modern reptile

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early tetrapod
modern reptile
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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
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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).
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From Rowe, T. (1996) Coevolution of the
Mammalian Middle Ear and Neocortex. Science
273651-654.
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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
273651-654.
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DEVELOPMENT
modern mammal
EVOLUTION
time
early therapsid
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MEDIAL VIEW
LATERAL VIEW
EVOLUTION OF THE LOWER JAW, JAW JOINT AND MIDDLE
EAR IN SYNAPSIDS
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TIME
EVOLUTION OF THE JAW JOINT BONES AND MIDDLE EAR
OSSICLES IN SYNAPSIDS
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Evolution of the Masseter Muscle in
Synapsids/Mammals just one of the things that
happened that helped to re-orient forces on the
jaws so that jaw-joint reaction forces were
reduced, allowing jaw-joint bones to become
smaller and smaller to increase sensitivity to
airborne sound waves, including higher frequency
sounds.
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Evolution of the Masseter Muscle in
Synapsids/Mammals seen in Transverse Section
Through the Jaw-Closing (Adductor) Muscles
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EVOLUTION OF THE SECONDARY PALATE IN MAMMALS