Re: Pelicosaurs: sprawling or not?

["Steve Brusatte" <dinoland@lycos.com>]

On Tue, 16 Apr 2002 11:39:37  
 David Marjanovic wrote:
>> I was under the impression that even monotremes have a semi-sprawling
>posture and that, among
>> synapsids, fully erect stance is seen only in therians. Would like to know
>if this is wrong.
>
>Multis and triconodonts (those few that are well enough known, that is...)
>sprawled likewise, and are commonly thought to be closer to Theria than to
>Monotremata. *Morganucodon* is also reconstructed as sprawling. So I think
>you're right. But I don't know how much mobility, maybe enough for a
>semi-erect stance, was possible in these animals.

Yes, Mike and David are correct here (AFAIK).  Monotremes do have somewhat of a 
sprawling gait, as likely did multis.  According to some things I have read, 
the initial shift from a true sprawling posture to a more upright posture is 
seen in the first therapsids.  Cynodonts later lost their lumbar ribs, which 
added to the effect.  However, only therians truly refined this entire system.

I've read Tracy's article (published in the October/November 2001 issue of 
Prehistoric Times). He offers two lines of support for a "more erect gait and 
not the more typical illustrated sprawling one" in pelycosaurs: footprint 
evidence and the position/size of the olecranon.  Others on the list are 
probably better off to answer what significance the olecranon has.  However, I 
do know the footprint evidence upon which Tracy based his hypothesis are some 
trackways of _Dimetropodus_, which show the prints closely placed next to one 
another.  I haven't seen the tracks myself, but, just to keep it "real:" 
obviously, we don't know if _Dimetrodon_ or any other pelycosaur actually made 
the _Dimetropodus_ tracks.  

Of course, there is also the possibility that some pelycosaurs could have 
evolved a more erect gait independent of the lineage that lead directly to 
mammals.  I doubt there is enough evidence to test this yet, though.

Tracy's hypothesis is interesting.  Hopefully we can illuminate it with new 
fossil material in the future!

Steve  

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