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RE: Maniraptor arms?



Just a thought... The question of how bipedal dinosaurs held/moved their
arms should benefit from consideration of other large bipedal vertebrates;
no, not so much us or the other apes (or bears, or Sam Johnson's friend's
dog) but kangaroos. 
What do roos do with their arms?  They're certainly capable of lots of
asymmetric actions such as reaching and grasping with one hand, crossing one
over the other, supinating one forearm to lick the wrist for evaporative
cooling, grooming the neck and flanks, asymmetric striking and grasping
during combat, mating etc. (Contrary to a late episode of the 1960s TV
series 'Skippy', however, landing a helicopter after pilot-collapse is not
really plausible.)
In bipedal (hopping) locomotion the arms are stereotypically held fairly
rigid and flexed in front of the chest, but this isn't necessarily
symmetrical either.  There are some species (e.g. one discussed in Tim
Flannery's book 'Country') where one of the arms (the left, I think) is
characteristically held further forward.  Aboriginal gestural 'language'
(used in hunting and dances) refers quite precisely to this species using an
asymmetric posture of the arms or a pair of fingers.
Grazing roos do a lot of slow 'pentapedal' locomotion (alternating hindlimbs
with forelinbs+tail) which is usually quite symmetric, though in tracks the
handprints may be offset either habitually or depending which way the animal
was looking, etc.
Of course, as well as the asymmetric flight strokes Mike H mentions, birds
do a lot of other asymmetric things with their wings, much as kangaroos (or
you and I).
Okay, let's apply the extant phylogenetic bracket (Witmer) to bipedal
vertebrates: birds, kangaroos and hominids are all capable of independent,
asymmetric postures and motions of the arms during locomotion as well as
other vitally important actions (feeding, mating, grooming; nobody's
ancestor couldn't carry out these functions).  So I don't see any reason to
expect (as some have implied) that the synchronous downstroke was FORCED on
bird ancestors by their anatomy; it was one of a range of options for
maniraptors, and seems to work pretty well for flying.

 
-----------------------------------------------
Dr John D. Scanlon
Palaeontologist, 
Riversleigh Fossil Centre, Outback at Isa
19 Marian Street / PO Box 1094
Mount Isa  QLD  4825
AUSTRALIA
Ph:   07 4749 1555
Fax: 07 4743 6296
Email: riversleigh@outbackatisa.com.au
http://tinyurl.com/f2rby


> -----Original Message-----
> From: Michael Habib [mailto:mhabib5@jhmi.edu]
> Sent: Saturday, May 12, 2007 2:56 PM
> To: dinosaur@usc.edu
> Subject: Re: Maniraptor arms?
> 
> On Friday, May 11, 2007, at 10:30  PM, Anthony Docimo wrote:
> 
> >
> > When a maniraptoran moved its arm, did the two arms move in sync with
> > one another, each a mirror image of the other?  (since it likely
> > wasn't in sync with the legs, as quadrapeds are)
> >
> >
> > I got the idea for this question by thinking of how birds fly by
> > flapping their wings largely in sync with one anther  --  opposed to
> > how a primate can hang by one arm, while eating with the other.
> >
> >
> > thoughts?
> 
> Well, I'm not sure how one would test for arm motion timing, but I
> would hypothesize that the arms were kept folded during walking/running
> and therefore did not move much at all as a product of locomotion (*).
> Whether the arms were used in synchrony or out of phase when grabbing
> for prey, obstacles, etc. would seem to matter primarily on what the
> animals were deploying the arms to do.  A predatory stroke would, for
> example, probably be synchronous against small prey (grabbing) and
> asynchronous when utilized to harm larger animals (slashing blows).
> 
> Birds actually do not always deploy their wings in synchrony.  They
> flap in synchrony under the most common conditions (leaping launches,
> level cruising flight, etc) but asynchronous flapping is seen during
> evasive maneuvers as well as some launching dynamics (namely large
> anseriform birds).
> 
> *The exception to the locomotion comment would be in maniraptorans with
> forelimbs capable of generating aerodynamic forces, in which case they
> could be deployed during very maneuvers, with the synchrony or lack
> thereof depending on the maneuver in question (example: leaps probably
> near synchrony, banking likely highly asynchronous or synchronous with
> asymmetrically deployment).
> 
> 
> Cheers,
> 
> --Mike H.