A review: Whatís a synapsid?
Synapsids are amniotes with one temporal fenestra
The only living synapsids are mammals. . .
. . . but between about 300 and 208 million years ago, synapsids were much more diverse than they are today.
The transition from ìreptileî to mammal is well documented, and itís informative about why modern mammals are the way they are. . .