Zygopophyses in dromaeosaurs

So having covered ornithischian ossified tendons it’s time to switch to the other side of the dinosaurian tree and take a look at the hyper elongate zygopophyses of dromaeosaurs. This is a bizarre feature of dromaeosaur tails where the (normally) little articulation points between vertebrate (the pre and postzygopophyses) are elongated such that they overlap other individual vertebrae rather than just articulating with each other. The postsygos (at the rear) are only mildly elongate compared to the truly mammoth prezygos which can overlap half a dozen or more bones.

Here, several have become disarticulated so that the individual shafts of the zygopophyses stick out free of the tail making them more visible. Just about visible (it’s not the greatest image) is the fact that these are part of the vertebrae and continuous with them and are not separate elements as are the ossified tendons of the ornithischians. A good look under a microscope makes that pretty clear, though of course a bad photo shot through glass in a museum doesn’t always. Also worth noting is the fact that under the tail, the chevron bones are also massively elongated too.

It’s tempting to assume that this makes the tail an incredibly stiff rod-like structure and this has been suggested in the past, but this is not likely to be the case. For a start this specimen is hardly unique in that the bones have separated out from each other even when the rest of the skeleton remains articulated, so clearly if even a fairly mild disturbance after death can move the bones around, then in life there must have been a degree of flexibility. Secondly, other specimens are preserved with the tail flexed to a degree suggesting that things are not that rigid. Finally, it’s pretty unlikely that the whole thing was exceptionally rigid as this would make it prone to breaking – even our own long bones can flex at least a bit when stressed without snapping, and some long and thin bones (like this one for example) pretty much must have had some flexibility or they’d break constantly – bones are not always very rigid (and indeed as an aside, bat bones are incredibly flexible). So while the tail was undoubtably stiffened, it was unlikely the flag-pole that some people think it was.

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9 Responses to “Zygopophyses in dromaeosaurs”

  1. 1 Darren Naish 10/02/2010 at 6:28 pm

    Hey Dave. Did you read this article…


    Sorry for self-promotion. We had a good discussion in the comments about stiffness (or not) of hyper-elongate zygs.

    • 2 David Hone 10/02/2010 at 9:00 pm

      Yes. Simply on the grounds that I have in fact read every single Tet Zoo post. Ever. All of them.

      However, I also can’t even alwasy remember what I have written about and regulalry have to go back through my archives to make sure I’m not repeating myself. Once I even finished an article and it was only going to post I realised I’d covered it before. It’s probably no surprise at all that I forogt about that one.

      Oh, and nothing wrong with the odd bit of self-promotion, especially when Tet Zoo is in hiatus.

  2. 3 Darren Naish 10/02/2010 at 9:09 pm

    Sorry Dave, I often forget that you are one of my longest-standing and most loyal of supporters… I really must start being much nicer to you🙂

  3. 5 Eric 11/02/2010 at 5:49 am

    Is there anywau that the Zygopophyses could be related to the origin of flight? I am not an expert in the origin of flight or aerodynamics at all but I would image that a stiff rod would help to control ones direction in the air.

    • 6 David Hone 11/02/2010 at 9:13 am

      In the short term (i.e. during a gliding phase) I can see that it could well help with steering (and Microraptor for example does have large flight-like feathers at the end of its tail, but then so too does the very non-flying Caudipteryx). Once powered flight evolves, I suspect that the extra benefit of steering is outweighed by the extra weight and the tail would be reduced or lost (as indeed it was in birds, bats and pterosaurs).

  1. 1 Dromaeosaur tails again « Dave Hone’s Archosaur Musings Trackback on 24/02/2010 at 4:04 pm
  2. 2 Linheraptor – another new dromaeosaur « Dave Hone’s Archosaur Musings Trackback on 19/03/2010 at 8:21 am
  3. 3 A post about a tail with no pun in the title « Dave Hone's Archosaur Musings Trackback on 07/07/2012 at 7:59 am
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