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Re: All those apsids

>Hi everyone.  Pardon my ignorance I'm only an undergraduate.  But I'm 
>VERY excited about dinosaurs and willing to learn.  I have question that 
>may-be someone can help me out with.  In my historical calss we learned 
>about he diffrent types of holes in reptile skulls.  Anapsid have no hole.
>They are the turtles.  Synapsids have one and are the mammal like reptiles.
>Diapsids have two they are the dino.  But then my professor mentioned 
>threapsids and said they were the mammal like reptiles.  I don't know 
>what's right.  HELP!!!  I'm confused.  Who's who and how many holes do 
>they have?  Thanks. 

I'm not sure that this classification by the number and position of skull
foramina is still accepted; I have the feeling that it has been modified
since my student days.  But Therapsids are actually a subgroup of Synapsids,
and have the synapsid condition (as do we, of course, as therapsid
descendants, though reduction and modification of the skull roof has
proceeded to such a degree that the "hole" has now taken over so much of the
side of the skull that, paradoxically, it looks like it isn't there at all!
(In fact many reptiles, such as snakes, have such highly modified skulls
that the textbook pattern is very difficult to discern).

Traditionally the main groups were:

Anapsida: no foramen, including not just turtles but all of what were once
called the "stem" reptiles.  Today I think we would call this a
"paraphyletic" group (?) as, in effect, it includes all reptilian lineages
exhibiting the "primitive" anapsid condition

Diapsida: both an "upper" and "lower" foramen, including the archosaurs and
the lineage leading to lizards, tuataras and snakes (thus all living
reptiles except turtles, and all birds (though again you might not guess
this looking at a bird skull)

Parapsida: only the "upper" foramen, including plesiosaurs, ichthyosaurs and
some other marine groups (placodonts, mesosaurs and nothosaurs, as I recall)
- this group, I think, is now regarded as composed of unrelated forms and
thus not a true lineage, but I stand to be corrected on this)

Synapsida: only the "lower" foramen.  This is not just the "mammal-like"
reptiles, which are the true therapsids; it also includes such things as the
pelycosaurs (eg Dimetrodon).

I've done this from memory, so if I've goofed feel free to correct me!

Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
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