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RE: Senter 2006, Confuciusornis, and humeral mobility



Jason Brougham wrote:

<When I initially was so bold as to blatantly state that Confuciusornis' second 
manual digit is not reduced, it was in the context of a discussion about 
function - specifically the possible flexion of this finger during the flight 
stroke. [...] I believe you had mentioned the lack of extensor pits and I 
thought your observations were along that same train of thought.>

  On a purely mechanical level, I equate physical mobility in a phalangeal 
joint to a few things, as I think Senter's work on manus flexibility (Bonnan 
and Senter, 2007; Senter and Parrish, 2006; Senter and Robins, 2005; Senter, 
2005, 2006 [two papers], and 2007) shows:

1. Arc of the interphalangeal and metacarpo-phalangeal joint, which limits the 
range of motion in general to the highest degree of arc.
2. Presence of bony "stops," usually in the form of "tongues" and the extent of 
an extensor or flexor pit (a phalanx typically has a longer flexor process 
anchoring to the flexor tendons on the ventral surface than the extensor, 
dorsal process; the extensor process fits into a dorsal extensor pit, which is 
inferred the limit of its motion [disarticulation is ruled out as 
unobservable], while flexor pits are rare -- the bone should just stop the 
phalanx from curving further).
3. Indications of cartilage (mottled and wrinkled surface texture), indicating 
how far the flexion or extension of the phalanges can move.

  Extensor pits thus indicate greater realms of motion, while longer tongues on 
the phalanges, associated themselves with greater curvature of the proximal 
phalangeal joint. The former appear lacking in *Confuciusornis sanctus*, while 
the latter are present (this is clear in both GMV-2132 and in Paul's figuring 
of the manus of the bird in _Dinosaurs of the Air_). A short flexor process is 
present in GMV-2132, and it appears raised and perpendicular to the phalanx -- 
it would imply to me a strong leverage point, and thus high flexibility; but 
the distal joint lacks such a process, and the joint, regardless of the flexor 
extend of the distal condyles, appears flat based on the proximal mdII-2 
morphology ... but this is again based on a single specimen, and it is crushed. 
I took the lack of pits and the latter morphology to imply a lack of 
flexibility, but I overstated my case (I really do not have a high sample-size, 
here).

<I assumed that you meant that it was reduced in a developmental/functional way 
- as the third digit is in the hand of Cathayornis or Longipteryx. [...] You 
were instead talking about reduction from the ancestral state. In that case, I 
agree that comparison to Archaeopteryx is best. But you are also equating 
hypertrophy to length, and there we do disagree.

<The humerus of the ostrich, for example, is much longer, proportional to the 
ulna and manus, than in the flying ancestor of ratites, yet it still must be 
described as reduced, mustn't it?>

  I think I get where you're going with this, but I am not talking about 
reduction in a conceptual sense in any other than mathematical terms. I would 
not call the humerus of the ostrich reduced if I were to compare it directly 
only to the basal flighted paleognath [setting aside the suggestion that 
various paleognath "families" may all be convergently flightless (Harshman et 
al., 2008)], and were I to use a bird with a relatively longer humerus (picking 
a non-limb element or measure in comparison). That's the thing: taken on their 
own, without regard to developmental processes and projections of an artificial 
evolutionary cline, picking two taxa, relegating one to the baseline, results 
in only one of three results: the second taxa is reduced from, is in agreement 
with, or is increased from the baseline. That I picked *Archaeopteryx 
lithographica* as a baseline can be of evolutionary concern, but I did so 
because it ran the potential of being the most stable baseline I could provide 
(having multiple and potentially ontogenetic proportions to compare), and 
because it could also be evolutionarily more useful than almost anything else. 
It's high degree of familiarity doesn't hurt, either.



Jason also wrote separately:



<The humerus of a hummingbird is also shorter, relative to the manus,
 or relative to the humeral width, or relative to the culmen to vent 
length, than the ancestral condition, but can we say it is reduced? 
Instead it seems to me it is specialized, sporting exaggerated 
processes, and even more high - performance than the ancestral 
condition.>



  Regardless, a reduction in any relative length is "reduction." Note 
that "reduction" must be relative to a typical baseline, but that 
baseline can be anywhere; and it can be across a series, such that an 
evolutionary lineage can result in a "reduction" _overall_ or in just 
one element towards the end. The _purpose_ of a part, or rather 
functional allowances it possesses or has lost from an earlier state, is
 irrelevant when all we are doing is assessing proportions and terms 
relating to them (including diameter for the "hypertrophy" metric).

Back from the original post, Jason again:

<So I suggest a truce. If using diameter alone is, let's say, grossly 
incorrect, then using length alone must be as well.>

  Truce met, and I agree. I like to be comprehensive in what my proportions 
_mean_ and how they relate to the systems in which they operate for me to use 
them (one reason I do not like seeing proportion characters in phylogenetic 
analyses), which I guess makes me a better biomechanist than a phylogeneticist. 
To paraphrase the Bard, "The context's the thing."

