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RE: loss of retractor muscles

>From David Peters...

Dear Tim,

The closest living analogs to those tiny bipedal prolacertiform sister
taxa of pterosaurs are the 19 living lizards capable of bipedalism to
various degrees (running only, standing and running, walking, standing
only). In each case undulation of the torso is reduced, undulation of
the tail is reduced (the two are typically in concert with each other),
caudofemoralis muscles are proximalized (is there such a word?), and the
anterior process of the ilium is elongated (but never by very much,
typically only a nub) when compared to non-bipedal lizards.  In
Cosesaurus, Sharovipteryx  and pterosaurs these characters are
hypertrophied beyond that of any living lizard, good evidence that they
were more than capable of bipedalism in the lizardy fashion (sprawling
femora). Evidently when the need to undulate is reduced, the big
undulating muscles, the caudofemoralis retractors, and their various
caudal anchors are reduced.

In dinos leading to maniraptors, the situation starts off advanced
beyond that of undulating lizards. I could only venture a guess in that
matter and that would be to follow Gatesy's work. Evidently the
maniraptorans were using their swinging tibiae more than their swinging
femora, hence eliminating the need for femoral retractor muscles and
their caudal anchors, the transverse processes and hemal arches. Just a

Hope this helps. Share with the group if you wish.

David Peters (not a Dr.)