Guest Post: Raptor Tails Declassified

Today Scott Persons returns to the Musings to talk more about theropod tails, their musculature and possible uses. This time around, it’s the oviraptorosaurs:

 

Previously on Archosaur Musings . . .

After my first post on dinosaur tails (which was mostly focused on the rear-ends of tyrannosaurs), I was asked a seemingly innocent question in the comments section by archo muser “Lucy”. She wrote:

“So what about oviraptorids and their reduced tails – do we assume they went the same way as emus? And do we know anything about the other maniraptors (I’m particularly thinking of dromaeosaurs and their famously odd tail design)?”

I suspect no malice on the part of Lucy and her question, but her inquiry had inadvertently put me in a corner. She had asked a good question . . . so good that I had already devoted two chapters in my master’s thesis to thinking about it and had two papers addressing the question in the works. So, I had the wherewithal to answer it, and that was my dilemma.

I am of the opinion that science should not be a covert affair. New scientific discoveries should not be closely guarded secrets. By its very nature, science benefits from transparency and from the unobstructed flow of ideas. And yet, in the modern system of journal-based publish-or-perish scientific academia, a certain level of discretion and intrigue is prudent.

Otherwise, you might get scooped. You might prematurely tell one too many colleagues what you are researching and what your results are, and, next thing you know, those ideas wind up in someone else’s publication. Maybe that someone else was already thinking the same thing and, upon hearing that they are not the only one following that particular line of thought, they rush to publish first. That leaves you with an outdated thesis and nothing new to report (and the best academic journals don’t do reruns). Consider, as an extreme example, what Alfred Russel Wallace’s place in the history books would be if he had kept his ideas to himself until going to print and hadn’t sent a certain letter to a certain long-bearded British naturalist. And sometimes, of course, the scooping can be downright nefarious.

Not that dino tail research is a ticket into the history books (I am just happy when I get asked to do a guest blog), but what seems like your own quiet little corner of paleo research can get scooped out from under you. It has happened to some of my friends. So, I did not want to tip my hand and give Lucy the answer that her question deserved. Instead, with all the aggravating crypticness of a magic eight ball, I gave what I’m sure was a dissatisfactory answer. I said, in effect, “Why, yes, dromaeosaurid and oviraptorids do have unusual tails, and, yes, I do think they were specialized for unusual functions. I am going to publishing on those topics soon. Ask again later.”

Both raptor tail papers have come to academic fruition, and I am now free of my research paranoia. The paper on dromaeosaurids (coauthored with my graduate supervisor Dr. Phil Currie) has been published in a special volume of Acta Geologica Sinica, and the paper examining oviraptorid tails (coauthored with Dr. Phil Currie and Dr. Mark Norell) is now available for early view in Acta Palaeontologica Plonica. If you are interested in what I think was going on with dromaeosaurid tails (and I think a lot was going on), then you can checkout the guest post that Dave asked me to do over at Pterosaur.net. That leaves us with the tails of oviraptors . . .

“Do we assume they went the same way as emus?”

Oviraptors have short tails (both in terms of the total number of vertebrae in their tails and in terms of tail length relative to body length). It has been argued that such short tails are evidence that the group did go the way of the emu — i.e., the ancestors of oviraptors had reduced tails for the purpose of flight and that this reduced-tail condition was retained after the group had become secondarily flightless. In fact, it has been argued by various scientists that many groups of fully-terrestrial dinosaurs may have descended from flight-capable ancestors. It is my opinion (though many wise and respectable scientists would disagree) that some of these arguments for secondarily-flightless dinosaurs are probably valid. However, in the case of oviraptors, I don’t think that the tail lends much support to the secondarily flightless hypothesis. Let me tell you why.

The wonderfully preserved skeleton of the oviraptor Khaan mckennai.

The wonderfully preserved skeleton of the oviraptor Khaan mckennai.

Those readers who made it through my first tail post will recall that I am interested in the musculature of dinosaur tails (as inferred through digital reconstructions based on skeletal attachment sites and comparative dissections of modern animals). In particular, I am interested in the caudofemoral tail muscles. Caudofemoral muscles are part of the locomotive system. They attach via tendon to the femur, and their retractions helped to propel a dinosaur forwards when walking and running.

In birds, the caudofemoral muscles are tremendously reduced and sometimes completely absent. That makes sense. To fly, a bird benefits from weight reduction, and what better weight could a bird reduce than that of a big muscle dangling off the rear that functions in land-based locomotion. Emu’s have reduced caudofemoral muscles (they compensate for this, as many birds do, by placing less emphasis on femoral retraction when they walk and run), but there is every indication that oviraptors did not have reduced caudofemoral muscles.

