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

K Kripchak <saurierlagen1978@gmail.com> wrote:

> Jason previously remarked about
> selection acting on existing variation to produce a diverse array of
> adaptions in paravians.  Exactly.

I don't disagree.  But we need to qualify this by pointing out that it
is not certain what these diverse array of adaptations were used for.
We cannot assume that these novel features in Paraves were employed
for arboreality (although they might have been).  It is tempting to
interpret the appearance of novel paravian features with the ability
to spend more time in the trees.  This sounds rather vague to me.
Such an hypothesis needs rigorous character-by-character

> So, we're back to my original question... How did theropods/birds
> become arboreal if they weren't up in the trees to begin with?

Here's one way of looking at it.  Certain small terrestrial
maniraptoran theropods spent some limited time in trees.  As they
spent more time in trees, they accrued novel arboreal characters.  The
current dispute is over at what stage in theropod evolution this
transition occurred, and what characters qualify as "arboreal".

We can all agree that theropods became arboreal; extant birds are
living proof of this.  Also, basal ornithothoraceans show unambiguous
evidence of perching, so arboreality apparently has a long history in
birds.  But when did theropods first initially venture into trees on a
part-time basis (partial arboreality)?  Was it early in Maniraptora -
if so, partial arboreality is primitive for Paraves (and so therefore
Avialae as well)?  Or did partial arboreality arise much later (= more
crownward), such well after _Archaeopteryx_ - perhaps just before or
around avialans like _Sapeornis_ and confucisuornithids?

> Like I
> remarked in a post from long ago, whales didn't just sprout flukes and
> flop into the sea.  We know they ended up in the ocean... But
> apparently, we should suspend believing whales were at one time not
> fully evolved for doing so, unless we can come with some reason why an
> animal might go into the ocean before it is fully aquatic.

One hypothetical scenario for the early evolution of whales goes
something like this: terrestrial artiodactyls initially ventured into
the water for food or for protection.  These small-ish quadrupeds were
amphibious or semi-aquatic, and had heavy limb bones (osteosclerotic)
to provide ballast for bottom walking.  So even at this early stage
they show features associated with spending prolonged intervals being
immersed in water.  Over time, these proto-whales became progressively
more adapted for an aquatic mode of life, and targeted aquatic prey.
The flukes arose much later.