Re: perching in pterosaurs

[Tim Williams <tijawi@yahoo.com>]

David Peters wrote:

> Earlier I asked Mike Habib about pterosaur ancestry. Whether
> archosaur or fenestrasaur, in either case the forelimbs were
> relatively small (witness Cosesaurus, Longisquama, &
> Sharovipteryx on one hand, Euparkeria and Scleromochlus on
> the the other). [snip] So in
> outgroups to pteros, we've got bipedalism, small
> forelimbs long hind limbs and big hips. If pterosaurs are
> going to quad launch, they can't inherit it from these
> guys.
> 
> And they don't. All on their own pteros develop
> elongated antebrachia. 


To quote Smith and Redford (1990), "Anatomy is not destiny".  So what if 
putative pterosaur outgroups taxa have shorter arms, longer forelimbs, etc?  We 
don't know what the first pterosaurs (or pterosauromorphs) looked like, and 
this is presumably the phase where the transition to aerial locomotion occurred 
(antebrachium became elongated, etc).  (Remember, there are some very 
short-armed coelurosaurs fairly close to the origin of birds - e.g., 
alvarezsaurs, compsognathids.)


If you are inferring that the quad-launch hypothesis for pterosaurs is 
contradicted by current cladograms that show bipedal taxa as outgroups, then 
you have misinterpreted the cladograms in question.

>From another post (also sent by DP):

> Take off the wing finger of any taxon from Longisquama to
> Sordes and you'll have an analog in basal primates from
> lemurs to tarsiers. Especially so among the short-necked
> pterosaurs. What are the differences in your opinion?
> Let's talk specifically. 


Let's start with wrists and ankles, and how relatively immobile they were 
compared to arboreal primates.


> I appreciate your thoughts, Tim. Have you considered the
> hypothesis that an elongated digit V acted as a universal
> wrench to enable perching? It worked like a reversed hallux,
> except opposite in every way. It was lateral, not medial. It
> opposed the branch surface by extension, not flexion. It
> contacted the branch surface with its inverted dorsal
> surface caused by hyperflexion of p5.2, rather than its
> ventral surface as in birds. And no claw was involved. 

It's an interesting idea; but it's difficult to accord with the proportions and 
morphology of the fifth toe.  For example, Clark et al. (1998; Nature 391: 
886-889) on _Dimorphodon_:

"The phalanges of the slender, elongated fifth digit, to which the wing 
membrane may have been attached, apparently did not participate in supporting 
the foot.  The robust metatarsal is positioned posterolaterally to the other 
metatarsals, and its plantar surface would contact the substrate in a 
plantigrade stance.  However, the nature of the MP joint does not indicate that 
the proximal phalanx was capable of projecting much further ventrally than 
metatarsals IâIV, and its length and slenderness would have provided little 
mechanical advantage in supporting the rest of the foot."


Clark &c then go on to discuss how _Dimorphodon_ might have used its other four 
toes in climbing ("...a capacity for grasping that is consistent with an 
ability to climb...").  But involvement of digit V in perching is a different 
matter altogether.


Cheers

Tim