Readers of my past post may also recall that caudofemoral muscles do not extend too far back towards the tip of the tail. Instead, most of their mass is located near the tail base (by the hips and legs). The shortening of oviraptor tails appears to have been restricted to the post-caudofemoral region of the tail – that is, vertebrae near the tip were lost — and I found no evidence that the caudofemoral muscles of oviraptors were reduced. Oviraptor tails were shortened, but not at the expense of muscles that helped them to move on the ground. Not like an emu.

 

Digital model of the tail skeleton and musculature of the oviraptor “Ingenia” yanshini. Three stages of reconstruction are shown: the tail skeleton modeled based on specimen measurements (A); the caudofemoral muscles (in red) modeled over the digital skeleton (B); and the full muscle reconstruction (C).

Digital model of the tail skeleton and musculature of the oviraptor “Ingenia” yanshini. Three stages of reconstruction are shown: the tail skeleton modeled based on specimen measurements (A); the caudofemoral muscles (in red) modeled over the digital skeleton (B); and the full muscle reconstruction (C).

Curiouser and Curiouser

What about the other tail muscles, are they reduced? Yes, and no. The other muscles of an oviraptor tail are considerably reduced in relative length, but not in relative mass. Wide caudal ribs (transverse processes), among other features of the tail skeleton, affirm that oviraptors had unusually robust tail muscles. Oviraptor tails were short, but stocky.

In an oviraptor tail, the individual vertebrae were short and the vertebral articulations suggest a high degree of flexibility in between the vertebrae. So, oviraptors had a lot of flexor points crammed into their short stocky tails. Per unit of tail length, an oviraptor tail may have rivaled all other theropods in its flexibility.

Then, we come to the very tip of the tail, and that’s when things really get strange. Back in 2000, it was announced that the oviraptor Nomingia had a tail that terminated in a series of fused vertebrae. This fused terminal tip was termed a “pygostyle”. Pygostyles are found in the tails of modern birds and are thought to be associated with anchoring a bird’s fanning tail-feathers. Did Nomingia have a tail feather fan? It seems likely. Direct proof that some early oviraptors had tail-feather fans comes from exquisitely preserved specimens of Caudipteryx and Similicaudipteryx, which actually have fossilized feathers preserved.

The pygostyle of the oviraptor Nomingia, composed of five fused tail vertebrae.

The pygostyle of the oviraptor Nomingia, composed of five fused tail vertebrae.

In our oviraptor tail paper, my coauthors and I announced the discovery of three new oviraptor pygostyles. One of these was from a second specimen of Nomingia, one was from Citipati, and one (the smallest of the three) was from Conchoraptor. These new pygostyles suggest that Nomingia may have been the rule, not the exception, and that pygostyles along with their accompanying feather fans were probably common features of oviraptors.

 

If You’ve Got It, Flaunt It

Muscular, flexible, and ending in a feathered flourish, what were oviraptor tails doing? I think they were flirting.

What good are tail feathers if you cannot fly? A lot of modern birds make use of tail-feather fans when they are on the ground. Think about a tom turkey or a peacock. Such birds use flamboyant feather-fans as display structures, most commonly, as courtship display structures. Now, imagine if the avian ancestors of peafowl and turkeys had not been forced to abandon the longer and muscular dinosaurian tail for the sake of aeronautics. What sort of a tail might these birds “want” in order to best woo potential mates and to wield their feathery instruments of seduction? Flexible, strong, and muscularly dexterous tails.

Life reconstruction of “Ingenia” yanshini, depicting a male “peacocking” to a female. Painting by the exceptionally talented Canadian artist Sydney Mohr.

Life reconstruction of “Ingenia” yanshini, depicting a male “peacocking” to a female. Painting by the exceptionally talented Canadian artist Sydney Mohr.

Lucy, I want you to know that this follow-up blog post was unsolicited. I trust that Dave humors me and posts it, and I hope you are satisfied with the answer. Sorry for the delay.

14 Responses to “Guest Post: Raptor Tails Declassified”


  1. 1 John 11/01/2013 at 2:04 pm

    Just out of curiosity, is it possible for Oviraptorsaurs to actively open or close these tail “fans”? Are there any possible features on the pygostyle that would or would not indicate such specialized muscular structure (something similar to M. bulbi rectricium in birds)?

