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Avian-Wrist Folding: Dictating Evolution of Wings, or the Reverse?

[Note: This post is labeled by the caveat that it is written by me, and my 
reputation (and apparent ego) preceed me.  This post is in no way meant to 
incur ill-intent or direct personal criticism on any individual, and is not an 
_argumentum ad hominem_.  You are warned.]

  Theories on the order of procession of the features that eventually resulted 
in fully volant, powered-flighted birds are numerous, but few are based on an 
argument of anatomical progression.  Paul (2002, _Dinosaurs of the Air_) 
reiterated an argument established almost two decades previous by the same that 
the phylogeny follows from the anatomy, that the most advanced flight apparatus 
preceeded in some dinosaurs their avian-attributes, making them convergent by 
extension of the hypothesis.  Gauthier, immediately before Paul's 1988 
_Predatory Dinosaurs of the World_, used his thesis (1984) and eventual seminal 
cladistic work (1986) to follow up a variety of arguments that Ostrom had been 
making throughout the late 60s and early 70s.  These theories predicated that 
the phylogeny of the animals proclaims that certain acquired features must have 
either preceeded or followed a point where avian volantism became possible, or 
in some cases did so repeatedly before one lineage perfected it as we see today.

  Eventually, the argument devolved into "grounds up" progression, generally 
pursuit predation leading to stabilization of the body while catching prey;  to 
"trees down" leading from parachuting to gliding as a means of locomotion (or 
also prey acquisition), and their various predictive capacities to resolve the 
phylogenetic frameworks ... but also to attempt to dictate them.  The groups 
now referred to somewhat derisively as "BANDit"s attempt to produce a framework 
for avian evolution that posits that a solely arboreal origin prevents theropod 
dinosaurs from being seminal to the avian morphology, regardless of 
convergences;  lately, this group has taken to incorporating various non-avian 
theropods into the mix, but only with many of them as secondarily flightless 
offshoots and not, actually, exemplary of the pre-avian morphology.  Other 
theories, generally forgotten, include the Orsen/Hopp hypothesis, which posits 
that wing folding for parental care was required with feathers to nest and then 
to brood.  Whether these anatomy-first or phylogeny-first perspectives are 
accurate is a fascinating field to explore, and both certainly give us food for 
thought and much to debate.

  However, a new contender comes into the fray:

  Sullivan, C., D.W.E Hone, Xu X. & Zhang F.-c. 2010. The asymmetry of the 
carpal joint and the evolution of wing folding in maniraptoran theropod 
dinosaurs. _Proceedings of the Royal Society B._ Published online March 3, 2010.

  Here, the authors argue that the carpal folding mechanism seems particularly 
suited to permitting the wrist from pulling the long wing feathers off the 
ground (which is oddly reminiscent of the Orsen/Hopp hypothesis, where the 
wrist folding permitted feathers to cover a specific area), due to the 
development of a high semilunate and high-angled radiale only after the 
evolution of long primary wing feathers.  As i said fascinating.

  As usual, however, I am intrigued by something in this paper that is not only 
not absent, but not elaborated on 9and I can only hope the authors intend a 
larger work to explore the flight apparatus and it precedents among theropods. 
Namely, that the authors dismiss the potential homology of the semilunate as 
relevant to the hypothesis:

  "A trochlear SLC broadly resembling the proximal-most part of the avian 
carpometacarpus is present in nearly all tetanuran theropods, although the 
degree of proximal convexity is sometimes small (Chure 2001; Rauhut 2003). It 
is possible that the SLC is not homologous throughout Tetanurae, in that 
different distal carpal elements may contribute to forming the SLC in different 
taxa or at least contribute to varying degrees (Chure 2001). For purposes of 
this study, however, the morphology of the SLC and its effect on the range of 
motion of the wrist are more important than its homologies."

  It should be of paramount understanding that the presence of a folding 
mechanism in taxa such as *Allosaurus* or *Tyrannosaurus*, both of which have 
distinct numbers of elements in the SLC "block" that forms the curve 
articulating with the radiale and thus permitting the wrist to adduct, occur in 
short-armed taxa, or persist in short-armed taxa;  that pre-avian long-armed 
taxa seem devoid of long feathers, or that advanced avian-like taxa (such as 
*Citipati*) possess extremely rounded SLC, arms nearly long enough to reach the 
ground (permitted by short legs), but may not have had long feathers (if 
*Caudipteryx* -- short-armed, long-legged and short-feathered -- is any 
indication) but preserved a complex mordern-style SLC that even appears to be 
fused into a carpemetacarpal unit in at least two taxa (*Heyuannia* and 
*Oviraptor*).  The authors' theory depends on the condition where the feather 
length required an advanced folding-mechanism for the wrist, to save the 
feathers from drag-damage as a benefit.  In this case, the authors reconstruct 
a minimal folding mechanism in *Microraptor* that shows that the arm would have 
to tuck in a particular way when passive to prevent the feathers fom dragging, 
and this is a fair argument.  What it does not tell us is that the argument for 
SLC homology is required to show that there was a trend toward feather-length 
and carpal block "roundness" throughout theropods from the point where 
theropods had feathers on their arms.  I cannot be certain that we are there 
yet, and the paper does not explore this option.

  The authors, however, do elaborate on the anatomy-first option for 
phylogenetic reconstruction in a sort, by presenting an analysis in which the 
two articulating facets of the radiale are mapped against a phylogeny.  This 
shows that the angle varies widely along the theropod lineage, from a low, 
nearly parallel angle in allosauroids (and basal tyrannosauroid *Guanlong*) to 
a high (or even extremely high) angle in therizinosauroids, oviraptorosaurs, 
and birds, separated by a lower angle in troodontids and dromaeosaurids. The 
avian angle is shown with *Meleagris* at 59, *Eoconfuciusornis*' at 55, and 
exceeded by *Caudipteryx* at 79 (!), while *Alxasaurus* measures in at fourth 
place with an angle of 39, greater than more basal *Falcarius* at 26.  Basal 
taxa are chosen, which is preferrable to more derived taxa, but this shows an 
extreme disparity along the lineage that belies any sort of trend unless one 
actually excludes non-paravians.  The value, then of the trend is disruptive to 
the theory, as feather length is mapped only for a dromaeosaur (medium-angle, 
but unknown feathers -- mapping *Microraptor* would be useful), some basal 
oviraptorosaurs (short feathers, high angles), and Avialae proper (which all 
have high-angles, large SLCs, and long feathers).

  The use of the radiale angle is a useful metric, however, and a tool that 
should be included to test its robusticity in further analyses;  as used, this 
angle is defined "as the angle between the proximal
face of the radiale (in avians, the facet for the ulna; in other theropods, the 
facet for the radius) and the facet that articulates with the SLC or 
carpometacarpus."  It should be noted that a shift in the articulations of the 
radiale, based on side in some cases for birds, implies a trend exists in its 
evolution, and this is also useful to apply tests to.

  A very interesting paper, and a good look at how particular bones impact our 
perspectives on broad topics.

  The paper that would seem unfamiliar to some of you (as it is so poorly 
cited) is this one:

  Hopp, T.P & M.J. Orsen. 2004. Dinosaur brooding behavior and the origin of 
flight feathers. In: _Feathered Dragons: Studies on the Transition from 
Dinosaurs to Birds_ (eds. by Currie, P.J., Koppelhus, E.B., Shugar, M.A. and 
Wright, J.L. pp.234-250.


  Jaime A. Headden

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

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