The Skull and Visceral Skeleton

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The Skull and Visceral Skeleton
What is a skull? Morphologists may refer
to Neurocranium Dermatocranium Splanchnocranium W
e will consider the skull as the skull minus
the jaws, which are derived from the
splanchnocranium
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The Skull and Visceral Skeleton
Neurocranium (aka enodcranium, chondrocranium,
primary braincase) Protects the brain
Endochondral ossification
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The Skull and Visceral Skeleton
Neurocranium origins Mesenchyme from neural
crest cells and mesoderm Cartilages Para and
Pre-chordal Otic, Olfactory, Optic nose ears
and eyes
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The Skull and Visceral Skeleton
Taxonomic variation of neurocrania Agnatha
remain loosely associated, fibrous cover
dorsally Cartilaginous fishes highly developed,
envelopes brain, NO bone Bony fishes remains in
primitive fish below dermatocranium, BUT teleosts
and tetrapods exhibit endochondral ossification
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The Skull and Visceral Skeleton
Neurocranial ossification centers Occipital,
Sphenoid, Ethmoid and Otic Occipital 1-4 bones
and one (reptiles, birds and early amphibians) or
two (modern amphibians and mammals) occipital
condyles What do these articulate with?
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The Skull and Visceral Skeleton
Neurocranial ossification centers Sphenoid in
humans is composite of neurocranium and
palatoquadrate. Remains as separate bones in some
species rather than wings. Sella turcica part of
this bone
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The Skull and Visceral Skeleton
Neurocranial ossification centers Ethmoid has
greatest tendencies to remain cartilage in
tetrapods. Comprised of ethmoid plate and
olfactory capsules In humans Interorbital cri
biform plate, cristi galli, conchae, and nasal
cartilages
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The Skull and Visceral Skeleton
Neurocranial ossification centers Otic bones
surrounding the membranous labyrinth can fuse
with occipitals or squamous (as in humans) to
form the temporal bone
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/Hum12wk.htm12weekHead
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The Skull and Visceral Skeleton
Dermatocranium Derived from dermal bone however,
modern tetrapods exhibit mesenchyme migration and
subdermal development 4 basic structures
1) Above/alongside brain, 2) upper jaw margin, 3)
primary palate and 4) opercular bones
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The Skull and Visceral Skeleton
Dermatocranium Palatoquadrate (cartilaginous
fishes) is replaced by maxilla and premaxilla
that fuses with other dermatocranial
elements Vomer(s), palatines and pterygoids part
of primary palate
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The Skull and Visceral Skeleton
Dermatocranium Operculars bones that cover and
protect the delicate gill structures of bony fish
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The Skull and Visceral Skeleton
Amphibian skulls Considerably modified from
labyrinthodonts, but still platybasic Only
apodans have fully ossified skull Why? Columella
ossified Otic capsule exposed dorsally and
laterally and in anurans large spaces exist
inferiorly too
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The Skull and Visceral Skeleton
Non-avian reptile skulls Stem reptiles similar
to basal amphibians Modern reptiles still have
some ancestral characteristics, namely well
ossified neurocranium and single occipital
condyle Parietal foramen (sphenodon and many
saurians) Why?
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The Skull and Visceral Skeleton
Apsidity Stem reptiles lacked temporal fossae
(An-apsid skull) Ancestors to mammals have a
single fossae (syn-apsid skulls) Modern reptiles
have two fossae (di-apsid skull) Extinct
dinosaurs have single superior fossae (Eury-apsid
skull) that may be convergent evolution
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The Skull and Visceral Skeleton
Secondary palate Development of secondary palate
makes internal nares positioned caudad
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The Skull and Visceral Skeleton
Cranial kinesis Movement of various functional
parts of the skull independent of other parts is
cranial kinesis Teleost, squamates and birds
well adept at this for example fish can move
maxillae and palate independent of the
neurocranium
Why?
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The Skull and Visceral Skeleton
Birds Modified reptile skull in 2 functional
regions1) brain box2) food getter Light weight
dermal bones and bigger space for large brain
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The Skull and Visceral Skeleton
Mammals Key features are single dentary
(mandible), modified secondary palate and 3
ossicles
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The Skull and Visceral Skeleton
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The Skull and Visceral Skeleton
Visceral skull Derived from pharyngeal arches
in fishes becomes the jaws and gill
arches Origins are neural crest cells Meckels
cartilage behind the palatoquadrate (jaw) is
homologous to the ossicles in mammals.
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The Skull and Visceral Skeleton
Articular cartilage becomes one of the 3 ossicles
in mammals
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The Skull and Visceral Skeleton
Meckels cartilage becomes one of the 3 ossicles
in mammals The columella (or stapes) is present
in basal groups.
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The Skull and Visceral Skeleton
Other pharyngeal arches become portions of 1)
Styloid process2) Hyoid bone 3) Thyroid
cartilage 4) Cricoid cartilage