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Have been following the thread on sauropod necks and rearing ability 
with interest. WRT Parrish's proposal that motion in the diplodocid 
neck was restricted, it may be of interest that, in the 3-D 
_Cetiosaurus_ mount on display at Leeds, John Martin found that the
zygapophyses effectively restricted the degree of motion, making the 
neck a tensile beam, rather than a mobile bird-like neck. As 
John and colleagues have proposed (_Oryctos_ 1: 113-120), maybe 
then some sauropods had well-braced, beam-like necks. Dorsal and 
lateral air sac complexes could have lightened and maintained this 
construction. It is well known that Greg Paul and others have 
contested this model and argue that sauropod necks simply cannot have 
been at all stiff, or restricted in their motion by such things as 
elongate cervical ribs or zygapophyseal articulations (see previous 
dino-l posts). One problem with stiff-necked sauropods is, how did 
they avoid predation from big theropods? Who knows.

Ron Orenstein wrote...

> The ancestors of sauropods were
> presumably bipedal; I assume no one argues that even prosauropods 
> were obligate quadrupeds.  

Though some recent analyses of sauropod phylogeny have supported the 
monophyly and sister-group status of Prosauropoda, I think it's fair 
to say that there is still some controversy - it's certainly there 
when you get talking to people (even if they won't put their ideas 
into print...:)) - and the idea that sauropods might fall out within 
a clade of big, graviportal prosauropods is not totally rejected. 
There are problems with this scenario: most notably the 'metatarsal 
problem', and some odd features in the prosauropod hand. The point 
here is that these sauropod-like prosauropods (the 
melanorosaurs - _Riojasaurus_ and relatives) probably were obligate 
quadrupeds. If they are sauropod ancestors, maybe sauropods did, 
then, evolve from obligate quadrupeds. Does everyone agree with this? 
(That is, that melanorosaurs were obligate quadrupeds).

Finally (for now), Ron also said...

> As all birds are obligate bipeds (at least on land)

BTW, possible obligate quadrupedality has been proposed for one bird: 
the bizarre Caribbean ibis _Xenicibis_. Honest.

"We gladly feast on those who would subdue us"