Bony claw ≠ actual claw

Just a quick post that ties into several themes previously explored here on the musings such as climbing dinosaurs, pterosaur soft tissues and interpreting fossils. This is one issue that often comes up in the palaeontolgoical literature and is well worth mythbusting in public. This is the idea that a bony claw of a skeleton provides and accurate representation of how that claw would look in life.

For those who don’t know, in most animals the last joint of the finger or toe is a specialised bone with the technical name of the ungual. Unguals are incredibly varied but their most notable feature is the soft tissue structure that typically accompanies them (a claw, hoof or nail, depending on species). In many, this sheath of keratin that covers the ungual and forms the actual external structure is closely tied to the bone that supports it, but critically, in many others it does not.

Claws and unguals in a pterodactyloid pterosaur
Claws and unguals in a pterodactyloid pterosaur

It is a mistake therefore to think that just because you have a nicely preserved ungual, you have a good idea of what the claw in life would have looked like, and thus can infer what it’s function might have been. To prove this point, here is a nice picture of a pterosaur foot with both the unguals and the keratin sheaths preserved. While the keratin (terminating at the white arrow) certainly follows the shape of the ungual (terminating at the black arrow), the dramatic elongation and increase in curvature is not expected and it would be hard to predict based on the bone alone. Even across the three toes shown here, the ungual of the second is almost identical to the first, but has a much shorted sheath and the third has a proportionally thin ungual but with a similar sized claw.

That really is it in short, don’t get carried away with interpreting an ungual as representing the claw in life. In many cases this is a reasonable supposition (horse unguals look just like hooves) but in others it is not, and work needs to be done to verify how closely these tie together, both in living taxa (which of course for the basis for most analyses of extinct animals) and those fossils for which both are available. This of course only refers to the overall outline or shape of the claw when seen from the side (since it is such flattened and well preserved fossils that preserve the actual claws), one must also consider the actual point of the claw (is it pointed or blunt?) and its ventral surface (rounded or sharp?) and neither of these can be determined from such fossils. You can look at the ungual itself, but no one seems to know how well they represent the state in life, so that’s perhaps not much help.

So the next time you see a study or report claiming something had especially short or sharp claws be warned. It might have pointed unguals, but that’s not the same as a sharp claw and the unguals might be short, but the claws could still have been long. This is of course relevant when it comes to functional analyses of things like climbing or predation – without supporting evidence from living animals, and ideally other fossils be wary of interpretations that are based on the shape of the ungual alone.

8 Responses to “Bony claw ≠ actual claw”

  1. 1 Zach Miller 12/11/2008 at 4:32 am

    I’m just gonna play devil’s advocate for a minute here. Couldn’t those super-long keratin sheath impressions be broken-off or sloughed-off as the animal rotted? The huge disparity between the ultra-long sheath of the leftmost claw and the much shorter sheath of the one right next to it make me think there’s something else going on. I can’t think of too many animals that have radically different-sized keratin sheaths over their claws.

  2. 2 David Hone 12/11/2008 at 8:29 am

    There might be a little erosion / prep artefact, but in short no I don’t think they are reduced. At least on this specimen there are large tracts of wing membrane still intact and articulated on the pterosaur and I doubt something as tough as a claw could be eroded and yet a realtively fragile wing remain in great condition. And there are at least soem animals that have different claws on their toes – look at anteaters – while the actual unguals are very different sizes, the size of the sheaths are also disparate and the main digging claw has a proportionally much larger sheath in the specimens I have looked at. This is another aspect of the problem, very little work has been done on claws in the literature and most of that has focused on claw length or curvature, but actual variation within and between species, and even on individula animals between claws remains largely absent so it’s actually pretty hard to make statements about claws. i have a couple of student projects ticking over looking at the basics of this variation, but it is soemthing i want to sit down and really tackle at some point.

  3. 3 Christopher Collinson 12/11/2008 at 1:05 pm

    “I can’t think of too many animals that have radically different-sized keratin sheaths over their claws.”

    Xenarthrans often do, but they also tend to have unguals that are widely different in size which also adds to the proportional size disparity of the keratin sheaths.

  4. 4 Christopher Collinson 12/11/2008 at 1:11 pm

    oh I almost forgot, what specimen is this? Is it the “Crato Azhdarchid” SMNK PAL 2342?

  5. 5 David Hone 12/11/2008 at 1:43 pm

    “Xenarthrans often do” yes, that’s whay I said “look at anteaters – while the actual unguals are very different sizes” 😉

    And yes, it is the Crato Azhdarchid, well one of them, it should actually be PAL 3830 but I’m not sure, it’s just a close up send to me by Ross Elgin for a paper on feet so I’m not sure. There are actually several like this in the collections in Karlsruhe. Finally in addition to my earlier comment, the two feet of this thing are almost identical in terms of the claws which is another reason one would argue agaisnt erosion / wear being responsible for the differences.

  6. 6 Christopher Collinson 12/11/2008 at 4:20 pm

    haha, I must of got distracted and I didn’t read past the part about the wing being preserved over the claw sheaths. Sorry :p

  7. 7 David Hone 12/11/2008 at 7:30 pm

    I’ll live! Nice to be right though, pretty much word for word you rewrote my comment for me! 😀

  1. 1 Pedal unguals « Dave Hone’s Archosaur Musings Trackback on 22/02/2010 at 8:50 am
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