1. The vertebrate skull consists of 3 parts: neurocranium, dermatocranium, and splanchnocranium.
Meckel's cartilage and replacement bone
Skeleton of the branchial arches
3. Neurocranium: primary braincase that (1) protects brain, (2) arises
as cartilage, which (3) is replaced by bone
(except in cartilaginous fish). Similar development occurs in all vertebrates.
Cartilaginous fish: components unite to form an adult chondrocranium which encloses the brain.
Lower bony fish: a cartilaginous neurocranium persists in adult chondrosteans
However, this cartilage is overlain with dermal bone.
Sphenoid centers - under the midbrain and pituitary gland include the basisphenoid
These together with lateral sphenoid elements (orbitosphenoid, pleurosphenoid) form
the adult sphenoid bone.
Ethmoid centers - anterior to sphenoid. Form the ethmoid plate
and olfactory capsules. Ethmoid tends to
remain cartilaginous even in mammals. Cribiform plate allows passage of olfactory nerves
to the olfactory epithelium.
Otic centers - several bones form here and may be replaced or fuse together,
prootics, opisthotic, and epiotics unite to form the petrosal bone which then
fuses with the squamosal bone to form a temporal bone.
4. Dermatocranium: these are the membrane bones of the skull and
may have originated in the bony dermal armor
of the ostracoderms.
The primary palate is still present in modern tetrapods as the roof of
the nasal cavity.
The oral and nasal cavities are divided by a secondary palate.
Skull and Visceral Skeleton II
Neurocranial-Dermatocranial complex of bony fish:
1. Chondrosteans: this superorder includes the spoonbill and paddlefish. In these animals the neurocranium remains cartilaginous throughout life. Traces of ossification occur in the otic capsules and in that portion of the sphenoid that contributes to the orbit of the eye. Dermal bones may obscure the neurocranium.
2. Holosteans: Bowfin and garfish have skulls similar to the chondrosteans with the neurocrania remaining mostly cartilaginous. Most obvious are the dermal bones which are sculptured to correspond to the underside of the dermis.
3. Teleosts: modern teleosts show skulls which are highly specialized and diverse, corresponding to the diverse feeding habits of this group. Bones associated with the jaws of a typical teleost include the maxillae, premaxillae, dentary, articular, quadrate and symplectic. Common roofing bones are the frontal, parietal, supraoccipital, and posttemporal.
4. Dipnoans: lungfish have similarities to all of the previous groups and yet show obvious differences. Typically the dipnoan skull is more conservative. The dermatocranium has evolved into only a few bony plates while the neurocranium remains cartilaginous.
The Neurocranial-Dermatocranial complex of modern tetrapods
1. Amphibians: neurocranium incomplete dorsally and largely cartilaginous. Articulating with the otic capsule is the columella which conducts sound from the eardrum to the capsule (comes from the hyomandibula). Dermatocranium lacks the bones that surround the orbit except for the lacrimal and prefrontal. Temporal bones are also missing or reduced. In the otic region, only the squamosal and quadrojugal remain. The primary palate has been altered to accomodate the eyes.
2. Reptiles: living orders show a well ossified neurocranium with a single occipital condyle and a larger number of membrane bones than amphibians. Many possess a parietal foramen, temporal fossae, and a complete secondary palate.
Temporal fossae: openings in the temporal region of amniotes bounded by one or more bony arches. Early stem reptiles had none (anapsid), which is also the condition in modern turtles. The synapsid condition involves a temporal fossa bounded by postorbital, squamosal, and jugal bones; today this is the zygomatic arch of the mammalian skull. The diapsid skull was characteristic of ancestral snakes and lizards. Extant snakes and lizards, have modified diapsid skulls.
Secondary palates: appear first in reptiles as a horizontal partition that divides the oral cavity into oral and nasal passages. In crocodilians, palatal processes of the premaxillae, maxillae, palatine, and pterygoid bones meet in the midline to form a secondary palate. In mammals, the premaxillae (not in humans), maxillae, and palatine bones form the secondary palate.
Cranial kinesis: independent movement of one or more parts of the neurocranial-dermatocranial complex. In the case of lizards, the quadrate, upper jaw, orbital bones, and the parietal bone may move as a unit, independent of the braincase.
3. Birds: similar to reptilian skull with modifications for flight and feeding. Some roofing bones lost; dermal bones reduced.
4. Mammals: here the dentary bone becomes the sole bone of the
lower jaw. Neurocranium incomplete with fontanels in newborns.
Bregmatic bones may ossify in the frontal fontanele of some species (a single bone is sometimes found in humans). Ossification centers in the neurocranium are similar to those previously described. Dermatocranium represented by pairs of premaxillae, maxillae, jugals, nasals, lacrimals, and squamosals. Frontals, parietals, and interparietals complete the series.
The Visceral Skeleton:
The splanchnocranium is the skeleton of the pharyngeal arches in fish (jaws and gill arches) and has given rise to some very interesting structural components in mammals.
1. Sharks: visceral skeleton consists of cartilage in each arch
as well as median basihyal and basibranchial cartilages in the floor
of the pharynx. First arch modified for feeding as the mandibular
arch. Consists of the palatoquadrate and Meckel's cartilages. The
second, or hyoid arch, components include the hyomandibular (dorsally)
and ceratohyal (lateral) cartilage. Articulation of the palatoquadrate
and Meckel's cartilages includes the hyomandibular in a movable joint.
The hyomandibula is bound by ligaments to the otic capsule and
thus suspends the jaws from the neurocranium: hyostylic jaw suspension.
Amphystylic attachment, where the palatoquadrate is attached at several
locations to the neurocranium, is seen in some ancient sharks. Autostylic
attachment occurs when the palatoquadrate is fused to the neurocranium.
2. Bony fish: embryonic cartilage is ensheathed by membrane bone.
Palatoquadrate is overgrown by premaxillae and maxillae. Palatal region replaced by palatine and ectopterygoids while the posterior tip ossifies to form the quadrate bone. The caudal end of Meckel's cartilage forms the articular bone, while the remainder forms the dentary, surangular, and angular bones. Hyoid cartilages form symplectic, interhyals, and epihyals. Articulation of the jaw may involve the symplectic and quadrate, or symplectic, quadrate, and lower jaw.
3. Tetrapods: modifications of visceral skeleton correspond with
adaptational changes for terrestrial life. Palatoquadrate and Meckel's
cartilage become ensheathed by dermal bones: premaxillae, maxillae,
and palatal bones (palatoquadrate); quadrate becomes site of articulation
with lower jaw in tetrapods below mammals (becomes incus in mammals);
Dentary, angular, surangular, splenial, coronoids, prearticulars,
and articulars form in Meckel's (articular bone articulates with
quadrate, except mammals where it forms the malleus). A new articulation
formed between the dentary bone and the squamosal, now known as the
temporomandibular joint. Note that mammals have only the dentary
bone forming the lower jaw.
Remember the the hyomandibula becomes the stapes (columella).
The remainder of the visceral skeleton contributes to the support of the larynx.