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Re: Yet more on pterosaur quad arm posture



Jason Brougham <jaseb@amnh.org> wrote:

> There is a danger here that we may become convinced that basal paravians did 
> not
> climb into trees. There is  also a danger that we may become convinced that 
> basal
> paravians were arboreal.


If basal paravians climbed trees, they would be "scansorial".  Glen
and Bennett (2007) underscore the difficulties associated with neatly
categorizing birds as arboreal or terrestrial, because many species
divide their time between the trees and the ground.  GSP believes that
_Microraptor_ spent most/all of its time in trees - so this would be
"arboreal" whatever definition one chooses.


However, so far there are no features in any basal paravian to
indicate that the hands or feet were adapted for grasping branches.
The contention that _Microraptor_ was a specialized arborealist
doesn't stack up against the anatomical evidence.  Yes, _Microraptor_
may indeed have climbed trees; but there is nothing in its anatomy to
suggest that it was specialized for living in trees.


Maybe _Microraptor_ spent most of its time on the ground, but
occasionally climbed trees to procure prey?  That would explain the
lack of arboreal adaptations; and its predatory features could be
exapted for scansorial behavior.  This kind of incipient arboreal
behavior would be difficult to discern from the anatomy, so it is
difficult to frame this question as a testable hypothesis.


> We all agree there was a transition at some point in the theropod lineage 
> that led to
> perching foot morphologies in birds, whether that happened within the Avialae 
> or
> earlier. We probably all agree that animals did not evolve perching feet AND 
> THEN
> climb up into trees.


This is a tricky one.  One hypothesis is that a grasping pes evolved
for a predatory function *prior* to perching (see Fowler et al, 2011).
 This predatory function (notably to help immobilize large prey)
promoted the lowering of the hallux, with the medially directed hallux
opposing the fourth toe, to effect a grip.  This sort of grasping pes,
evident in certain deinonychosaurs, would have had limited (if any)
use in perching; but it could have been the starting point for
evolving a true perching foot.


A perching (branch-grasping) function requires a few refinements to
the pes - including a much longer hallux that was positioned even
further down on the foot (ideally articulating at the same level as
the other three toes), and retroverted (reversed) such that the more
posteriorly directed hallux opposes the third toe (= anisodactyly).
Fowler &c even propose that predation favored the evolution of this
anisodactyl grasping morphology, but there is no direct evident of
this in theropod phylogeny.  (Unless, of course, the grasping foot of
confuciusornithids and sapeornithids is a predatory, not arboreal,
adaptation... but even I would not go so far as to argue that!)


An alternative pathway for the evolution of the perching pes is that
the hallux was fully descended before it was posteriorly-shifted.  The
foot of _Epidendrosaurus_ shows this configuration.  This pedal
morphology may not be much good for grasping (no digital
opposability), but it is consistent with scansorial behavior where all
four toes are used to grip an inclined or vertical substratum.


> They had to be adapting to living in trees when arboreal features were 
> selected for.


Or, they were capable of venturing in trees when arboreal features
were selected for.  If you equate "capable of venturing in trees" with
"arboreal", then we are in agreement.  But this definition of arboreal
is far too permissive for my liking.


> I would say that Dr. Williams is right that (despite many claims) no one has 
> firmly
> proven arboreality in Paravians below Avialae, but he uses a strict 
> definition of
> arboreality (as foraging primarily in trees)


(There are some who call me... Tim. )


> and is only comfortable assigning that behavior to animals with reversed 
> halluces.


That's not really what I meant.  I'm just *very* surprised that basal
paravians that are purported by some to be fully arboreal (like
_Microraptor_ and _Archaeopteryx_) have a pedal morphology that is
fully consistent with terrestriality.  If these paravians were
arboreal (such as habitually perching or roosting), why wasn't the
hallux fully descended and retroverted?  As I said, even
_Epidendrosaurus_ shows a fully descended hallux (though it shows no
indication of being reversed.)


> I'd say his reasoning there is
> unassailable, and conservative in the best sense of the word. However I think 
> he
> sometimes confuses those with whom he corresponds, who may use a far more
> inclusive definition of the word 'arboreal'.


Definitions aside, I would say that it is very odd that animals that
are alleged to have spent most or all of their time in trees have (at
best) only incipient adaptations for arboreality. But if these
theropods only occasionally ventured in trees, then it makes sense.
Which brings me to this...


> I am personally more interested in incipient tree climbing behaviors. That is 
> to say,
> tree - climbing behaviors that occur without clear, skeletal, arboreal (sensu 
> stricto)
> adaptations.


The thing is, a whole raft of basal paravians are alleged to have
spent most or all of their time in trees: _Microraptor_,
_Sinornithosaurus_, _Archaeopteryx_, _Jeholornis_, etc.  Why were all
these so-called arborealists 'stuck' in the incipient stage of
arboreality?


> There could have been a very long evolutionary transition that involved
> tree - exploiting behaviors among the many small - bodied, winged, basal 
> paravians.


Yes, it might have been a long evolutionary transition.  But if
incipient arboreal behavior began at the base of the Paraves (let's
say early/mid Jurassic), why do certain mid-Cretaceous forms like
_Microraptor_ have such "incipient" (read: piss-weak) arboreal
adaptations if they were supposedly highly specialized for living in
trees?  I think the most parsimonious explanation is that
_Microraptor_ was not arboreal.


> It is also eminently possibler that tree - utilization could have been quite 
> sudden within
> basal avialans, in which case it may have required flight to proceed.


This is an interesting hypothesis.  If (and this is still a very big
IF) these basal paravians (including basal avialans) could take off
from the ground, then perching and roosting behaviors might have come
*after* the advent of flight.  Thus, theropods may *never* have
actually been adapted for climbing trees.  On the line to birds, they
went from terrestrial bipeds to perching/roosting bipeds without a
transitional quadrupedal climbing phase.


> It is true to say that several empirical studies of claw curvature and foot 
> proportions
> are consistent with the possibility that these small, basal, paravians spent 
> some time
> in trees and foraged primarily on the ground. Their measurements tend to 
> overlap
> those of ground birds that roost or spend some small percentage of time in 
> trees,
> though the comparisons are imperfect. Glen and Bennett. (2007) Foraging Modes 
> of
> Mesozoic Birds and Non-Avian Theropods. Birn-Jeffrey et al. (2012) Pedal Claw
> Curvature in Birds, lizards and Mesozoic Dinosaurs.


Yes, I agree.  Nevertheless, the birds that do forage on the ground
and perch in trees (a) can fly up into the tree crown, and (b)
typically have a reversed and opposable hallux.  These studies on claw
curvature do not tell us how basal paravians climbed trees, or how
they locomoted among the branches.  (Of course, if these basal
paravians had good flight abilities, then climbing is not required to
reach branches, and there was no need to use tree branches as an
elevated platform for launch.)


> I, for one, would like to acknowledge the fact that the evidence is open, and 
> refer to
> the research of those who have done the hard work of publishing papers on this
> subject. We already know what one another's inclinations are.


I have yet to see a paper that has presented a convincing case for
arboreality in _Microraptor_.  Yes, certain papers have drawn
attention to derived characters associated with pedal phalangeal
proportions, claw curvatures, and femoral insertion into the
acetabulum - all supposedly "arboreal" characters.  But since some
other (much larger) dromaeosaurs like _Deinonychus_ have these too,
I'm reluctant to accept these as evidence of arboreality.





Cheers

Tim