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Re: LATE SURVIVING CYNODONT (lenghty and boring)




On Sat, 14 Jun 1997 NJPharris@aol.com wrote:

> In a message dated 97-06-14 12:36:41 EDT, radass@orion.neca.com (Ron Dass)
> writes:
> 
> >  And do monotremes excrete uric acid like birds and my geckoes or urea like
> 
> > me?
> 
> Can't help you there.

As I understand it urea excretion is primitive for amniotes and uric acid 
excretion is a derived feature of the Reptilia (which includes Aves). 
Therefore I would expect monotremes to have the primitive condition of 
urea excretion like other mammals.

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

Actually, despite the egg laying, the embryology of monotremes is more like 
that of other mammals 
in that the egg is retained in the female reproductive tract long after 
fertilisation (the condition has been independantly evolved in some 
squamates). Interestingly because this condition is variable in the 
outgroups to Amniota, wether immediate laying or egg retention is the 
primitive condition for Amniota cannot be determined. Laurin and Reisz (1997)
have advanced the hypothesis that egg retention is primitive for Amniota 
and the membranes of the amniotic egg evolved to facilitate this, rather 
than survival on land. 

> >  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).

Not quite true. Steropodon galmani is a Cretaceous monotreme from 
Australia. It appears to have had less specialised teeth than the 
Cainozoic extinct platypi (of which there are two Australian species 
besides the South American one) Though only known from an incomplete jaw, 
Steropodon doesn't appear to have sported a platypus like bill. Also 
known from the Cretaceous of Australia is the monotreme, Kollikodon 
(informally "Hotcrossbunodon") which has really freaky teeth that appear 
to be designed for crushing hard food items. It isn't closely related to 
modern monotremes, it is an indication that during the Mesozoic australia 
was filled with a diverse array of monotremes (expect more on Australian 
Mesozoic mammals soon). 

Adam Yates