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Re: pterosaur femora sprawl

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?



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