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Re: Dinosaur Dreaming report 2003




David Marjanovic wrote:
> 
> > This specimen indicates that the lower jaw of
> > Teinolophos was comprised of four bones; the dentary, and facets that
> > indicate three additional bones that had been reduced to splints.
> 
> What could those be? The coronoid, the splenial and the ossified Meckel's
> cartilage (called mentomandibular in frogs)? Or [shudder] are they in the
> back of the jaw, which would indicate that the mammalian middle ear evolved
> twice?

Here's a quote verbatim:

--
In the immediate ancestors of the mammals, there was not just one bone
(the dentary) in the lower jaw as is true of all living mammals, but
seven. In becoming mammals, some of these additional bones were simply
lost while others were incorporated into the structure of the middle
ear.

What this specimen of T. trusleri shows is evidence for three additional
bones in the jaw besides the dentary. The evidence consists of facets on
the dentary where these additional bones were lodged in life.

This is the first monotreme that shows evidence for the presence of any
bone in the lower jaw besides the dentary. One of the three additional
bones became part of the middle ear of all the living mammals. This
means that the monotremes must have split apart from the common stock of
marsupials and placentals at a primitive stage where the transition in
function of this bone had not taken place.
--

Or in other words: either the middle ear of monotremes and other mammals
developed along similar lines independantly (assuming that monotremes
have the same structure of inner ear bones); or surely it's possible
that Teinolophos represents a dead-end branch of monotremes that
retained primitive charactersitics long after other monotreme species
had already given rise to the placental/marsupial line? Just because
it's a monotreme doesn't mean it is ancestral to modern monotremes (or
placentals/marsupials for that matter).

> > periglacial (or cryoturbational) periods
> > [...] longer and slightly warmer intervening periods
> 
> Wow! Glacial cycles in the Mesozoic! -- This sounds like the temporal
> resolution of the sediments is very fine.

I'm not sure they are actual glacial cycles - cryoturbational layers
are being refered to as 'periglacial structures'. Cryo layers seem to be
interspersed with layers that do not contain evidence of cryoturbation
(ie. perhaps warmer phases before and after). This may indicate slight
fluctuations in climate (as you would expect), from 'not quite cool
enough' for cryoturbation to take place, to 'just cold enough'. Or it
may not - cryoturbation may have occured pretty much all the time, but
that doesn't mean that cryo structures will always be preserved.
Changes in the surrounding terrain may also prevent cryoturbation
from occuring at various times (I'm guessing forest environments are a 
few degrees warmer than open plains), even if the average climate
remained similar.

The slightly warmer Kilcunda Cliff layers (with their slightly higher
biodiversity of pollen/spores) seem to about the same age, and in about
the same location, as Flat Rocks (which is surrounded by many cryo 
layers). Given the temporal resolution of strata of this age, they could 
be anything from roughly contemporaneous, to a few million years apart. 
All this research suggests is that floral diversity was greater in
warmer  areas (or times) than where and/or when it was colder - not 
exactly  unexpected results. Of course, this is just the biodiversity 
of pollen and spores, not of actual plants. Preservational biase may 
be at work (as always).

________________________________________________________________

Dann Pigdon                   Australian Dinosaurs:
GIS / Archaeologist         http://www.geocities.com/dannsdinosaurs
Melbourne, Australia        http://www.alphalink.com.au/~dannj/
________________________________________________________________