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Notes from the Dark Side



     
     Now that I've seen the published version of the recent Jones et al. 
     Nature paper on relative leg lengths in non-avian dinosaurs and birds, 
     and digested some of the commentary thereon, I can offer my spin on the 
     various matters.  One colleague privately characterized the rather 
     heterogeneous group of authors as "injudicious."  I think "ecumenical" 
     might be a more appropriate choice of words.  The co-authors are not of 
     one mind about all of the issues raised by the paper, and the way it was 
     written reflected necessary compromises among points of view.
     
     My first thought is that some of the comments about the paper strike me 
     as a classic case of the tail wagging the proverbial dog.  The title of 
     the paper is: "Cursoriality in bipedal archosaurs", not something like 
     "Why _Caudipteryx_ really isn't a non-avian theropod (with hugs and 
     kisses to Alan Feduccia)".  The thoughts about the phylogenetic 
     affinities of said beast are, in my opinion, a minor part of the paper. 
      I'm not trying to be disingenuous here:  Nobody on this list will be 
     astonished to learn that some of the authors of the paper lean toward 
     the idea expressed in what was not the title of the paper, but that is 
     NOT the main point that this article is all about.
     
     My involvement in this project stemmed from another study, which was 
     actually finished earlier, but which has not yet appeared in print.  
     This paper, co-authored with Gatesy, Holtz, Hutchinson, and Robinson, 
     is about theropod locomotion, and should appear soon in the American 
     Zoologist feather symposium.  As I worked on it, I noticed that 
     non-avian theropods (NATs) and ground birds differ in the length of 
     the hindlimb relative to the length of digit III (a crude proxy for 
     that part of the foot responsible for making footprints), with birds 
     having a relatively longer leg length.  However, if one takes into 
     account the fact that most of the action in the avian leg is at the 
     knee, rather than the hip, the "effective" limb length in NATs and 
     ground birds is much the same.  I thought this was interesting, but 
     wondered what would happen if one used a better proxy for animal size 
     than digit III length, such as body trunk length.
     
     As it happened, Terry Jones and John Ruben were interested in the same 
     question, but in the context of the matter about which I was not being 
     disingenuous in a preceding paragraph.  We agreed to work together on 
     this project, and the rest is history.
     
     Now, in my opinion there are two components to the Nature paper.  One 
     of these is the question of whether at least most bipedal non-avian 
     dinosaurs (NADs) differ from ground birds in the length of the leg 
     relative to the trunk (and how this might affect the matter of 
     effective limb lengths).  The second is how _Caudipteryx_ (and NATs 
     thought to be close relatives of said beast) fit into the picture.  I 
     will consider these matters separately, because I do not believe that 
     they must stand or fall together.  Let us examine each of these in 
     turn.
     
     1) Relative leg lengths in NADs and ground birds.  Tom Holtz 
     criticized us for using skeletal reconstructions (in part) as our 
     sources of data.  Indeed, we are guilty.  He mentions several theropod 
     taxa which were probably not well enough preserved to permit good 
     reconstructions of the trunk and hindlimb, and I thank him for 
     correcting us.  He also argues that we have introduced circularity 
     into our argument by using "imaginary measurements which are 
     themselves scaled by the artist from other known specimens."
     
     It is undeniably true that these kinds of data are not ideal.  The 
     question is, are they good enough for addressing the question posed?  
     Granted that they are almost surely not exact, are they so inexact 
     that this would cause the relationship between hindlimb length and 
     trunk length to be indistinguishable in NADs and ground birds?  
     
     In considering incomplete specimens, let me suggest that our argument 
     can be circular, in the context of the question being asked, only if 
     those incomplete specimens were reconstructed in such a manner as to 
     make them them fit our preconceptions of what their limb length-trunk 
     length relationships should be.  In contrast, if, say, an incomplete 
     specimen was given additional vertebrae in the reconstruction because 
     a close relative is known to have had said vertebrae, that will 
     constitute inexactness, but not circularity.  Because all of the 
     reconstructions were published prior to our study, I think that the 
     latter situation is what applies here.
     
