Re: pteros have lift-off

[Tim Williams <tijawi@yahoo.com>]

Augusto Haro wrote:


> David Peters seems to be right to me in that bipedalism
> represents the ancestral state for Pterosauria if they are 
> ornithodirans (the currently more favored hypothesis).  


Following the tree of Brusatte et al. (2008, Science), Pterosauria is the 
sister taxon to Dinosauromorpha.  I agree that being at least facultatively 
bipedal is the ancestral state for Dinosauromorpha, since this is the posture 
inferred for _Marasuchus_ and _Lagerpeton_.  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.   

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. 
 

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.


> By this reason, one can say that quadrupedalism should be
> even more
> suggestive in pterosaurs because of the much longer and
> stouter forelimbs.


You may well be right.  But the earliest known pterosaurs are highly derived 
morphologically, and in the absence of intermediate forms it is not possible to 
be too confident regarding the behavior(s) of pre-flight pterosaurs (or 
pterosauromorphs).  


Cheers

Tim