Re: Bird /dino questions -Reply

[James Norton <JNorton@MAILBOX.UNE.EDU>]

It's been my understanding that the forward shift in the center of gravity
in birds due to the loss of the mass of the tail was(is) compensated for
by a shift in the orientation of the leg bones.  The femur of a resting
(standing) bird is close to horizontal, bringing the knee closer to the
center of gravity.   The movement at the hip joint is reduced in favor of
more movement around the knee and more distal joints of the leg and
ankle, compared to a theropod, which would use the tail as a
counterbalance and would retain a greater range of motion around the
hip.

Is this a correct interpretation?

JM Norton

>>> "Matthew Troutman" <m_troutman@hotmail.com> 03/26/98 11:00am
>>>

>Maybe a dumb question, and I'm groping for something here, but do we 
see
>a correlation between reducing tail length and lightening of the skull
>anywhere in the evolution of birds? Just that, aerodynamically, I
>imagine you want fore and aft equilibrium about the COG for flight (the
>wiings?), so each loss of tail length should be reflected in a
>corresponding loss of skull weight (or to a shorter neck, up to a
>certain point, after which you _have_ to reduce skull weight becuase 
the
>neck vertebrae cannot be made any shorter)?.

    Early birds did not have the lightening of the skull and reduction 
of the tail correlation. Rahona and Archaeopteryx still had long caudal 
series with many of the basic dromaeosaur features. Archaeopteryx 
retained a postorbital and many other bones that ornithurines lack. 
Though enantiornithines did reduce their tail to a pygostyle, their 
skulls were basically similiar to that of Archaeopteryx. Confuciusornis 
did have a relatively short neck, though many enantiornithines did not. 

>Also, flying seems to imply that, if the COG is moved forward to the
>wings, then an upright stance when grounded is obliged, in order to
>bring the COG back over the (rear) legs?  Voila, bi-pedalism could be 
>the consequence of flight?

    Actually, when a bird folds its wings, it is forced to stand near 
vertically. So it seems when a bird folds its wings the center of 
gravity shifts. This may be another explanation for wing folding.

MattTroutman


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