Vertebrate Skeletal Anatomy II - The Skull
Gnathostome braincase development
Context I: Developmental regions
At the most basic level, the initial chondrifications (cartilage formation) of the neurocranium define the anteroposterior regionalization of the skull, with:
- Ethmoid region: Nasal capsules and supporting structures.
- Sphenoid region: Eyes and associated braincase structures derived from trabeculae - the primordial paired rods. In adults, this region supports the eyes and provides passage between the brain and olfactory capsules for the olfactory nerves.
- Otico-occipital region: ("Occipito-otic region" of some authors. Get used to seeing both terms.) Otic capsules and the brain, proper. Derived from primordial otic capsule, parachordal cartilages, and occipital arch cartilages.
But don't forget the spaces in between:
- Between the sphenoid and otico-occipital region:
- The hypophysis (pituitary gland) occupies the recess where the trabeculae and parachordals meet. The small polar cartilages give rise to the pila antotica that form the back of the sella turcica (Turkish saddle) in which it sits. ("st" in linked image.)
- The ventral cranial fissure separating ossifications of the braincase between the sphenoid and otico-occipital sections.
- Between the otic capsules and occipital arch: The lateral occipital fissure.
Major units of the skull are preformed in cartilage. Remember: all cartilages except for those of the otico-occipital region are preformed in migrating neural crest cells. The major units:
- The neurocranium: containing special sense capsules, brain, and supporting structures.(In blue - right)
- The mandibular arch: The paired elements of the jaws. (In yellow - right).
- Palatoquadrates: - upper jaws that articulate with the neurocranium dorsally.
- Meckel's cartilages: that articulate with the palatoquadrates.
- The hyoid arch: The paired elements supporting the jaws (although radically repurposed in land vertebrates!). (In green - right).
- Hyomandibulae: - that articulate with the neurocranium dorsally.
- Ceratohyals: that support the mandibular arch medially.
- Basihyals: Midline element to which both sides of the hyoid arch articulate.
In animals like the shark Clamydoselache these components and their relationships are clear. In creatures like land vertebrates, they are not obvious, but they still provide the key to understanding cranial anatomy. Sources of obfuscation include:
- The integration of these functional units and blurring of their boundaries.
- The presence of superficial dermal elements of the skull.
- The fact that bony elements that ossify from them do not have a one-to-one correspondence with the cartilages from which they form.
We use the example of Greererpeton burckemorani, a creature near the common ancestry of land vertebrates. As such, it displays all of the cranial elements of Sarcopterygii for which homologies are securely known, but very few of them are high transformed from their ancestral state.
We continue to color-code:
- Blue: Ossifications of the neurocranium (dermal or endochodral)
- Yellow: Ossifications of the mandibular arch (dermal or endochodral)
- Green: Ossifications of the hyoid arch (endochodral)
- Pink: Ossifications of the skull roof (dermal)
What immediately distinguishes Greererpeton is that the endochondral elements of its skull roof, jaw, and palate are covered in dermal elements. The images at right show dorsal and ventral views of the skull with jaws removed, and standard abbreviations of visible elements.
In dorsal view:
- f: frontal.
- it: intertemporal.
- j: jugal.
- l: lacrimal.
- m: maxilla.
- n: nasal.
- p: parietal.
- pf: postfrontal.
- pm: premaxilla.
- po: postorbital.
- pp: postparietal.
- prf: prefrontal.
- qj: quadratojugal.
- sq: squamosal.
- st: supratemporal.
- t: tabular.
- Temporal series:
This complexity is easier to address if we break it down into regions:
- d: dentary: the primary tooth-bearing bone. Dentaries meet on the midline in a symphysis.
- sp: splenial: Abutting the dentary ventrally. Sometimes the splenial participates in the symphysis.
- psp: postsplenial: Continuing the splenial series posteriorly.
- an: angular: Forms the posteroventral angle of the jaw.
- sa: surangular: Forms the dorsolateral margin of the jaw posterior to the dentary.
- Adductor fossa: the opening through which the muscles that close the jaw insert onto the inner surfaces o the dermal bones and onto Meckel's cartilage.
- art: articular: Endochondral. Direct ossification of Meckel's cartilage that articulates with the quadrate ossification of the palatoquadrate.
- part: prearticular: Forms the medial margin of the adductor fossa.
- co: coronoids: A series of elements medial to the dentary that often bear teeth as well.
- a fenestra: By odd happenstance, Greererpeton has a fenestra (window) on its medial jaw surface. Fenestrae can occur anywhere in the dermal roof of vertebrate skulls, and are often of great value in determining what you are seeing.
In ventral (AKA "palatal") view, the relationship of the neurocranium, palatoquadrates, and dermal skull roof can be seen.
Ossification of the palatoquadrate:
- Endochondral: Direct endochondral ossifications of the palatoquadrate cartilage:
- q: quadrate: Articulates with the articular ossification of Meckel's cartilage to form the jaw joint.
- ept: epipterygoid: Ossification of the dorsal margin of the palatoquadrate. (In some vertebrates, it reaches the dermal skull roof).
- Dermal: Membranous ossifications of the ventral surface of the palatoquadrate. Note - any of these might bear teeth:
- v: vomer:
- pal: palatine:
- ect: ectopterygoid:
- pt: pterygoid: A large and complex bone. On the midline, they enclose an interpterygoid vacuity that may be wide, narrow, or closed.
Ossifications of the neurocranium:
- Dermal: Membranous ossification of the ventral surface:
ps: parasphenoid: Lines the braincase ventrally, extending anteriorly as a long parasphenoid rostrum
- Endochondral: Only one is clearly visible in thsi view:
se: sphenethmoid: Forms a trough in the sphenoid region through which the olfactory nerves pass between the brain and nasal capsules.
- Midline elements:
- ps: parasphenoid: (light blue) Dermal ossification covering the floor of the neurocranium.
- bs: basisphenoid: Forms the floor of the otico-occipital region. From it project paired basipterygoid processes ("bpt" in linked image) that articulate with the palatoquadrate.
- b: basioccipital: Forms the occipital condyle through which the neurocranium articulates with the vertebral column.
- Paired elements:
- eo: exoccipitals: Ossifications of the occipital arch forming the lateral margins of the foramen magnum through which the spinal cord exits the neurocranium.
- op: opisthotics: The posterior half of the otic capsule. Typically, the opisthotics reach out toward the skull roof through laterally projecting paroccipital processes.
- pr: prootics: The anterior half of the otic capsule.
- The neurocranium contacts:
- the skull roof through the paroccipital processes of the opisthotic. Together they enclose a pair of posttemporal fenestrae
- the palatoquadrate through the basipterygoid articulation (not visible in this view). Together they enclose a cranio-quadrate passage
- sta: stapes: What is the green thing? (Shown on one side only.) In fact, it is the hyomandibula, which in land vertebrates is repurposed for use in hearing and called the stapes. (It focuses vibrations picked up by the mandibular arch onto the fenestra vestibuli, a persistent opening in the lateral occipital fissure.) In Greererpeton it probably serves both its original and its new function to some degree.
Test yourself with Proterogyrinus scheelei:
Test yourself with extant vertebrates: