Vertebrate Skeletal Anatomy II - The Skull

John Merck


Gnathostome braincase development
from Palaeos

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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:

But don't forget the spaces in between:



Clamydoselache

Chondrifications:

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:



Eusthenopteron foordi

In animals like the shark Clamydoselache these components and their relationships are clear. Among osteichthyans, we add a layer of dermal bones, but these are still associated with the major functional regions we've discussed.

In creatures like land vertebrates, they are not obvious, but they still provide the key to understanding cranial anatomy. Sources of obfuscation include:

Caution!


Greererpeton burckemorani

Greererpeton:

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:

What immediately distinguishes Greererpeton is that the endochondral elements of its skull roof, jaw, and palate are covered in dermal elements that frame major openings in the skull roof including:

The palatal (ventral) aspect shows how ossifications of the skull roof, palatoquadrate, and neurocranium connect, but not too closely. Openings of the palate include:

Finally, the posterior view reveals:

In the course of vertebrate evolution, these openings are modified and, often, joined by new openings that we will need to keep track of. That's a story for later.


Acanthostega gunnari

The Neurocranium

We address Greererpeton's cranial osteology from the inside out. The neurocranium of Acanthostega shows in dorsal and ventral view the familiar trough-like shape.


Greererpeton burckemorani
Greererpeton's braincase in lateral view reveals: Paired elements of the otic capsules: And of the occipital arch:

And midline elements the braincase floor:



Paleoherpeton decorum
Elements of the neurocranium: Dissecting away the palatoquadrate, we see the neurocranium with its characteristic elements in lateral view in context:


Paleoherpeton decorum

The Palatoquadrate

Parts of the actual palatoquadrate cartilage ossify as two endochondral elements in tetrapods. This is visible in the skull of an early tetrapod with the cheek and posteroventral orbital series removed.



Greererpeton burckemorani
In ventral (AKA "palatal") view, the relationship of the neurocranium, palatoquadrates, and dermal skull roof can be seen.

Ossification of the palatoquadrate:

The elements of the palatoquadrate are firmly sutured to the margin of the dermal skull roof, but they maintain mobile articulations with the neurocranium at the basipterygoid articulation.



Greererpeton burckemorani

Meckel's cartilage:

Dermal elements sheathing Meckel's cartilage are visible:



Greererpeton burckemorani
The medial aspect of the jaw reveals:



Greererpeton burckemorani
The skull in posterior view: Here we see all the interacting regions. These tend to meet at distinct and limited contacts:
Test yourself with Proterogyrinus scheelei:

Test yourself with extant vertebrates: