Re: Synapsids vs. Diapsids??

[chartiem@ERE.UMontreal.CA (Chartier Michel)]

> 
> The episode of Paleoworld last night focused on the Dimetrodon, and
> its role as fore-runner of madern warm-blooded animals; interesting
> show.
> What I found confusing though, being new to paleontology, was the
> distinction made between the two groups which the show said split
> from one ancestor: The synapsids, which the show says included 
> protomammals, and all of the current mammals. The other groupw
> was diapsids, which included dinos and reptiles and, I owuld
> guess birds?
Like I said in the previous post, there was no split between synapsids and
diapsids. They do have a common ancestor but only a distant one: this common
ancestor is not exclusive to synapsids and diapsids. Diapsids have a closer
ancestry with chelonians (turtles) than with mammals and their kin.
Diapsids include archosaurs (dinosaurs and kin) and their descendants, and
lepidosauromorphs (lizards, tuataras and snakes and their ancestors). They
do not include reptiles, at least not all of them. For example, turtles 
are reptiles but are classified as anapsids (along with a host of other
primitive and extinct groups).

> The term "diapsid" was defined as meaning "two holes", and they 
> showed a skull of some kinf which had two holes which the jaw 
> muscles apparently squeeze through whan the animal bites. The 
> problem with this was that this method limits the size of the
> brain.
> They then defined "synapsid" as meaning "one hole, and showed
> how this allowed bigger brain development to occur, and all that
> goos stuff. As an example, they showed a human skull, and darned
> if it didn't have a hole on each side of the head... Now one hole
> plus one hole = two holes, right? I found this kind of confusing
> and contradictory, since it seems then that both synapsids and
> diapsids have two holes. Is the difference then in the location
> of the holes? It looked to me like the difference was that 
> synapsids have them on the sides of the head, where they don't 
> get in the way of the brain, whereas diapsids have them at the 
> back of the skull, so that they stretch across the brain. 
> Can someone help clear this up for me? 
> Thanks.
> Sean
> =====================================================================
> |    Sean R. "Snake" Kerns              e-mail: sean.kerns@sdrc.com |  
> |    DoD# 1052   '48 CJ-2A   '79 F-250 4x4 429   '93 750 Virago     |
> |    Structural Dynamics Research Corporation    '79 AQHA           | 
> |    These opinions aren't SDRC's...  They may not even be MINE...  |
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> 
The synapsid condition means that the animal has only one temporal opening
on each side of the skull, for a total of two (hence a single pair of 
openings). The diapsid condition means that the animal has two temporal
openings on each side of the skull, for a total of four (two pairs of
temporal openings). The human skull has indeed two temporal openings, one
on each side of the skull (note that this condition is highly modified
from the one present in early synapsids).
You have to be very careful in assuming that the synapsid condition favored
the enlargement of the brain. The development of temporal openings does not
relate directly to that. The reasons why these openings evolved has to do
with the mechanical stress that the jaw muscles impose on the skull. The
muscles act on particular areas on the sides and top of the skull. Bone is
deposited most thickly along these lines of stress, which create ridges or
crests. Muscles have a stronger attachment on ridges and edges than on
smooth surfaces, so that the concentration of forces will be stronger on
these thickened areas. The thin areas of bone between ridges are very 
sensitive to the stress and can crack, especially when these areas are
crossed by sutures as was the case in early amniotes. The stress imposed
on these weak areas can be dissipated by the development of more-or-less
round openings whose margins serve as attachment sites for the muscles:
you thus eliminate the weak areas by replacing them with openings, resulting
in a better and more effective distribution of mechanical stress. As for
why the numbers of openings differ in both groups, this is still a mystery.
It may be due to the different configuration of the skull in their
specific ancestors. For details, check Dr. Robert Carroll's book,
Vertebrate paleontology and evolution (1988), chapter 10.
The development of a large brain is still difficult to explain in the light
of all this. Just consider that therapsid's brains were no larger than
other reptile's brains. The enlargement of the brain is a relatively recent
trend in mammalian evolution.

Sorry if this was too long

Mike

>