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:

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:

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. The images at right show dorsal and ventral views of the skull with jaws removed, and standard abbreviations of visible elements.



Greererpeton burckemorani
In lateral view: Dermal elements sheathing Meckel's cartilate are visible:


Greererpeton burckemorani
The medial aspect of the jaw reveals:


Greererpeton burckemorani

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

Ossification of the palatoquadrate:

Ossifications of the neurocranium:



Paleoherpeton decorum
Elements of the neurocranium: Dissecting away the palatoquadrate, we see the neurocranium with its characteristic elements in lateral view. Some are paired and some are midline. All but the parasphenoid are endochondral:

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: