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Re: Jeholornis




David Marjanovic <david.marjanovic@gmx.at> wrote:

> I wouldn't be surprised. Tail reduction seems to be a common trend > in coelurosaurs.

I'd rather say tail elongation is one... :-) (basal dromies, basal
oviraptorosaurs...)

Two characters are actually at play here: (1) number of caudal vertebrae; (2) overall length of the tail. The number of caudal vertebrae in non-ornithothoracine maniraptorans is fairly stable at around 23-30, though the length of the tail relative to the trunk varies enormously. Dromies and basal birds, as David suggests, re-elongated the tails, perhaps due to their novel use in locomotion, such as dynamic stabilizers.


_Caudipteryx_, _Nomingia_ and derived therizinosauroids have absurdly short tails. In these short-tailed theropods it is difficult to conceive how the center of mass could have remained close to the hips, unless...
(a) It didn't (as suggested by Jones, Ruben &c for _Caudipteryx_) and these critters adopted an avian-style bipedal posture in response to a forward CM (or vice versa?). Unlike Jones, Ruben &c, I don't think this necessarily implies secondary flightlessness.
(b) Increased pelvic mass and girth shifted the CM backward - quite possible for bulky herbivores such as therizinosauroids;
(c) These theropods were not cursorial, and so did not need a stable bipedal posture (and therefore a pelvic CM) because they never actually ran. Again, this is quite possible for the ponderous therizinosauroids, especially if they spent most of their waking hours squatting on their haunches grabbing branches with their clawed hands (as proposed by the Russells some time ago.)




Tim






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