Trialestes (was Re: PDF-request: Original description of Lagosuchus and/or Marasuchus)

[Tim Williams <tijawi@yahoo.com>]

David Peters <davidpeters@att.net> wrote:


> What three to five genera preceded Marasuchus in the
> lineage of the Dinosauria?


What David M. said.  The terms "ancestor" and "sister taxon" are not 
interchangeable.


> Testing indicates Turfanosuchus and Trialestes are in there
> for starters. 


Ah yes.  _Trialestes romeri_...


What specimen(s) are you using for _T. romeri_?  Aside from the holotype (PVL 
2561), two specimens (PVL 2559 and PVL 3889) have been referred to _T. 
romeri_.  The holotype (an incomplete skeleton that includes cranial elements) 
and PVL 3889 (an incomplete postcranium) have been assigned to the same taxon 
(_T. romeri_) based on the striking similarity of the forelimb elements.  PVL 
2559 is a partial pes.  All three specimens were found in the same horizon.


However, PVL 3889 appears to come from a dinosaur, based on features such as a 
perforated acetabulum, inturned femoral head, and mesotarsal ankle.  It is 
therefore possible that the holotype comes from a dinosaur too.  Alternatively, 
the _T. romeri_ holotype is a sphenosuchian-grade crocodylomorph, and PVL 3889 
is a dinosaur, and the two just happen to have uncannily similar forelimb 
morphology & proportions (e.g., antebrachium longer than humerus).


So _T. romeri_ must be treated with caution when putting together an OTU.  The 
hypodigm is currently uncertain.


Cheers

Tim