     If it had just been me, I would not have fitted a regression line to 
     the points for extinct forms, for the very reason that some our data 
     points were indeed rather fuzzy.  But we can check the effects of 
     this inexactness on our results.  First, I redid the theropod 
     regression, tossing out the points we shouldn't have used, on the 
     basis of Tom's comments.  Then I did it again, using only those taxa 
     for which, as best I can tell, we can be reasonably confident that 
     the limb length-trunk length ratio isn't too far from reality: 
     Eoraptor, Coelophysis, Dilophosaurus, Sinraptor, Allosaurus, 
     Compsognathus, Sinosauropteryx, Gorgosaurus, Gallimimus, 
     Struthiomimus, and Deinonychus.  Although the regression coefficients 
     changed a bit,there was no major change in the results.
     
     I therefore think that this is a robust, real result: most bipedal and 
     facultatively bipedal NADs had relatively shorter legs, compared with 
     their body trunk lengths, than do ground birds, and this surely will 
     be related to the way they moved.  This result fits nicely with what 
     was seen in my other paper, in which leg lengths were compared with 
     digit III lengths.  Of course, it may be that with further work, Tom 
     or somebody will find data that falsify these conclusions of the two 
     papers.  If so, I will mourn the death of yet another beautiful 
     hypothesis and move on.
     
     2)  Sinornithoides, Bambiraptor, and Caudipteryx: Having considered 
     the dog, I turn now to the tail.  This is where my co-authors and I 
     could be on thinner ice, in my opinion (and that is just MY opinion, 
     not that of my co-authors).  We excluded S and B from our regression 
     equation on the grounds that these might be juveniles, and we were 
     trying to remove ontogeny as a variable (but note that in our paper 
     we explicitly stated that we were doing this, and why, so give us 
     good marks for candor).  I argued that we should nonetheless plot the 
     S and B points on our graphs, even if we didn't use them in our 
     regression, but was outvoted (see paragraph one, above, 
     "compromises").
     
      Based on some work I did over the summer, however, I am not confident 
      that S and B can be disregarded in this kind of analysis, even if they 
      are based on juveniles.  I looked at hindlimb length relative to trunk 
      length in an ontogenetic series of alligators, from very small to quite 
      large individuals (this study is to be published in the Journal of the 
      Paleontological Society of Korea, part of a symposium volume).  With 
      increasing alligator size, limb length does indeed become proportionately 
      less.  Unfortunately, this trend is not very strong, becomes obvious only 
      across the entire size range of gators.  There is very little change 
      between half-grown (in linear dimensions) and big gators.
     
      If the same was true of S and B, then unless these specimens were 
     quite small individuals of their species, it is possible that their 
     limb length:trunk length ratios may not have been that different from 
     adults.  
     
     On the other hand, it could be argued that because gators are aquatic 
     quadrupeds, they might not show as marked ontogenetic changes in 
     limb:trunk proportions as terrestrial bipedal archosaurs, but it would 
     take another study to determine if this is the case.  
     
     It is worth noting here that in the abstracts for last year's SVP 
     meeting Foster and Chure reported that a young individual of 
     _Allosaurus_ had a leg length, relative to ilium length, that was 33% 
     longer than in adults, suggesting the possibility of an ontogenetic 
     drop in leg length relative to body trunk length as well.  However, 
     these authors also indicated that the cursorial limb proportions of the 
     juvenile are lost by the time the beasts are about half-grown. 
     
     It is therefore possible some NADs may indeed have departed from what is 
     seen in most others, and have been more like ground birds in their 
     limb:trunk proportions--and, as Tom noted, it is mighty suspicious that 
     the guys who are doing this are among the forms thought, on other grounds, 
     to be closely related to birds (it's interesting, though, that Deinonychus 
     and Velociraptor seem to be more like typical NADs in this regard).  And 
     if you want to stir the pot even more, try putting the Elzanowski and 
     Pasko reconstruction of Archaeopteryx, and the Chiappe reconstruction of 
     Confuciusornis, on the graph--they plot more like typical NADs than like 
     ground birds.  Go figure.
     
     To sum up, I still think it likely that Caudipteryx had a style of 
     locomotion more like a ground bird than like most non-avian theropods.  
     This could very well mean that it derived from volant ancestors, as we 
     argued in our paper.  But it could also be--again as we noted in our 
     paper--that Caudipteryx (along with S and B?) was indeed a member of a 
     plexus of NADs that had independently (note the comments in the preceding 
     paragraph about Archaeopteryx and Confuciusornis) adopted a style of 
     locomotion similar to what ground birds later employed.  I don't have a 
     firm opinion on this, and the Nature paper didn't, either (see, one last 
     time, paragraph one, above, "compromises").

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