[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: LATE SURVIVING CYNODONT (lenghty and boring)

Ron Dass wrote:
> >  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?
NJPHarris (Nick Pharris?) wrote: 
> 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.

Very right. The stapes we mammals inherited from our pre-Synapsid amniote 
ancestor since this small bone is also present in extant Theropsids, 
lizards and birds. Articular became malleus, quadrate became incus, 
and the other postdentary bone, the  angular with its reflected lamina became 
the tympanic bone, which holds the eardrum.
According to a theory advocated by Allin, the intermediate cynodont 
status with its already reduced postdentary bones already played some 
role in conducting sounds from the eardrum (behind the squamosal) to 
the fenestra ovalis in the braincase (one problem with this theory: 
the high inertia of this intermediate structures only allowed 
conduction of very low frequency sounds).
The evolution towards the mammalian middle ear ossicles was clearly 
related to the acquisition of better high frequency sound perception. 
This could be linked to the evolution to a nocturnal habit, together 
with an  insectivorous diet (insects make high-pitch sounds) (of 
course this is speculation, but plausible).

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

I don't think so. Extant reptiles cannot concentrate their urine more 
then their blood, their urine thus is relative hypotonous. However 
they can excrete their excess nitrogen without losing body water by 
excreting nitrogen as a uric acid paste. Mammals only excrete a minor 
part of their nitrogen as uric acid, they have the ability of 
concentrating their urine to hypertonous by the evolution of the 
Henle loop in their nefron.
> >  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.

Yes. Monotremes also lack the Musculus Supraspinatus which is present 
in all other mammals. Otherwise monotremes are very similar to other 
mammals in their physiology and biology, except from their 
reproductive system. 

> >  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).

The platypus does have some teeth as a juvenile, but it quickly loses 
them. The fossil record of monotremes is very very poor indeed; as far 
as I know off my head, there are only some Miocene teeth and three 
genera from the Cretaceous of Australia (Steropodon, Kallimodon and a 
third one) all based on isolated teeth. However at least one of these 
Cretaceous genera reached the size of a cat, which was respectable 
for a mammal in those times. They probably occupied more habitats 
than nowadays, the platypus and the echidnas being relicts that 
survived by specialisation towards a very peculiar niche.

Pieter Depuydt.