Re: Emus a Model for Dinosaurs?

[Tim Williams <twilliams_alpha@hotmail.com>]

Michael Skrepnick wrote:

> of particular interest is their assertion that center of gravity remains 
> the same in Caudipteryx as in
> all other non-avian theropods and if so negates the necessity of a shift in 
> hindlimb locomotion from the hip to the knee. . [snip] If the reduction in 
> caudals was not enough to affect hindlimb movement in Caudipteryx itself,

The neornithean "body plan" (at least where volant forms are concerned) is 
characterized not only by a shortened tail, but also by a larger head, 
longer forelimbs, and expanded pectoral musculature +skeleton.  All these 
features conspire to drag the CM well forward of the hips.  These features 
have been reversed somewhat in ratites and other flightless cursorial birds 
- except for the tail, which remains short.  Ratites do show a caudal 
lengthening of the pelvis though.

As for _Caudipteryx_, it has rather short forelimbs by coelurosaurian 
standards.  Also, I wonder how useful its forelimbs were, given that they 
were: (a) covered in feathers; (b) useless for flight; (c) functionally 
didactyl.  I really don't think the forelimbs were used to catch prey.  This 
speaks to a reduced pectoral musculature compared to other maniraptorans 
(avian and non-avian).  So the shorter tail may be correlated with a less 
weighty shoulder+forelimb region.  Just an idea.

> I  wonder if there is any impact in oviraptorosaurs like Nomingia having 
> further reduced caudal series / pygostyles. . .

_Nomingia_'s tail was certainly short (24 vertebrae), but it was also looks 
quite 'hefty': big, flaring chevrons; median dorsal crest (made up of pre- + 
postzygapophyses) running down the last third of the tail; and the 
blade-like pygostyle.  If the tail was feathered, the rectricial frond or 
fan would added extra weight too.

> are any secondarily flightless theropods ( or those initially evolving 
> flight ) subject to a shift in hindlimb locomotion, or is this strictly 
> relegated to neornithes / advanced birds?

Excellent question.  This requires more study.  It's unclear how or when 
this shift occurred in avian evolution, but it was certainly 
post-_Archaeopteryx_, and probably close to the base of the Pygostylia 
(Confuciusornithidae and higher), not surprisingly.

Cheers

Tim

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