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Re: Emus a Model for Dinosaurs?

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.



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