinodus’s apparent closest relatives are known from Scotland and North America. It was most likely on a side branch from our actual ancestors.

      1. Early tetrapods from Scotland are mentioned at several points in the article (one is the wonderfully named Eucritta melanolimnetes (“the true creature from the black lagoon”), but without doing a lot more reading around I’d hesitate to guess which is more closely related to Joe Random Mammal, or how closely related they (Eucritta, or the North American genera are) are to each other.
        The etymology for Eucritta is a masterpiece and deserves repeating :

        Etymology. Eu (Greek) true; critta (American vernacular): creature; melano (Greek): black; limnetes (Greek): living in a pool or marsh, from the black lagoon (in reference to the locality of East Kirkton).

        Aparrently field conditions in the East Kirkton quarry were … “challenging”. If only every species name were so memorable.

  19. The biomechanics and physics are above my pay grade, but the comparison to fractures of the radius that occur when a human falls on an outstretched arm seems dodgy to me. We humans are bipedal, and our upper extremities have a pretty impressive range of motion, so our reflexive protective movements and postural adjustments would seem to be different from those of a proto-amphibian with limbs in a “sprawling position.” If the animal fell off a ledge, would it stick out its forearm and hand to catch itself? Or would it just fall on its side and perhaps fracture a bone that’s proximal to the radius? I’d want to know more about how “sprawling” tetrapods such as salamanders, lizards, etc. behave during a fall (not by actually dropping them – that would be mean!), before I’d buy the falling-on-an-outstretched-arm scenario.

  20. I wonder if the 5 digits on each limb shown in the drawing is accurate. This could be very significant, since the earliest of the tetrapods had 8 or 9. If it really had 5 that could indicate that Ossinodus was well into tetrapody. I guess I should check the paper.

    1. This could be very significant, since the earliest of the tetrapods had 8 or 9.

      6, 7 and 8 I’ve heard of ; 9? Do you have a link for that?

  21. I looked at the original description of Ossinodus pueri, and the authors didn’t have enough digit bones (only 3) to indicate what the digit number was.

  22. Using an isotropic model for bone seems a bit iffy, as bone in general has definite anisotropy. Other

  23. Apologies if this is a stupid question but

    The fossil was of course found in sedimentary rock, i.e., it was buried underwater.

    Is it really the case that all sedimentary rocks, or at least, all fossil bearing sedimentary rocks form under water? Or is it that we draw that conclusion from the particular type of sedimentary rock?

    1. I suppose that wind-born sediments (forming soils known as “loess”) might eventually form rock. I don’t know it this would become fossil bearing, though.

    2. Is it really the case that all sedimentary rocks, or at least, all fossil bearing sedimentary rocks form under water?

      No.
      And to GBJames’ question, yes, terrestrial sediments can be fossil bearing.
      The best known examples of terrestrial sediment-entombed that springs to mind are the dune-sand (not loess!)buried theropods named Oviraptor when discovered in 1924 (with a caution that the name may ” entirely mislead us as to its feeding habits and […] character”). More recent work in the area has revealed a lot more genera of fossil dinosaurs (and mammals IIRC) in the area.
      Horner’s dinosaur egg discoveries (in many intervals now) are all terrestrial. It’s a bit difficult to do gas exchange through water, as the extant theropod dinosaur genera clearly show.
      There are numerous reports of footprint fossils (ichnofossils) in terrestrial sediments.

      1. Thanks for the answer. I was pretty sure that was the case, but the “i.e.” in JAC’s post kind of implied that I might be wrong.

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