RE: for David M.: Scleromochlus ankles + Brusatte et al. supertree

[Tim Williams <twilliams_alpha@hotmail.com>]

David Peters wrote:


> Everyone has known for a long, long, time, pterosaurs don't quack
> like an archosaur.


Everyone except me, apparently.  I thought pterosaurs can quack like an 
archosaur, and (more importantly) can walk like an archosaur - especially 
ornithodirans.  


> To quote an esteemed expert on pterosaurs, Chris Bennett 1996 (and
> also quoted by Hone and Benton 2007): "“… few characters can be found
> that unite [pterosaurs] with any other clade among the archosaurs.“


Be aware that Bennett (1996) actually *excluded* hindlimb characters from his 
analysis.  Given the important role accorded to hindlimb characters in 
archosaur evolution (especially those pertaining to the ankle), this might be 
viewed as a little unfair. 


Bennett's rationale was that hindlimb characters were associated with bipedal 
digitigrade locomotion, and were therefore homoplastic between dinosaurs and 
pterosaurs.  Given that this was an untested *a priori* assumption, the 
exclusion of hindlimb characters makes this study less esteemed than it might 
otherwise have been.  Moreover, it's unsurprising that "few characters" were 
found to unite pterosaurs with other dinosaurs, given that many/most potential 
synapomorphies were chucked out before the analysis was even run.  


> To quote another esteemed expert on pterosaurs, David Unwin 2004, "
> But there are difficulties with this suggestion [that Scleromochlus
> is the closest known relative of pterosaurs] and with the more
> general idea that pterosaurs are ornithodirans."


I don't think these difficulties (should they exist) are insurmountable.  Also 
it's worth noting that pterosaurs can be ornithodirans without first 
establishing a _Scleromochlus_-Pterosauria sister group relationship.


> Now would be a good time to listen to what Chris and David say.


It might be worth giving Kellner a hearing too, lest you be accused of 
cherry-picking quotes.  Says Kellner (2004):


"Assuming that the PAJ [pterosaur ankle joint] is representative of 
Pterosauria, it adds support to a close relationship of this clade of volant 
reptiles and Dinosauromorpha. The lack of an astragalar posterior groove and 
the reduction of the calcaneal tuber (absent in Pterosauria) are some of the 
features shared among members of both clades." 


This is based on examination of the well-preserved ankle-joint of two 
pterodactyloid species.  As Kellner argues, there's no reason that this tarsal 
morphology (PAJ) isn't typical of pterosaurs, including basal forms which have 
ankles that conform to PAJ, although the details are often absent or poorly 
preserved.


Cheers

Tim

_________________________________________________________________
See how Windows Mobile brings your life together—at home, work, or on the go.
http://clk.atdmt.com/MRT/go/msnnkwxp1020093182mrt/direct/01/