Re: Feather Flap

[Tim Williams <twilliams_alpha@hotmail.com>]

Scott Hartman wrote:

> I would side with Tim in that seed-eating is not necessarily indicative of 
> arboreal habbitats. Seeds can and do fall to the forest floor, where even 
> fully arborreal volant birds will go after them if their density and 
> gathering costs are more favorable than the seeds still in the trees.

Yes - and it might be seasonal too.

> Jeholornis does not seem to exhibit any particular adapations to 
> arboreality either. I'm more agnostic on omnivopterids. Certainly a little 
> higher up the tree (word-play intended) you start seeing early birds aquire 
> characters that are specializations for abroreality, so it doesn't seem to 
> take too long once birds really get "going" in their radiation.

You have to go quite a way up the tree (word-play still intended) before you 
find a fully reversed hallux.  This character appears quite late in avian 
evolution, in fairly derived euornitheans and avisaurid enantiornitheans.  
Confuciusornithids and a bunch of enantiornitheans (e.g., _Sinornis_) show a 
medially (or slightly posteromedially) directed hallux, which might have 
been used for grasping branches without being specialized for it.

Then again, a large proportion of known Mesozoic birds appear to have been 
shorebirds, so a specialized perching pes may not have been too necessary.  
For example, _Gansus_ has a fully reversed hallux, but the hallux is also 
rather short.  One possible explanation is that _Gansus_ evolved from 
arboreal birds, but the hallux was secondarily shortened in the line leading 
to _Gansus_.

Jim Cunningham wrote:

> >  However, goats do not habitually climb trees and they show no specific 
> > adaptations to either scansoriality or arboreality.
>
> And I never meant to imply that they did.  I was making the point that you 
> don't have to have specific adaptations to either scansoriality or 
> arboreality in order to get into the trees......  :-)

Didn't mean to single you out, Jim.  :-)   I agree with you.  I once climbed 
a tree quite easily.  Falling out was even easier!

For microraptorans, _Archaeopteryx_ and certain other basal birds, if they 
did habitually take to the trees we might expect to see arboreal 
adaptations.  It's arguable whether changes to phalangeal proportions, claw 
curvature, etc qualify as arboreal adaptations.  A specialized perching pes, 
however, is proof positive of arboreality.  As the hallux shifted all the 
way down on the foot, and became larger and fully reversed, it becomes clear 
that the pes was used for grasping.  But in fossil taxa that have one or 
more of these characters incipiently expressed, then the evidence for 
arboreality is more ambiguous.

Cheers

Tim

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