Re: On the Issue of Sprawling Dromaeosaurs

[Tim Williams <twilliams_alpha@hotmail.com>]

David Marjanovic wrote:

> I still don't know why everyone seems to assume that *Microraptor* and the
> like were arboreal. The 1st toe was a bit longer than usual, but still
> short, and its position is unknown; the phalangeal proportions are still on
> the terrestrial side;

I don't know that "everyone" assumes that _Microraptor_ was arboreal, but a 
lot of people believe it probably spent at least part of its time in trees 
(including myself).  The elongate penultimate phalanges and the slender and 
recurved pedal claws are comparable to modern perching birds; and although 
the orientation of the hallux is unclear, the first metatarsal and the first 
pedal digit have shifted further down on the foot.  This allowed improved 
opposability, but it is not specialized for anisodactyl perching.  Then 
again, a specialized perching pes had to come from something, and the pedes 
of _Microraptor_ and _Archaeopteryx_ show an incipient stage in its 
development.

> the body size means nothing; and the foot feathers
> could or could not have hindered running, we don't know, because we don't
> know if the feathers were foldable (among other things).

The "foot feathers" might or might not have hindered running.  But what were 
they there for, if not for aerial locomotion?  Even those who argue that 
_Microraptor_ might have been a powered flier have not gone so far as to 
suggest that _Microraptor_ was capable of a ground-level takeoff.  
Therefore, _Microraptor_ required an elevated platform from which to launch 
itself into the air.  In light of the derived pedal morphology, trees appear 
to me to be a pretty good candidate for this.

True, body size alone means nothing.  Until we find a _Microraptor_ specimen 
with its foot wrapped around a fossilized tree-branch, we have no direct 
'proof' that _Microraptor_ spent time in the trees.  But combining all the 
characters mentioned above (small body size; pedal moprhology; plumage), one 
has to wonder if all these *derived* theropod characters might be tuned 
toward the same ecomorphology.  I think we can build a highly circumstantial 
and parsimonious case that they are - in this case, tree-climbing + aerial 
locomotion.


Tim

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