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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. 
>
>Bite Is might

(I like that!) :-)

First: the temporal fenestrae are the openings behind the eyesocket, and are
associated with the jaw muscles. Fenestra is Latin for "window", by the way
(as in defenestration).

The lateral temporal fenestra (or sometimes, infratemporal fenestra) is the
one of the side of the skull; the supratemporal fenestra is on top of the
skull.

The different groups are named according to the presence or absence of
these features, as well as their development.

Now, the "apsids"!
Synapsida ("fused arch", so called because they were erroneously thought to be
descended from diapsids) have only the lateral temporal fenestra present. In
modern systematics, Synapsida is defined as mammals and all animals closer to
mammals than to reptiles (defined below). The Therapsida is the group of
advanced synapsids, and now includes the mammals. In most therapsids, the back
of the skull is transformed so that the base of the lateral temporal fenestra
is pulled outwards, becoming the cheek bones. The whole fenestra is aimed
upwards, then.

[Some people use "Theropsida", or Mammal Faces, for Synapsida.]

Reptilia is now defined as the most recent common ancestor of turtles,
lepidosaurs (tuatara, lizards, and snakes), and archosaurs (crocs & birds),
and all descendants of that ancestor.  Sauropsida ("Lizard Faces") are
reptiles and all taxa closer to reptiles than to mammals.

Within Reptilia are two major divisions.  The Anaspida ("no arches", turtles
and extinct relatives) are notable for lacking either type of temporal
fenestrae, but this condition is also present in more primitive sauropsids.
Anapsids are recongized on derived features of the skull, vertebrae, and limbs.

Diapsida ("two arches) have both the lateral and supratemporal fenestrae.
These include tuataras, dinosaurs, crocs, etc.  Some diapsids (lizards
(including snakes), birds, plesiosaurs, ichthyosaurs, etc.) lose one or the
other fenestra (by losing the arch beneath it).

Hope this helps.  A drawing would probably be much more useful, though...

Thomas R. Holtz, Jr.
Vertebrate Paleontologist
Dept. of Geology
University of Maryland
College Park, MD  20742
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Fax: 301-314-9661
Phone:301-405-4084