The Vertebrate Skull consists of: 

  1 - neurocranium (also called endocranium or primary braincase)

  2 - dermatocranium (membrane bones)

  3 - splanchnocranium (or visceral skeleton)
 

Neurocranium:

1 - protects the brain

2 - begins as cartilage that is partly or entirely replaced by bone (except in cartilaginous fishes)
 

•  Cartilaginous stage:
• neurocranium begins as pair of parachordal & prechordal cartilages below the brain
• parachordal cartilages expand & join; along with the notochord from the basal plate
• prechordal cartilages expand & join to form an ethmoid plate
• Cartilage also appears in the
• olfactory capsule (partially surrounding the olfactory epithelium)
• otic capsule (surrounds inner ear & also develops into sclera of the eyeball)

• Completion of floor, walls, & roof:
• Ethmoid plate - fuses with olfactory capsules
• Basal plate - fuses with otic capsules
• Further development of cartilaginous neurocranium = development of cartilaginous walls (sides of braincase) &, in cartilaginous fishes, a cartilaginous roof over the brain

Bony fishes, lungfishes, & most ganoids - retain highly cartilaginous neurocranium that is covered by membrane bone

Cyclostomes - the several cartilaginous components of the embryonic neurocranium remain in adults as more or less independent cartilages

Other bony vertebrates - embryonic cartilaginous neurocranium is largely replaced by replacement bone (the process of endochondral ossification occurs almost simultaneously at several ossification centers)
 

Neurocranial ossification centers: 

1 - occipital centers

• cartilage surrounding the foramen magnum may be replaced by as many as four bones:
• basioccipital
• exoccipital (2)
• supraoccipital
• Mammals - all 4 occipital elements typically fuse to form a single occipital bone (pictured below)
• Tetrapods - neurocranium articulates with the 1st vertebra via 1 (reptiles and birds) or 2 (amphibians and mammals) occipital condyles (see human skull below)

 

2 - Sphenoid centers form:

• basisphenoid bone (anterior to basioccipital)
• presphenoid bone
• side walls above basisphenoid & presphenoid form:
• orbitosphenoid
• pleurosphenoid
• alisphenoid

3 - Ethmoid centers tend to remain cartilaginous & form

• anterior to sphenoid
• cribiform plate of ethmoid & several conchae (or ethmoturbinal bones)

The ethmoid region is clearly visible within the bisected skull above. In most mammals, the nasal chamber
is large & filled with ridges from the ethmoid bones called the turbinals or ethmoturbinals. These bones are covered with
                  olfactory epithelium in life and serve to increase the surface area for olfaction (i.e., a more acute sense of smell).
                  Another ethmoid bone, the cribiform plate, separates the nasal chamber from the brain cavity within the skull.

4 - Otic centers - the cartilaginous otic capsule is replaced in lower vertebrates by several bones:

• prootic
• opisthotic
• epiotic

 
• One or more of these may unite with adjacent replacement or membrane bones:
• Frogs & most reptiles - opisthotics fuse with exoccipitals
• Birds & mammals - prootic, opisthotic, & epiotic unite to form a single petrosal bone; the petrosal, in turn, sometimes fuses with the squamosal to form the temporal bone

DERMATOCRANIUM - lies superficial to neurocranium & forms:

1 - bones that form the roof of the brain & contribute to the lateral walls of the skull

2 - bones of the upper jaw

3 - bones of the palate(s)

4 - opercular bones

 Basic pattern of "roofing bones":

• crossopterygians - a series of paired & unpaired bones along mid-dorsal line of skull (below left)
• labyrinthodonts - unpaired bones lost & a series of paired bones resulted (nasals, frontals, parietals, & dermoccipitals) (below right)
• Fontanels = 'soft spots'
• occur when neurocranium is incomplete dorsally (e.g., teleosts & tetrapods)
• can be felt in head until the membranes under the skin have ossified

Bones of the upper jaw

• Pterygoquadrate (palatoquadrate) cartilage - 1st upper jaw that vertebrate embryos develop
• Cartilaginous fishes - palatoquadrate is the only upper jaw that develops
• Bony vertebrates - the palatoquadrate becomes covered with dermal bones (premaxillae & maxillae) that make up the adult upper jaw

 

• Sharks - cartilaginous
• bony vertebrates - membrane bones form
• Birds, mammals, & some reptiles - a secondary ('false') palate develops creating a horizontal partition that separates the oral cavity into nasal & oral passages. The secondary palate is formed from processes of the premaxillae, maxillae, and palatines.

Opercular bones

• Operculum = fold of the hyoid arch that extends back over the gill slits in holocephalans & bony fishes
• Tetrapods - no vestiges of opercular bones remain

Back to Skeletal System I

To Skeletal System III

Related links:

Skulls and jaws

Back to BIO 342 Syllabus