Re: LATE SURVIVING CYNODONT (lenghty and boring)

[NJPharris@aol.com]

In a message dated 97-06-14 12:36:41 EDT, radass@orion.neca.com (Ron Dass)
writes:


>  >Another example: the transition from the primitive articular-quadrate jaw
>  >hinge to the 
>  >mammalian dentary-squamosal articulation went via a series of 
>  >intermediate forms who possessed a double hinge (Probainognathus, 
>  >Pachygelenus,...)
>  
>  I believe this is the "two part" jaw I referred to in my post. Yes? 
>  Is the posterior part of this double hinge homologous to one of the middle
>  ear bones (hammer, anvil, stapes) in advanced mammals?

OK.  The primitive amniote jaw joint is formed by the articular bone, in the
lower jaw, and the quadrate bone, which attaches to the braincase.  In
pre-mammal evolution, the dentary bone (the bone which holds the teeth and
the mandible of modern mammals) grew up and around the old jaw joint on the
outside until it reached the squamosal bone, where the articulation still is
in modern mammals.  Some protomammals (my favorite of which is
_Diarthrognathus_--"two-jointed jaw") show this double jaw joint:
 articular-quadrate on the inside, dentary-squamosal on the outside.  The
articular and the quadrate then migrated into the mammalian inner ear to
become the hammer and anvil, respectively.

 
>  I think I read somewhere that monotreme means
>  "one hole" and refers to the fact that the platypus has a cloaca rather
than
>  the separate anus and penis/vagina characteristic of other mammals (hope
the 
>  communications decency police don't smash down my door ;-). Is this true?

Yes; however, this is also true of marsupials.

>  And do monotremes excrete uric acid like birds and my geckoes or urea like

> me?

Can't help you there.

>  The answer to that might indicate a broad physiological gulf between 
> monotremes
>  and other mammals, much more so than viviparity.

My questions would involve the very primitive shoulder of the monotremes,
which preserve a coracoid and an interclavicle, both lost, AFAIK, in marsups
and placentals.


>  Incidently, are any fossil monotremes known that are generalist "normal"
>  herbivores and carnivores or are they all highly specialized wierdos
>  like the platypus and echidna. 

Unfortunately, the fossil record of monotremes is almost nonexistent apart
from a jaw from South America, nearly identical to a modern platypus apart
from its teeth (it had them).

NP