How did baby Utahraptor caudals articulate? The answer will sicken and disgust you
December 3, 2022
Over on Mastodon (sign up, it’s great!), Jim Kirkland posted a baby Utahraptor caudal vertebrae for #FossilFriday. Here it is:
And after a bit of virtual prep work:
My first reaction was just “That’s pretty!“. My second, which I admit should have been my first, was “Wait a sec — how the heck do those things articulate?”
The issue is that both the prezygs and the postzygs overhang the centrum by so much. If we imagine three of these babies consecutively, there are basically two options.
First, the centra articulate closely, with what we might feel intuitively is a reasonable cartilage gap; and the zygs cross over:
Does something like this ever happen? Not in sauropods, for sure, but it could be correct — if the zyg facets are some way short of the tips of their processes, so that the most distal parts of each process are pre-epipophyses and epipophyses rather than prezygs and postzygs per se.
The other interpretation is this, with the zygs overlapping near the end as in sensible dinosaurs, and much more spaced out centra:
If this is right, then (in this respect) baby Utahraptor tails resembled camel necks in having big intervertebral spaces, which in life were filled with big cartilage plugs.
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SPOILER SPACE
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Have a think about this before reading on.
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SPOILER SPACE
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OK, here is the horrible truth.
Dromaeosaur tails do overlap their zyg processes as in the first mock-up above: but they do much, much worse than this!
Here is the truly perverted figure 37 of Ostrom’s classic 1969 monograph on Deinonychus — the publication that catalysed the whole Dinosaur Renaissance:
As you can see, the zygapophyseal processes are grotesquely elongated, and overlap in long stiffening bundles with those of successive vertebrae (part C of the figure). The actual zyg facets are small, and close to the origins of these processes (see parts A and B of the figure). And the chevrons are also hideously protracted beyond their natural length to form stiffening bundles beneath the tail that complement those above the tail.
To add insult to injury, the chevrons even face in the wrong darned direction, extending anteriorly along the tail rather than posteriorly as in all decent animals. Yes: in Ostrom’s illustrations, we’re seeing the vertebrae in right lateral view, i.e. anterior is to the right.
All of this confirms that I was so, so right two decades ago to focus so completely on proper dinosaurs instead of these nasty mutant ones. Ugh.
Sauropod Vertebra Picture of the Week 
December 3, 2022 at 4:03 pm
**clutches pearls**
December 3, 2022 at 10:40 pm
to me, a very stiff tail seems a sensible anchor for the caudal-femoralis muscles, if an animal needs a burst of speed to chase something down – or to escape being chased. Don’t hadrosaurs also have very stiff tails?
The seemingly easy response is that sauropods presumably did not have stiff tails because they took a different approach; vast numbers of juveniles, coupled with huge adult body size.
December 4, 2022 at 2:45 am
Hadrosaurs do indeed due to these: https://ucmp.berkeley.edu/diapsids/ornithischia/ossten.gif
December 4, 2022 at 8:19 am
Right, LeeB — but my understanding is that, where velociraptorine theropods do this with elongated zygapophyses, hadrosaurs do it with ossified tendons. Different mechanisms; similar effects.
December 4, 2022 at 3:56 pm
[…] Just to wash our mouths out after all the theropod-related unpleasantness yesterday: […]
December 5, 2022 at 5:46 am
Also worth noting is that these “stiffened” tails are only stiff in a dorsoventral direction. They were still plenty whippy and bendable horizontally.
December 5, 2022 at 6:08 am
Jura, what makes you say that? I’m not disputing it, I’m just not what about their structure makes them horizontally flexible.
December 5, 2022 at 6:35 am
Overlapping the zygapophyses and chevrons keeps the vertebrae from moving too far in a dorsoventral direction, which works great when you want to keep a tail (or spine) from falling down, but there isn’t really anything there to stop the tail vertebrae from moving in the mediolateral plane aside from muscle and ligament attachments. Chris Organ tested this in Ornithischians (Organ 2006) and found ossified tendons to do effectively nothing to reduce mediolateral flexibility. I don’t think there has been a specific study for theropods, but I do remember the now rather famous image of a preserved Velociraptor tail (IGM 100/986) showing how sinuous these tails could get (Norell and Makovicky 1999 Fig. 21). The authors noted that such substantial sinuosity did not seem to affect prezygapophyseal placement.
December 5, 2022 at 6:51 am
Thanks, Jura, that’s really interesting. There is certainly nothing about the elongated-zygs-and-chevrons system that would make me think, from first principles, that they stiffened vertically but not horizontally; but there’s no gainsaying that IGM 100/986 photograph!
If we’re talking about “Thoracic Epaxial Muscles in Living Archosaurs and Ornithopod Dinosaurs”, then Organ (2006) seems to be talking about the torso rather than the tail, bar a brief mention on page 789. Do I have the wrong paper?
December 5, 2022 at 6:55 am
The reference is close. This was his tail-specific one. This is my bad. I really should have provided full references for everyone.
Organ CL. 2006. Biomechanics of ossified tendons in ornithopod dinosaurs. Paleobiology 32(4):652–665.
Norell MA, Makovicky PJ. 1999. Important features of the dromaeosaurid skeleton II: Information from newly collected specimens of Velociraptor mongoliensis. AMNH Novitates 3282.
December 5, 2022 at 6:56 am
Ah, thank you for the Organ reference. (I found the Norell/Makovicky one OK!)