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RE: Yet more on pterosaur quad arm posture

Precisely. A corollary of the roosting hypothesis is that roosting exapted the 
lineages of avialans that went on to evolve reversed halluces (and maybe even 
fully powered flight), by placing them in the environment where perching toes 
were selected for in the first place.

From: owner-DINOSAUR@usc.edu [owner-DINOSAUR@usc.edu] on behalf of Tim Williams 
Sent: Thursday, July 11, 2013 7:48 PM
To: dinosaur@usc.edu
Subject: Re: Yet more on pterosaur quad arm posture

Scott Hartman <skeletaldrawing@gmail.com> wrote:

> Yes, I think we all agree that animals with derived flight but are not
> scansorial can engage in brief activity most anywhere (building
> ledges, trees, etc) since they do not have to climb them.

Yes, powered flight opens up a whole world of opportunities.  As David
M. said, a great many terrestrial or semiaquatic birds can roost in
trees (or on ledges or on fences) simply because they can do so
without climbing.

> How this
> applies to non-flying theropods or theropods at an early flying stage
> is less clear, unless you are advocating a WAIR ingress to the
> roosting site, which I don't think anyone would take issue with.

In general, I take issue with WAIR as an early flight behavior for
bird ancestors.  Several studies have criticized WAIR as an incipient
flight behavior.  WAIR requires an advanced flight stroke with an
incipient wing.  By contrast, basal avialans had a fully-developed
wing, but this was apparently combined with a primitive flight stroke
(if they were capable of a flight stroke at all).  Or at least, a
flight stroke very different to that of today's birds.

> Regardless, the act of roosting is not scansorial activity, and would
> not result in any sort of flying-squirrel selective pressures -
> instead it would seem that the vast majority of selective pressure
> would still occur on the ground, where the organisms have to eat,
> mate, and flee danger for most of their lives.

Exactly.  The corollary of this is that i
its time in trees, that's where the selection pressures would be
applied.  I can accept that _Microraptor_ and kin were capable of
venturing up into vegetation, although spending most of their time on
the ground (including when nesting or sleeping).  But if these
theropods spent most or all of their time in trees, they would be
under strong selection to acquire appropriate "arboreal" adaptations,
such as for grasping branches.  However, these traits are not apparent
in basal avialans or non-avialan theropods (although _Bambiraptor_ has
been regarded as having a somewhat opposable manus).  But no way were
any of these non-avialan theropods adapted for living in trees.