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



Jason Brougham <jaseb@amnh.org> wrote:

> Of course, in the ontogeny of the Hoatzin, the animal goes from being an 
> arboreal quadruped to biped.


Feduccia (among others) has argued that the juvenile hoatzin serves as
an analog for quad climbing in _Archaeopteryx_.  I think it's a crock.
 The hoatzin has a specialized grasping foot with a massive hindtoe
(not surprising, because it's an arboreal bird!).  In juveniles, these
feet are huge in proportion to the rest of the body.  Although the
short stumpy wings are equipped with claws, the feet are actually
majorly responsible for climbing.  This morphology is nothing like
that of  _Archaeopteryx_, or any non-avialan theropod.  Remember, the
proportions of most non-avialan theropods and basal-most avialans are
terrestrial/cursorial.


> But no one can dictate whether or how fast halluces reverse and descend. 
> [snip] One, strongly grasping, toe
> bent 180 degrees backward is not an easy or fast step in evolution, because 
> it is outside the range of normal
> variation, and a halfway version is not super helpful.


I agree that this step is neither easy or fast.  I disagree that a
"halfway version is not super helpful."  Some basal avialans appear to
have a hallux that was directed posteromedially (= in between medially
directed and reversed, so in effect a "halfway version").  So
presumably this configuration served some function.  Might have been
useful for grasping thick branches?


> A longer toe or toes with stronger flexor tubercles would be the more likely 
> first steps.


This really depends on what the toes were used for.


>Those who persist in saying that they get to decide when reversed halluces 
>must evolve are looking the wrong
> way through the telescope - back from a  world where it has already evolved, 
> and biased by that historical
> contingency. All historical events seem inevitable AFTER that fact, just look 
> at the last American election
> and, after election night, how many people suddenly knew it all along!


The pre-election polls were pretty consistent in predicting an Obama
victory.  A great many people did know it all along, because they
understood the polling data.  (Check out Nate Silver's blog, 538, as
an example.)  Others preferred not to believe the polls - even up to
and including election night when the results were rolling in.  Some
of these folk had pride of place on FoxNews.


> Longer arms, fingers and hands may be selected for to help stabilize the body 
> in trees far more readily from
> existing ranges of variation and, lo and behold, basal paravians have a rich 
> diversity of just those adaptations!


As do therizinosaurs and ornithomimosaurs.  I could find just as many
so-called "arboreal" characters in _Deinocheirus_ as in _Microraptor_.
 Rozhdestvensky's reconstruction of _Deinoicheirus_ as a giant
sloth-like arboreal quadruped was preposterous, right?  But
_Deinocheirus_ has just as many "arboreal" characters as _Microraptor_
(although not necessarily the same ones).


> Everything from Epidendrosaurus' crazy finger to Yixianosaurus' long hands.


I'm intrigued why the hyperelongated finger of _Epidendrosaurus_ is an
arboreal feature.  As for _Yixianosaurus_, the manus has strong
predatory characters, as noted by Dececchi et al. (2012).


Long arms and hands might be related to improved grasping when
climbing trees.  But other explanations deserve a hearing, especially
given that increased forelimb length is a primitive coelurosaurian
trait, and not limited to small paravians.


> And ALL basal paravians that i
> know of have long arms relative to sister outgroups like, say, Caudipteryx or 
> Compsognathus or Coelophysis.


The forelimbs of _Caudipteryx_ or _Compsognathus_ (indeed, all
compsognathids) are secondarily shortened.  So they perhaps these taxa
aren't an ideal frame of reference.


> So y'all can keep arguing about what should have happened, in your opinion, 
> or start thinking harder about
> what we know DID happen.


Umm... yeah.  That's what I'm trying to do.  I'm pointing out that
scansoriality and arboreality are not the only explanations for the
appearance of novel (and often weird) features in those theropods
closest to the origin of birds.  I'm more than happy to put
non-avialan theropods in trees.  But it should be for the right
reasons.  On that point...


Don Ohmes <d_ohmes@yahoo.com> wrote:

> As I pointed out long ago, beyond the basic ability to climb, it is 
> impossible to delineate -- even in the case
> where the "venturing" is nightly.


Yes, which makes such a hypothesis difficult to test.


> M gallopavo a good example of ground-foraging animal that perches nightly, 
> yet has a foot that gives no
> evidence of that habit -- assuming it is descended from a smaller 
> pheasant-like ancestor.


The foot retains evidence of a "partially arboreal" habit (roosting,
in this case).  The turkey has a large and reversible hindtoe.  It is
more elevated than in tree-foraging birds, but nonetheless capable of
effecting an anisodactyl grasp (even if it the hindtoe isn't always
used during roosting).


>>So unless one has a compelling reason to put
>>non-avialan theropods in trees, why do it?
>
> As Orenstein mentioned -- PREDATOR AVOIDANCE.


I originally meant "compelling" from a morphological perspective.  But
since you mention "predator avoidance", yes this does sound like a
good reason for driving a small theropod up a tree.  (Hoping, of
course, there aren't larger predatory arboreal mammals already in the
tree.)






Cheers

Tim