The Skull and Visceral SkeletonWhat is a skull?

Morphologists may refer to…

Neurocranium

Dermatocranium

Splanchnocranium

We will consider the “skull” as the “skull” minus the jaws, which are derived from the splanchnocranium

The Skull and Visceral SkeletonNeurocranium (aka enodcranium, chondrocranium, primary braincase):

* Protects the brain

* Endochondral ossification

The Skull and Visceral SkeletonNeurocranium origins:

Mesenchyme from neural crest cells and mesoderm

Cartilages… *Para and Pre-chordal

*Otic, Olfactory, Optic

“nose” “ears” and “eyes”

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

The Skull and Visceral Skeleton

The Skull and Visceral SkeletonNeurocranial 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?

The Skull and Visceral SkeletonNeurocranial 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

The Skull and Visceral Skeleton

http://www.theodora.com/anatomy/images/image153.gif

Neurocranial ossification centers:

*Ethmoid has greatest tendencies to remain cartilage in tetrapods.

Comprised of ethmoid plate and olfactory capsules

In humans… *Interorbital

*cribiform plate, cristi galli, conchae, and nasal cartilages

The Skull and Visceral SkeletonNeurocranial ossification centers:

*Otic bones surrounding the membranous labyrinth can fuse with occipitals or squamous (as in humans) to form the temporal bone

http://embryology.med.unsw.edu.au/wwwhuman/Hum12wk/Hum12wk.htm#12weekHead

Dermatocranium

Derived from dermal bone… however, modern tetrapods exhibit mesenchyme migration and subdermal development

4 basic structures…

The Skull and Visceral Skeleton

1) Above/alongside brain, 2) upper jaw margin, 3) primary palate and 4) opercular bones

The Skull and Visceral SkeletonDermatocranium:

Palatoquadrate (cartilaginous fishes) is replaced by maxilla and premaxilla that fuses with other dermatocranial elements

Vomer(s), palatines and pterygoids part of primary palate

The Skull and Visceral SkeletonDermatocranium:

Operculars… bones that cover and protect the delicate gill structures of bony fish

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

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?

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

The Skull and Visceral Skeleton

Secondary palate:

Development of secondary palate makes internal nares positioned caudad

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

The Skull and Visceral Skeleton

Why?

Birds:

Modified reptile skull in 2 functional regions…1) brain box2) food getter

Light weight dermal bones and bigger space for large brain

The Skull and Visceral Skeleton

Mammals:

Key features are single dentary (mandible), modified secondary palate and 3 ossicles

The Skull and Visceral Skeleton

The Skull and Visceral Skeleton

Visceral skull:Derived from pharyngeal arches… in fishes becomes the jaws and gill arches

Origins are neural crest cells

Meckel’s cartilage behind the palatoquadrate (jaw) is homologous to the ossicles in mammals.

The Skull and Visceral Skeleton

The Skull and Visceral Skeleton

Articular cartilage becomes one of the 3 ossicles in mammals…

The Skull and Visceral Skeleton

Meckel’s cartilage becomes one of the 3 ossicles in mammals…

The columella (or stapes) is present in basal groups.

The Skull and Visceral Skeleton

Other pharyngeal arches become portions of:1) Styloid process2) Hyoid bone3) Thyroid cartilage4) Cricoid cartilage