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Re: Perching/climbing Dromaesaurs



Jessica Wagar GOBI 2010 <gobi2010@hotmail.com> writes:
> I was wondering if there is any evidence that Dromaeosaurs (Did I get 
> that right?-as in Deinonychus,Velociraptor, Dromaeosaurus,etc) could 
> perch like birds? 

Dromaeosaurs were not built for perching.  A modern perching bird has a
hallux (first toe) which is long and originates at the ground level, the
base of the foot.  It is reversed with respect to the forward toes, much as
our thumb opposes our fingers when we grasp a branch.  A dromaeosaur's
first toe (or #1 pedal phalanx, as they say) is not directed in opposition
to the forward toes (phalanges) and originates too high up the foot to
assist in grasping.  _Archaeopteryx_ has a reversed hallux, although its
position is a bit higher than would be ideal.  It could have clung to a
branch, but not as securely as some birds do.  

Regarding Alan Feduccia's analysis of perching vs. ground-dwelling bird
claws in birds, _Archaeopteryx_, with its needle-sharp, curving claws, was
deemed to be a percher.  You can see comparative illustrations on page 320
of Bakker's _The Dinosaur Heresies_ (1986) and on page 52 of _Discover_
magazine, January 1994.  A ground-dwelling bird, such as the pheasant or
lark in the foregoing examples, has toes that are straighter and worn dull
by frequent contact with the ground.

Interestingly, Gregory S. Paul points out that Derek Yalden demonstrated a
likeness in _Archaeopteryx_' claws to those of climbing animals in 1984
(mentioned on page 209 of _Predatory Dinosaurs of the World_).  Paul's
analysis of the _Archaeopteryx_ fossils also has led him to interpret this
first known bird to have been a poor runner, better suited to flying,
climbing, leaping and (possibly) swimming.

And Paul restores _Archaeopteryx_ with a raised #2 pedal phalanx. 
Regarding the claw of the #2 phalanx of dromaeosaurs, Philip J. Currie
writes in _Encyclopedia of Dinosaurs_, page 195:

"This claw is strongly recurved and was more than twice as long as the
other claws on the foot.  Because of its sharp point and knife-like lower
edge, it was held off the ground in normal situations.  Although
dromaeosaurid footprints are unknown at present, the raised position of
this claw can be seen in several articulated skeletons."   

Dromaeosaurs feature manual and #2 pedal unguals which have been
interpreted to have been sharp and very highly curved, much more so than
those of _Archaeopteryx_.  These claws compare most favorably to the claws
of cats, which strongly suggests the possibility of a climbing function. 
If extant birds can climb trees without the aid of forelimbs, why shouldn't
dromaeosaurs (small ones, at least) be able to accomplish climbing with the
long, well-clawed forelimbs they possessed?  Does a small monitor lizard
"look" arboreal?  None-the-less it is!  Whether early, small dromaeosaurs
arrived at their peculiar anatomy in response to a progressively arboreal
lifestyle or not, it does not seem so farfetched that their climbing
potential was enhanced by their sharp, recurved claws, strong legs, long
forelimbs, balancing rod tails, sturdy torsos, furculae, and
posteriorly-directed pubes.  In spite of public perceptions, the
proportions of their hind limbs do not show an adaptation for high speed
running, such as we see in the ornithomimids.

Getting back to the question of perching, modern perching birds show
further adaptations that are not to be neglected.  Not only are their limb
and hip bones fused together to a greater extent than those of dromaeosaurs
and early birds -- lending additional lightness, compactness, and
stability,         but their feet automatically clench tight to a branch
when their bodies are at rest.  This is because the tendons which flex the
toes run along the outside of the heel and knee, so that any bending of
these joints conveys a clenching reflex to the foot.  This is obvious when
a perching bird walks on the ground; each time the foot is picked up off
the ground, the bending of the knee and ankle joints cause the bird's foot
to flex, just as it would were it squatting down on a branch.  Birds which
don't perch, most obviously the large ratites, have toes which droop a bit
as they pick up their feet in walking; the toes do not clench.  So it was
with theropods, as modern paleolife restorations illustrate.  Bakker's 1969
"running _Deinonychus_" illustration for Ostrom's monograph (also on view
in _Dinosaurs Past and Present_, Volume 1, page 41) shows fully flexed toes
on the foot which is up; theropods are no longer believed to have exhibited
such toe flexion as we see in perching birds.

I don't know when birds first acquired the perching reflex, but it seems to
be a very valuable adaptation.  Imagine squatting on a spindly branch high
up in a tree and falling asleep with no fear of falling!  I think of their
system as being something like a delicate set of Vise-Grip (TM) pliers,
tightening in response to the weight of the bird's body.    

-- Ralph Miller III     gbabcock@best.com

Pleasant dreams.