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Betty's question regarding stiff tails

Regarding the stiffness of the tail in _Deinonychus_, the long thin tail of
bipedal lizards has been shown, by Hamley, to act as a laterally swinging
metronome, swinging in phase with the lateral undulation of the body and the
hind limb cycle. Hamley (1990) demonstrated that the stride frequency is
affected by the length, mass and stiffness of the tail. A reduced mass or
increased stiffness increased the stride rate.  This may have been the
evolutionary driving force behind the peculiar caudals of dromaeosaurs
(including _Archaeopteryx_) and pterosaurs (including _Sharovipteryx).

The situation is taken to the extreme in post-Jurassic birds and

An important difference between lizards and dinosaurs, of course is this: in
lizards the hind limbs are nearly horizontal, in dinosaurs vertical.  Gatesy
has shown that in dinosaurs, due to their vertical femora, the tail must have
been pulled down with every extension.  So lateral undulations did not occur
in dinosaur tails during locomotion, only perhaps in corrective weight shifts
to maintain balance during moments of instability.

In pterosaurs, the caudofemoralis muscles are greatly reduced (as evidenced
by their greatly reduced caudal transverse processes and hemal arches).
 Thus, despite semi-erect, semi-sprawling limbs, lateral undulation became
greatly reduced, another piece of evidence pointing toward bipedalism.  

Conclusion: The thin, stiff tail of pterosaurs is a terrestrial feature, not
an aerodynamic one.  No wonder pterodactyloids got along so very well with
only a teenie tiny one.

David Peters