The ancestral synapsid (mammal-like
reptile) jaw has four elements, with the articulation formed between the
Articular
and Quadrate bones, which is therefore referred to as a Quadro-Articular jaw. In therapsids
(the ancestors of mammals), the joint has been simplified to an articulation
between the Dentary and Squamosal bones, which is called a Dentary-Squamosal
jaw. The Articular and Quadrate bones have moved into the middle ear as
the Malleus and Incus, respectively. The squamosal also contributes
to the zygomatic arch, which is the mammalian cheek bone.
Text material © 2005 by Steven
M. Carr