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early tetrapod
modern reptile
Modern Lepidosaurs (Reptiles) In Comparison to Synapsids/mammals:
Jaw joint bones (articular and quadrate, + some others) shown in green. Note how
close the squamosal (upper jaw) and dentary (lower jaw) are to the quadrate and
articular (respectively) of the jaw joint. In mammals, the dentary and squamosal
replace the articular and squamosal as the jaw joint bones, freeing the ancestral
joint bones to become incorporated into the middle ear hearing mechanism.
Likewise, note position of angular (develops reflected lamina in synapsids and
becomes ectotympanic bone of modern mammals). [angular = ectotympanic;
articular = malleus; quadrate = incus; columella = stapes]
Modern Birds:
green = quadrate and articular of jaw joint. Although birds are highly
derived reptiles, they retain the same basic jaw joint anatomy as in the lizard (previous slide).
From: Rowe, T. (1996) Coevolution of the Mammalian Middle Ear and Neocortex. Science 273:651-654.
Ontogeny
recapitulates
phylogeny in
mammal jaw/brain
development. In
other words,
embryonic
development of an
opossum jaw/brain
passes through
stages very similar
to the evolution of
the mammalian
jaw/brain. Brain
expansion seems
to account for the
transition of jaw
elements to
association with
the otic (ear)
region of the skull!
From: Rowe, T. (1996) Coevolution of the
Mammalian Middle Ear and Neocortex. Science
273:651-654.
D
E
V
E
L
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P
M
E
N
T
modern mammal
time
E
V
O
L
U
T
I
O
N
early therapsid
MEDIAL VIEW
LATERAL VIEW
EVOLUTION OF THE
LOWER JAW, JAW
JOINT AND MIDDLE
EAR IN SYNAPSIDS
TIME
EVOLUTION OF THE
JAW JOINT BONES
AND MIDDLE EAR
OSSICLES IN
SYNAPSIDS
angular
Evolution of the Masseter Muscle in
Synapsids/Mammals: just one of the
things that happened that helped to reorient forces on the jaws so that jaw-joint
reaction forces were reduced, allowing jawjoint bones to become smaller and smaller
to increase sensitivity to airborne sound
waves, including higher frequency sounds.
Evolution of the Masseter Muscle in Synapsids/Mammals seen in
Transverse Section Through the Jaw-Closing (Adductor) Muscles
EVOLUTION OF THE SECONDARY PALATE IN MAMMALS