re: Synapsids and Diapsids

[biojfm@vaxc.hofstra.edu (John F. Morrissey)]

Regarding synapsids and diapsids and holes in the head:

The confusion begins with the word "holes."  These aren't holes, they are
"dents" in the sides of the skull.  Turtles (anapsids) have none, synapsid
reptiles and us mammals have one dent on each side of our skulls, and
diapsids have two per side.  The presence of our one pair of lateral
depressions (technically termed temporal fossae; a fossa is a depression in
a bone) results in an arch of bone remaining where our profile used to end
(prior to the development of the dent).  This arch is made of processes
from two bones (the zygomatic and temporal = jugal and squamosal of
reptiles) and is called the zygomatic arch (= "cheek bone").  Therefore, us
critters with one lateral fossa (and thus one arch) are called synapsids
(meaning "with an arch").  The dentless turtles are anapsids (= "without an
arch") and the double-dented reptiles are diapsids (= "two arches," their
second arch being made of processes from the postorbital and squamosal). 
So, those terms refer to the bony arches that remain after the temporal
region of the skull invaginates, either once or twice, not the fossae
themselves.

By the way, I thought that the K in "K/T" was from "kreta" (meaning
"chalk," and referring to the planktonic ooze common at the boundary). 
Other subscribers have suggested different sources for the K (= different
words that mean chalk).  Which is correct?

Thanks for the interesting discussions.


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John F. Morrissey
Assistant Professor
Department of Biology
114 Hofstra University
Hempstead, NY 11550-1090
516-463-5517
FAX: 516-565-0098
E-mail: biojfm@vaxc.hofstra.edu
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