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Arboreal Theropods: The prize at the bottom of the cracker jack box

A very long time ago, I went on a rant, asking for insight on how
theropods, including birds, became adapted for life in the trees if
they were never in the trees to begin wtih.


I bring this up, yet again, because of the latest 12-round throw-down
on the one-time existence (or not) of arboreal theropods.  To recap
what I was previously driving at, here was my main point (which I
believe is still the 800lb gorilla in the room):

"It’s basic; Selection only works if there is a need. How did the need
for the traits we deem as “requirements” for arboreality begin if
protobirds weren’t up in the trees before they needed the traits to be
up in the trees???  Where’s the logic? Awfully convenient that many
ended up in the trees only AFTER they were evolved to allow such
behaviour, right?  So, we end up with arguments that boil down to
saying; "Birds/theropods were unlikely to have become arboreal without
cursorial basal birds/theropods possessing the capabilities, traits,
and behaviours for becoming arboreal.  However, the capabilities,
traits, and behaviours for becoming arboreal wouldn't have been
selected for until cursorial basal birds/theropods became arboreal."
A Catch-22."

And now we have this: "Aditya Barve, Andreas Wagner. A latent capacity
for evolutionary innovation through exaptation in metabolic systems.
Nature, 2013; DOI: 10.1038/nature12301".  Yes, it deals with E. coli
bacteria... but the results of the study have far-reaching
implications, most notably for the importance of exaptation. The
authors' conclusion? Exaptation may exceed adaptation several-fold,
further muddying the already turbid waters of distinguishing adaptive
from exaptive traits.

What's this have to do with rants about arboreal theropods?  Well, it
further complicates our desire to neatly package up a testable
hypothesis because the traits you interpret as being adaptation (or
conversely maladaptation) for an arboreal lifestyle may not be all
that in a box of cracker jacks (man, I just dated myself...)

How do you derive interpretations of traits?  By comparison with the
traits of known, living animals, of course... that are arboreal,
terrestrial, and scansorial.  The problem is that you already know the
answer to the question of the relative importance of your identified
traits... the "selective advantage" you assign to those traits.  With
extinct taxa, particularly those that represent animals "in
transition", that's a luxury you actually don't have.  It could very
well be that a trait you deem a selective advantage, like, let's say,
a certain toe position, nail curvature, or joint mobility, was in fact
of lesser importance at that particular time for that particular
animal.  And it's even worse because the opposite may also be true...
What you've dismissed as minor may have been key at that time for that
animal to live whatever lifestyle it was living.  Oh, but the
nightmare continues!  In the end, a minor, neutral trait, sitting
latent for generations, and that you glossed over, may later emerge
with great importance.

To what degree natural selection is driven by selective advantage is
far from cut and dry.  So back to my previous rant... The main point
hasn't changed: Selection only works if there is a need.  But now
consider that pre-adaptive traits may not only be more common than
adaptive traits, but they may harbor greater selective importance in
the long run... Using the existence of mobile shoulders, or the
existence of perching toes, or whatever, as the linchpin selective
advantage from which to build positive conclusions on "the moment of
arboreality" may not actually be as strong a foundation as you think.
Being small and having long, strong arms with big freakn' claws may
have been pre-adaptive enough for first scratching out a niche in the
trees. And unlike many other courses of reasoning, it actually
provides an escape from that Catch 22.

And this is marvelous... Now I'm hungry for cracker jacks...