synapsids vs. diapsids?

[George Engelmann <engelman@cwis.unomaha.edu>]

To Sean. The "holes" referred to are the temporal fenestrae, of which the 
synapsids have one on each side (being bilaterally symmetrical) and diapsids
have two one each side (at least primitively). All are posterior to the orbit
and represent an innovation in comparison with the skulls of "amphibian" 
ancestors in which the space from which the temporal musculature of the jaws
originate is completely enclosed by the dermal bones of the skull. The single 
opening on each side of the skull of synapsids is in an anatomically similar 
position to the lower of the two openings on each side of the diapsid skull. 
the functional significance of the openings may be similar in both cases but may
have more to do with lines of action of the muscles and stresses or the lack 
thereof on the supporting skeletal elements than with the bulging of muscles 
under 
contraction. Possibly temporal fenestrae are a response to increasing 
complexity of jaw function and musculature with the specific adaptation
varying from lineage to lineage. It seems unlikely to me that this difference
played a major role in preadaptation for increased brain size as the relatively
large-brained birds managed to evolve within the diapsid lineage. The 
particular adaptive path followed by some synapsid lineages, possibly driven 
by specialization of jaw function among other things did lead to the skull 
architecture of mammals (changes in jaw structure and function are also
responsible for incorporation of some of the bones other vertebrates use to 
make the jaw work into the ears of mammals.), but greatly increased cranial
capacity is a rather late development on this line. Unfortunately, I missed 
that Paleoworld so I don't know what was said. I hope this helps.
GFE