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Re: Origin of Birds-arboreal biped

Jack, jconrad wrote(12/09):
>_Scleromochlus_ is preserved only as an impression.  Dr. James Clark has
>told me that about all we have to argue with on this little creature are
>limb proportions. 

Well, I suppose that limb proportions are a great start when it comes to 
proposing a hypothetical bird ancestor.

Pieter Depuydt wrote(12/10):
>Scleromochlus as a "dune jumper", an agile creature adapted to 
>locomotion on loose sand slopes, parallelling present day desert 
>lizards (agamids?), based on the taphonomy and paleoenvironment of 
>the Lossiemouth sandstone: i.e. an oasis or wadi like environment 
>surrounded by sand dunes.
>In the SVP memoir 2, Sereno published a new reconstruction of 
>Scleromochlus, rather different from the classical "gerbil or 
>rabbit"- like one we all know. This new reconstruction shows a very 
>lightly build, bipedal animal, very similar in body proportions to 
>Marasuchus (formerly 'Lagosuchus').
>But again, S. is very imperfectly known, its place within Ornithodira 
>(whether closer to Pterosauria or Dinosauria) is not established, and 
>it might even turn out not to be an ornithodiran at all.

Wow! I'll have to track down Sereno's SVP memoir!  Even if this animal 
can not be determined to be arboreal the mere existence of something that 
size, at that place in time, and at least possibly bipedal lends more 
credence to speculations of similar forms that did take to trees and 
develop flight.

Stanley Friesen wrote (12/11):
>One new (to me) fact I found in there: it is *minute*, about sparrow sized,
>if I remember aright.  So, despite it being "cursorial" in anatomy, I do
>not see how we can rule out arboreality in it.  At that size an arboreal
>lifestyle need not leave essentially any major skeletal traces.

Exactly!  If one watches a jay, for example, moving through a thicket it 
becomes apparent that it does not need to even open its wings all that 
much; it is able to hop and jump easily from branch to branch, 
occasionally throwing a wing out for balance.  Many "more cursorial" 
birds, such as roadrunners, chachalacas, thrashers, etc. also are capable 
"thicket-hoppers".  This seems to be a viable niche for a protobird.  I 
suppose an interesting question could be regarding the evolutionary 
sequence of feather development and obligate bipedalism; which came first?

Gary Bloomfield

Gary Bloomfield, paleobirder
Bloomfield Studio