The Skull and Visceral Skeleton

1. The vertebrate skull consists of 3 parts: neurocranium,  dermatocranium, and splanchnocranium.

2. Classification:

                Palatoquadrate cartilage and replacement bones

                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.

                     Cyclostomes: individual components remain more or less independent throughout life.

                     Cartilaginous fish: components unite to form an adult chondrocranium which encloses the brain.

                      Lower bony fish: a cartilaginous neurocranium persists in adult chondrosteans and holosteans.
                      However, this cartilage is overlain with dermal bone.

                            Occipital centers - as many as four areas around the foramen magnum (2 exoccipitals, basioccipital,
                            and supraoccipital)

                             Sphenoid centers - under the midbrain and pituitary gland include the basisphenoid and presphenoid.
                             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, eg. the
                              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.

                In crossopterygeans and primative tetrapods, these membrane bones included a parasphenoid
                and paired volmers, palatines, pterygoids, and ectopterygoids.

                 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.