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Re: Fwd: Aw: RE: Arboreal Theropods: The prize at the bottom of the cracker jack box



Don Ohmes <d_ohmes@yahoo.com> wrote:

> Analog, schmanalog. The mechanics of standing on a horizontal rigid branch 
> are the same as standing on
> flat ground -- whatever your neutral pose is
>
> It is irrelevant, ecologically...


It is?  You're a biped (I assume).  Give it a try.  Is standing on a
branch really as easy as standing on flat ground?  :-)


Seriously, one reason why I'm cool on the idea of roosting behavior in
non-bird maniraptorans is that there is no direct evidence for it.
Birds that roost in trees *typically* have a grasping pes, even if it
is not optimized for perching; the hallux is reversed, and able to
oppose the other three toes in anisodactyl grasping.  I know not all
roosting birds do this.  But to argue that this feature was not
required for roosting in non-bird maniraptorans strikes me as special
pleading.  (_Hypsilophodon_ was once regarded as an arboreal biped
based on similar flimsy evidence.)


The "roosting" hypothesis also holds that "proto-wings" were used by
maniraptorans to return to earth after roosting.  I like the notion
that "proto-wings" are tied to terrestriality (in this case, returning
to ground-level) rather than specialized arboreal behavior (for which
non-bird maniraptorans were clearly not adapted).


However, the problem as I see it is that long forelimb feathers that
form winglike structures ('pennibrachia') are present across a wide
range of coelurosaurs.  Even ornithomimids had them.  _Velociraptor_
might have had them too, based on the presence of quill knobs along
the ulna.  Clearly, these theropods were not climbers or roosters.
Nor were those smaller maniraptorans where the forelimbs were too
short to be of much use in climbing, such as _Caudipteryx_, _Mahakala_
and all but the most basal troodontids.  In fact, the troodontid _Mei_
is preserved in a posture consistent with sleeping on the ground.  So
not only do we lack evidence for roosting in non-bird maniraptorans,
but we have evidence for *not* roosting.


While on the topic, I'll conclude this post by throwing a bucket of
cold water on some of the features that are alleged to support
arboreal and flight abilities in non-bird maniraptorans...


First, powerful forelimbs - supposedly an indicator of refined flight
abilities in non-bird maniraptorans.  The derived trodoontid
_Linhevenator_ has short but very powerful forelimbs (based on the
robust humerus and massive deltopectoral crest).  This undercuts the
view that such features always imply good flight abilities.


Second, long distal hindlimb elements (a striking characteristic of
microraptorines) is said to be indicative of advanced  arboreal
abilities.  The dromaeosaurid _Tianyuraptor_ (a basal microraptorine?)
has short forelimbs, but elongated lower hindlimb elements as in
(other) microraptorines.  This challenges the hypothesis that a longer
distal hindlimb is always associated with arboreality.  Short-armed
_Tianyuraptor_ was clearly not arboreal, or volant.






Cheers

Tim





Cheers

Tim