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



Alrighty... First things first...

Kris Kripchak wrote:
>Selection only works if there is a need.<

Let me clear up the semantics... Swap "need" with "pressure."  A
pressure is due to needs.  There would be no pressure without
differing needs for different environments... And no, evolution is not
directional, but evolution is mainly due to selection, which is very
often "directional."  Therefore, I'm not saying selection is acting on
a need.  Selection is acting on variation, with pressure, from both
internal biological needs and external environmental forces, driving
the evolution of advantageous traits that either meet the need for
survival or individual preferences (i.e. behavior... mate selection,
acquiring a favorite food item, etc).  Jason previously remarked about
selection acting on existing variation to produce a diverse array of
adaptions in paravians.  Exactly.

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?  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.

Tim Williams wrote:
> However, if you are saying some small theropods were arboreal
> quadrupeds, then this requires arboreal characters.  This requires
> "specialized traits" (to borrow your terminology).  The theropod
> cannot be an arboreal quadruped (sensu stricto) with just incipient
> characters "borrowed" from other ecologies, if it's spending most (or
> all) of its time in trees.

It is difficult to completely resolve the niche being filled until
AFTER the evolution of "highly specialized" traits... advantageous
traits that make their owner better suited... better adapted than its
ancestors... to that niche.  How then can we rely on "highly
specialized" traits to resolve whether or not the ancestors of
arboreal animals were arboreal, which they must have been for the
selection of those arboreal traits in the first place!?  Traits
exapted for a new environment wouldn't become specialized until after
the niche is refined.  So, in what I think is the answer to your
question, the theropods in question would have had trees as part of
their niche, but not as their purely refined niche.

There are degrees of being aquatic, as well as degrees of being
arboreal... and we need to come to terms with that.  The "perfect"
correlation?  It doesn't exist.  If an animal is not max-selected for
a niche?  Then we say it may as well be totally unselected for it, and
therefore incapable.  Back to those whales again....

V/r,
Kris





On Fri, Jul 19, 2013 at 6:00 AM, Tim Williams <tijawi@gmail.com> wrote:
> Kris, it took me a while to realize what you meant by "rated PG-13
> strip tease".  :-)
>
> In brief, there is an ecological distinction between the ability to
> climb trees, and arboreality.  Arboreality is living and foraging
> mostly in the trees (at least as applied to arboreal mammals, which
> tend to be quadrupedal).  Goats can climb trees, but goats are not
> arboreal.  I can climb (some) trees, but I am not arboreal.  Sloths
> and tree-kangaroos are arboreal, because they live and forage in
> trees.
>
> If you are saying that small theropods could climb trees using
> non-arboreal characters, you'll get no arguments from me.  This
> hypothesis cannot be refuted, because it is essentially untestable.
> Heck, maybe even coelophysids could climb trees, given the right set
> of circumstances
>
> However, if you are saying some small theropods were arboreal
> quadrupeds, then this requires arboreal characters.  This requires
> "specialized traits" (to borrow your terminology).  The theropod
> cannot be an arboreal quadruped (sensu stricto) with just incipient
> characters "borrowed" from other ecologies, if it's spending most (or
> all) of its time in trees.
>
>
>
>
>
>
> On Fri, Jul 19, 2013 at 1:12 AM, K Kripchak <saurierlagen1978@gmail.com> 
> wrote:
>> Tim,
>>
>> I completely understand your points.
>>
>> Consider that our theropods were able to scamper about semi-well in an
>> arboreal environment.  Once that environment became more of a key
>> niche, selection refined those exapted traits.  The thing is... We
>> like to only focus on those now-refined traits (again exapted, not
>> adaptive) as the hallmarks of being arboreal.  Furthermore, these
>> animals are inconveniently extinct.  We cannot observe their
>> behaviors.  As such, it is extremely difficult to completely resolve
>> the niche they were filling until AFTER they become more highly
>> specialized for that niche.
>>
>> We are trapped by circular reasoning; "Theropods in the trees would
>> probably not have been specialized for such a limited niche, and would
>> appear to be non-arboreal to those only using highly specialized (and
>> modern) traits to determine arborealism."
>>
>> In other words, animals with a wide range of habitat can live in
>> places they are not highly specialized for... And... one would
>> naturally rule out fossil animals living in these wide ranges if only
>> highly specialized traits are used as the markers for an animal that
>> ventured into that particular aspect of its environment.
>>
>> My bottomline is that traits exapted for a new environment will not be
>> specialized until after their niche is refined.  The theropods at the
>> center of this debate would have had trees as part of their niche, but
>> not as their purely refined niche... And we are left with those
>> theropods being ruled out as arboreal with circular arguments based on
>> specialists traits.
>>
>> Tim Williams wrote:
>>
>>> But thereafter... if an animal spends more of its time in trees, then 
>>> selection would be expected to favor the acquisition of full-blown arboreal 
>>> characters at the expense of terrestrial ones. By "arboreal characters" I 
>>> mean those for quadrupedal locomotion in a three-dimensional substrate 
>>> (such as tree-crowns).  This is where the mobile shoulders/ankles and 
>>> grasping hallux come into play......<
>>
>>> Also Kris, the aforementioned arboreal scenario requires that small 
>>> theropods went from being terrestrial bipeds --> arboreal quadrupeds --> 
>>> arboreal bipeds.  This transitional phase of "arboreal quadruped" is often 
>>> glossed over.......<
>>
>> Eh... I don't necessarily see a problem.  In fact, the hodgepodge of
>> traits possessed by the beasties we've already found provide echoes of
>> those transitions... and this also means that arguing "this should
>> happen before this happens" doesn't hold much water for me.  Just look
>> at all the freakn' variation in those beasties we already know about!
>> This is intrinsically linked to something else that I mention with
>> great trepidation... "There's the trait you are looking for!  But I
>> don't think it means what you think it means because of where you
>> found it."  I'm sorry, but a number of these animals straddle lines,
>> fitting either here, or there, depending on the meaning assigned to
>> certain traits, some of which also straddle lines.  There is
>> ambiguity, plain and simple, as there well should be with animals "in
>> transition", settling into a new niche.  But yes... for the sake of
>> being empirical, we must identify, classify, and assign meaning.  Damn
>> you Science, and your need for measurable evidence from which to base
>> testable hypotheses...
>>
>> I liken the use of the fossil record to resolve such evolutionary fog
>> to a rated PG-13 strip tease: It offers a tantalizing glimpse, but it
>> never, ever gives a clear look at all the goods.  Instead, you are
>> left frustrated, filling in the blanks with related information, never
>> knowing if you actually end up with the accurate picture.
>>
>> And Ron... I 100% agree that there were certainly many, many different
>> "pressures" influencing behaviors that took them into the trees (and
>> even back down again), probably across many different lineages and at
>> many different times throughout their evolutionary histories.
>>
>> V/r,
>> Kris