    • 2 Scott Persons 12/01/2013 at 11:06 pm

      That would be cool to know. I don’t.

      Perhaps oviraptor fans were stuck in an “open” position. If there were
      no associated opening and closing muscles, the pygostyles still may
      have served an important function: as rigid “handles” for the fan. If
      vertebrae at the tail terminus had been tiny and un-fused, the tail’s end
      might have been prone to going limp and wobbly as the fan was waived
      through the air. The vertebral fusion may have prevented that.

      — Scott

      • 3 John 13/01/2013 at 5:01 am

        Thanks for the reply, that make sense. The only reason that I asked is because when I look at the tail of Yixianornis grabaui, the form of its tail aren’t that different to some of the oviraptorsaurs (except that it might be less elongated) yet Yixianornis has a tail fan which “presumably” has the ability to open and close.

        Wouldn’t it be great if we could find out one day (wishful thinking)?

  2. 4 atheistpassivist 11/01/2013 at 2:08 pm

    I’m not Lucy, but that’s an interesting article. Thanks!

  3. 5 Kilian Hekhuis 11/01/2013 at 2:09 pm

    That comment hijacked an unrelated WordPress account of mine, sorry.

  4. 6 Craig Dylke 12/01/2013 at 2:16 am

    Cool stuff as usual Scott.

    So as an ignorant artist, am I to take it from those computer simulations that Oviraptors had tremendously (downwardly) fat tails? If I were reconstructing one would I outline it like C, or is that exaggerated by the computer?

    Very interesting and neat implications for the group.

  5. 8 Mickey Mortimer 12/01/2013 at 11:36 am

    Since this never got answered on the pterosaur.net post, I’ll ask again here-

    Are caudal transverse processes actually thought to be homologous to ribs, and thus be caudal ribs? It was my impression they were homologous to diapophyses, and the ribs were merely lost. They are never(?) found disarticulated from the neural arch in dinosaurs AFAIK, and they seem to grade into sacral diapophyses, not sacral ribs.

    Also, do you have the specimen number for the Anchiornis specimen illustration on pterosaur.net? I have a larger photo that shows the plaque says IVPP V_____, but I can’t quite make it out. Maybe V13065?

    • 9 Scott Persons 12/01/2013 at 11:10 pm

      Caudal “ribs” are sometimes found disarticulated from the neural arch
      in skeletons of very young dinosaurs.

      Developmental studies on baby and embryonic crocodilians have given
      some support to the conclusion that caudal “ribs” are really ribs and
      not diapophyses. But this is an issue about which reasonable
      paleontologist disagree. My advisor, Dr. Phil Currie, favors using the
      term caudal “transverse processes”, although he thinks that a dino’s tail often has a mixture of ribs and diapophyses. Whereas, Dr. Robert
      Bakker thinks that caudal “ribs” are indeed ribs, and that calling them
      anything else constitutes sloppy anatomy.

      For now, I prefer the term caudal “rib”, because I find the developmental evidence more compelling.

      I’m afraid that I cannot help you out with the specimen number. Dave,
      do you know it?

      — Scott

      • 10 David Hone 12/01/2013 at 11:33 pm

        I’ve got a photo of it on here, but I can’t read the number either: http://archosaurmusings.files.wordpress.com/2011/03/anci.jpg

      • 11 Mickey Mortimer 15/01/2013 at 2:20 am

        Thanks for the reply. I was unaware of the separate ossification center in crocodilians. I searched a bit for data on birds, but couldn’t find any. Do they exhibit the same pattern? I did find Gallina and Otero (2009), who show sauropods had separate ossification centers too, and propose the structures in sauropods are caudal ribs with diapophyseal laminae contributing in some taxa. If this holds true in theropods, I guess they would be caudal ribs with diapophyseal contributions in taxa with caudal laminae like megaraptorans and oviraptorids.

        Gallina and Otero, 2009. Anterior caudal transverse processes in sauropod dinosaurs: Morphological, phylogenetic and functional aspects. Ameghiniana. 46(1).

    • 12 Kilian Hekhuis 14/01/2013 at 9:56 am

      I’m reading V16035, but it’s indeed almost impossible to make anything of it.

  6. 13 Mark Robinson 16/01/2013 at 3:32 am

    The type specimen is IVPP V14378 so logically the number of a subsequently discovered specimen will be greater. Is this just LPM-B00169 re-located and renumbered?

    It does look most like V16035 to me but caveat emptor and all that.


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