Re: Archaeopteryx chimaera

["John V Jackson" <jjackson@interalpha.co.uk>]

On the subject of the backward pointing pubis in various forms, may as well
chuck this out.  (Original source of a lot of it was PDW of course.) :




"The reason seems linked to balance considerations in birds.  The shorter
and
stockier the body, the further forward the centre of gravity is brought
relative to the wings. The tail provided lift in addition to the wings but
with more drag, so it was better to rely on the tail for lift as little as
possible, which in turn meant the centre of gravity had to be moved
forwards.  (Quite soon, the very early bird Confuciusornis demonstrated it
had completed this task by removing almost all its tail feathers except for
two decorative streamers.)  Of course the tail was drastically shrunk in
Archaeopteryx, in length as well as girth, saving considerable weight.  (Why
even fully terestrial theropods didn?t do this earlier might seem to some a
bit of a mystery!)  However, with a much lighter tail, there would have been
a tendency to topple forwards when on foot.  This problem could also be
solved by the solution to the ?in flight? balance problem: shortening the
trunk, but this time the crucial effect was to move the centre of gravity
backwards relative to the point of contact with the ground (ie the feet).

"Of course if the trunk is shortened, a new place had to found for some of
the innards: between the legs - which provides an explanation for the
backwards pointing pubis in birds, the ?raptors?, and to a certain extent
Archaeopteryx.

"(Unfortunately the legs then presumably had to be somewhat wider apart,
which mechanical considerations suggest would in turn lead to inefficiencies
on foot.  This might explain why, if the secondary flightlessness theory is
true, the arctometatarsalians and the oviraptor family must have moved the
pubis forwards again, and though of course modern birds whose wide-hipped
design is irreversible do include some very fast runners, some see troodon
and the ostrich mimics as faster runners, at least than the ?raptors?.)

"Maybe the ?on foot? balance problem was more significant than the ?on the
wing? one, but it may be of some interest to note that until the wings
started to create proper aerofoil lift, airborne balance may have had less
importance.  As wings bearing long bristles/quills would be no more
efficient than a
bristly tail, there would be less incentive to shorten the body.

"It seems though that we can take the retroverted pubis of Deinonychus  and
its similar hipped relatives as evidence of a flying background."



JJ

http://www.geocities.com/CapeCanaveral/Hall/2099/DinoKabin.html