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In a message dated 97-07-24 12:22:27 EDT, Tetanurae@aol.com writes:

<< A probable explanation for this is that when small, basal maniraptorans
 climbing around in the trees, they got the 90 degree tail bend trick which
 also lightened their tails, which necesitated the retroversion of the pubes
 for proper balance. >>

Most of the early changes in theropod tail anatomy can be related to the use
of the tail in directional gliding control by "small, basal maniraptorans."
Specifically, this includes distal tail stiffening as well as proximal tail
flexibility. Proximal tail flexibility can also be useful in an arboreal
lifestyle outside gliding, and the concomitant strengthening of the
caudifemoralis musculature certainly didn't hurt theropods' cursorial
abilities. Pubic retroversion, however, may simply be related to giving the
body a more aerodynamic shape, as it occurs in Theropoda only in the most
avian-like forms.