Re: pterosaur femora sprawl

[Michael Habib <mhabib5@jhmi.edu>]

> If we look at birds and bats, the other volant tetrapods, both got 
> their wings after a bipedal phase (bats inverted).

Bats hang, sure, but I wouldn't call them bipedal.  And we don't have 
enough information on their ancestry (given the enormous range of 
competing phylogenetic hypotheses on bat origins) to gauge what the 
ancestral state was.  Besides, even though bats rest while hanging from 
the hindlimbs, they climb quadrupdeally.  In fact, bats have 
essentially arboreal limb proportions and structural strengths 
(especially pteropodids): they are quantitatively like suspensory 
primates in the forelimb structure from the shoulder down to the wrist. 
 It only diverges greatly from arboreal expectations in the hand 
(Swartz and Middleton, 2008).  The bat hindlimb is actually quite 
unsuited for bipedal locomotion - the rotated femoral head makes for 
good hanging but poor walking.  They're really quite locked into quad 
walking now.  That doesn't mean that their ancestors were quad walkers, 
but I do fail to see why we should assume a bipedal intermediate 
(especially given that none of the potential outgroups to bats are 
bipedal).

I also note that launch mechanics suggest birds as the only volant 
group with bipedal ancestry: bats launch quadrupedally (if they can 
launch from the ground at all), and pterosaurs very much seem to have 
done the same.  Birds are therefore the only one of the three that use 
a hindlimb dominated launch initiation.

> Peters 2000 showed, via Langobardisaurus/Cosesaurus/Rotodactylus and 
> Sharovipteryx that a bipedal phase was also a part of the pterosaur 
> story. There are no published alternate scenarios for the rotation of 
> the planted hand. And still no alternate genera proposed that test 
> closer to pterosaur origins.

The phylogenetic argument is stronger: if you're correct about the 
placement of pterosaurs, and their immediate outgroups are bipedal, 
then that would at least partially suggest bipedal ancestry.  However, 
I'm a bit confused on your last point: how is it that your favored 
outgroups can be said to be objectively closer than those favored by 
other phylogenetic hypotheses?  Are you referring to branch support 
methodologies?

Cheers,

--Mike


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181
habib@jhmi.edu