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Re: First Mammal



 Jeff Poling wrote:
> > 
> >    Anybody happen to know if the oldest true mammal was a semi/sprawler?
> > 

Jonathan Woolf wrote:
> Last I knew, _Morganucodon_ was supposed to be the oldest mammal with
> good postcranial remains.  My references include a couple of
> restorations of it as a semi-sprawler, and also a couple of restorations
> of it with an upright posture something like a modern shrew.  I suppose
> you pays your money and you takes your best guess. <G>
> 
> -- JSW

The only Late Triassic/Early Jurassic true mammals from which more or 
less adequate postcrania (yes, here we go again) are known AFAIK are 
Morganucodon/Eozostrodon from the British fissure infillings and 
Megazostrodon from the South African Upper Elliot Formation.
They were studied by Jenkins and Parrington (publication in the  Phil 
Trans Roy Soc as far back as in 1976) who concluded that the limb
position was more or less similar as in extant small mammals as 
shrews; except that the front limbs were still somewhat more 
sprawling, reminiscent of the primitive condition in non-mammalian 
cynodonts. The same slightly sprawling stance of the forelimbs is also 
observed in Monotremes (but which are rather specialized mammals). 
The oldest well-studied (in terms of postcrania) Therian mammals 
(such as Zalambdalestes) show the fully developed mammalian stance: 
four limbs tucked in under the body.
Earlier reports of Multituberculates mentioned a rather primitive 
sprawling position of the forelimbs, but study of a well preserved 
multituberculate pectoral girdle (Bulganbataar I think) by Sereno and 
McKenna revealed they were as advanced as Therian mammals in their 
front limb position. The same authors concluded that 
Multituberculates were closer related to Therian mammals than 
previously thought and that a fully mammalian front limb position 
evolved somewhere before Late Jurassic times.

Remark:  evolution of limb posture in therapsids to the mammalian 
condition went apparently in two times: 
Middle Triassic cynodonts as Cynognathus and Diademodon already
combined upright, quite mammalian hindlimbs with the primitive sprawling
position of the forelimbs, as well as later chiniquodontid and 
gomphodont cynodonts.The most derived non-mammalian cynodonts,
the tritylodonts were in limb position similar to the earliest 
mammals (morganucodontids).

Pieter Depuydt