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Re: Ah ha! That's where therizinosaurs came from
Dr. Ronald Orenstein wrote:
> Has anyone come up with an adaptive explanation for the degree of fusion
> and finger reduction in the avian hand (including the hand of living
> ratites) that does NOT involve supporting a more or less rigid feathered
> wing? I suppose someone will point to alvarezsaurids, but the similarity
> does not seem very great (and I suppose they could be secondarily
> flightless too). This is not a question of reducing forelimbs (as seen in
> tyrannosaurids), which I suppose can be explained as a consequence of a
> shift in a bipedal animal from manipulating or catching food with the hand
> to using the mouth - but of the very specific type of fusion seen in
> modern birds.
Somebody (in this case me) will point to the fused carpometacarpus in
the oviraptorosaurs _Avimimus_ and _Heyuannia_, and in the
deinonychosaur _Balaur_. In these cases, fusion of the
carpometacarpus might very well be associated with lack of use.
I'd be hesitant to endorse an aerodynamic explanation for the
development of the avian-style carpometacarpus. Because there is no
doubt that ratites evolved from flighted ancestors, it's a moot point
whether the avian-style carpometacarpus originally evolved for a
specific flight-related purpose.
> I admit this point isn't proof of anything (it might be
> characterized as an argument from incredulity) but surely it is at least
> suggestive of secondary flightlessness in ratites. Could the same thing
> be said of the pygostyle, or could this be explained as an adaptation to
> support and manipulate a display structure?
One hypothesis is that the evolution of the pygostyle was initially
driven by truncation of the tail, and fusion of the distal caudal
vertebrae was a byproduct of this process. This is one explanation
for the development of the pygostyle in avialans, and the
pygostyle-like structures in other taxa (_Nomingia_, _Beipoaosaurus_).
In modern birds, the pygostyle has an important flight-related
function insofar as it supports the rectricial fan. But it is not
clear when this flight-related function arose; it may be a fairly late
innovation (at or around the level of basal
euornitheans/ornithuromorphs?). AFAIK, there is no evidence of basal
avialans or enantiornitheans using large tail feathers for any
function other than display.