Re: pteros have lift-off

[Augusto Haro <augustoharo@gmail.com>]

Tim Williams wrote:
>
> Following the tree of Brusatte et al. (2008, Science), Pterosauria is the 
> sister taxon to Dinosauromorpha.

Well, grossly speaking. As you say below, in reality, Pterosauria is
the sister taxon of Scleromochlus in that analysis.

> However, bipedal behavior may be derived for Ornithodira, given that the 
> sister clade to Ornithodira
> (called Crurotarsi in the phylogeny of Brusatte &c) was primitively 
> quadrupedal.

True, do not deny that, but say that you have less steps admitting
Ornithodira as primitively bipedal (I mean, bipedalism appearing at
the most basal node within Ornithodira) and Pterosauria bipedal too,
than admitting a quadrupedal Pterosauria. This is mainly so because
the two successive outgroups of Pterosauria (namely Scleromochlus and
Dinosauromorpha) are likely primitively mainly bipedal, and the
quadruped Crurotarsi come just after these in current studies.

> If _Scleromochlus_ (probably also a facultative biped, at least) forms a 
> clade with Pterosauria to the exclusion of Dinosauromorpha
> - in other words, if _Scleromochlus_ is a basal pterosauromorph - then this 
> would lend support to ornithodirans (and therefore
> pterosaurs) being ancestrally bipedal.  (This is the position recovered by 
> Brusatte &c, in Fig S1, for example).  But given how
> uncertain _Scleromochlus_'s phylogenetic position is, this is an open 
> question.

Well, as far as I know, the alternative is the position recovered by
Benton 1999 (in Philosophical Transactions, I think), where it is the
sister taxon of all other ornithodirans. I think this does not change
the counting of steps I gave before, if my maths are right. If
Scleromochlus was closer to dinosaurs, which I didn't find anywhere,
then yes, considering pterosaurs quadrupedal will not add an extra
step.

> Even if bipedality (even facultative bipedality) is primitive for 
> Ornithodira, it does not rule out quadrupedal posture being primitive for
> pterosaurs.  As you say, some ornithodoran clades returned to quadrupedality 
> ("silesaurs", and multiple dinosaurian lineages), and
> this could have happened in ancestral pterosaurs too, on the way to evolving 
> powered flight.

Yes, but I left quadrupedal dinosaurs aside of the equation because
all them are relatively derived (I mean, Sauropodomorpha more derived
than the ex-Thecodontosaurus, thyreophorans more derived than
Lesothosaurus, etc.). The more primitive dinos seem to have been
basically biped.