Bonnan, M. F. and P. Senter. 2007. Were the basal sauropodomorph dinosaurs 
*Plateosaurus* and *Massospondylus* habitual quadrupeds? _Special Papers in 
Palaeontology_ 77:139-155.
Harshman, J., Braun, E. L., Braun, M. J., Huddleston, C. J., Bowie, R. C. K., 
Chojnowski, J. L., Hackett, S. J., Han K.-l., Kimball, R. T., Marks, B. D., 
Miglia, K. J., Moore, W. S., Reddy, S., Sheldon, F. H., Steadman, D. W., 
Steppan, S. J., Witt, C. C. & Yuri T. 2008. Phylogenomic evidence for multiple 
losses of flight in ratite birds. _Proceedings of the National Academy of 
Sciences, Philadelphia_ 105(36):13462-13467.
Senter, P. 2005. Function in the stunted forelimbs of *Mononykus 
olecranus* (Theropoda), a dinosaurian anteater. _Paleobiology_ 31:373-381.
Senter, P. 2006. Forelimb function in *Ornitholestes hermanni* Osborn 
(Dinosauria, Theropoda). _Palaeontology_ 49:1029-1034.
Senter, P. 2006. Comparison of forelimb function between *Deinonychus* and 
*Bambiraptor* (Theropoda: Dromaeosauridae). _Journal of Vertebrate 
Paleontology_ 26:897-906.

Senter, P. 2007. Analysis of forelimb function in basal ceratopsians 
(Dinosauria: Ornithischia). _Journal of Zoology_ 273:305-314.
Senter, P. and J. M. Parrish. 2006. Forelimb function in the theropod dinosaur 
*Carnotaurus sastrei,* and its behavioral implications. _PaleoBios_ 26(3):7-17.
Senter, P. and J. H. Robins. 2005. Range of motion in the forelimb of the 
theropod dinosaur *Acrocanthosaurus atokensis,* and implications for predatory 
behaviour. _Journal of Zoology_ 266:307-318.

Cheers,

Jaime A. Headden
The Bite Stuff (site v2)
http://qilong.wordpress.com/

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


"Ever since man first left his cave and met a stranger with a
different language and a new way of looking at things, the human race
has had a dream: to kill him, so we don't have to learn his language or
his new way of looking at things." --- Zapp Brannigan (Beast With a Billion 
Backs)





----------------------------------------
> Date: Tue, 29 Mar 2011 21:21:28 -0400
> From: jaseb@amnh.org
> To: qi_leong@hotmail.com
> CC: jaseb@amnh.org; dinosaur@usc.edu
> Subject: RE: Senter 2006, Confuciusornis, and humeral mobility
>
> Hi Jaime,
>
> Yes I did misunderstand your notation. I took your use of Md for Mc. My
> apologies.
>
> Now let's get to our real misunderstanding.
>
> When I initially was so bold as to blatantly state that Confuciusornis'
> second manual digit is not reduced, it was in the context of a discussion
> about function - specifically the possible flexion of this finger during
> the flight stroke.
>
> I assumed that you meant that it was reduced in a developmental/functional
> way - as the third digit is in the hand of Cathayornis or Longipteryx. I
> thought you were suggesting that it was reduced perhaps because it
> received less of the mechanical stress (stress placed on it by muscular
> tension, force directed through it by loaded feathers, etc.) that induces
> modeling and haversian remodeling. I believe you had mentioned the lack of
> extensor pits and I thought your observations were along that same train
> of thought.
>
> You were instead talking about reduction from the ancestral state. In that
> case, I agree that comparison to Archaeopteryx is best. But you are also
> equating hypertrophy to length, and there we do disagree.
>
> The humerus of the ostrich, for example, is much longer, proportional to
> the ulna and manus, than in the flying ancestor of ratites, yet it still
> must be described as reduced, mustn't it? So perhaps the converse can be
> true as well: that a bone can be shorter yet more robust, more voluminous,
> thicker - walled, perhaps more densely reinforced by internal trabeculae,
> with large flanges that probably support the follicular ligaments of
> primary feathers, and therefore more functional importance overall. I am
> not saying that I have evidence of most of these features in the case of
> PhII-1 of Confuciusornis, but all are possible.
>
> When considering functional reduction or function - related hypertrophy,
> the distinctness of the articular processes and muscle insertion points is
> probably also key. Again, in fig 39 of Chiappe et al. 1999 I see that the
> proximal end of Ph II - 2 is much broader than any other penultimate
> phalanx, and the distal end of Ph II - 1 seems correspondingly broad. Both
> show large condyles.
>
> I agree that it would be nice to see more specimens. And I'm not certain
> that the second digit could flex powerfully, it just looks possible to me.
> I am looking at the two specimens preserved in IVPP V 11374 (Fig. 173 in
> The Jehol Fossils, Academic Press (Elsevier) 2003). They show also that
> the proximal PhII 2 and distal PhII 1 are very broad, though perhaps
> flattened.
>
> But I certainly do concede that the second digit of Confuciusornis is much
> shorter, proportional to the other fingers, than it is in Archaeopteryx.
> Sorry to put you to so much trouble measuring so many specimens when I
> meant to concede that point without hesitation. And I am fascinated by
> your observation about the ungual of digit II sitting in a bowl-like
> cotylus.
>
> So I suggest a truce. If using diameter alone is, let's say, grossly
> incorrect, then using length alone must be as well.
>
> Thank you for this stimulating exchange. You are a most knowledgeable
> anatomist and patient in explaining your points.
>
> -Jason
>
>
>
>