GEOL 431 Vertebrate Paleobiology

Spring Semester 2023

Vertebrate Skeletal Anatomy II - The Skull

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Now for the nitty-gritty of the elements of the skull-roof. Among tetrapods, these have distinct identities and homologies. In evolutionary time, they can be:

• maintained in identifiable form,
• lost,
• lose their identity by fusing with adjacent elements.

As with last week, Greererpeton provides the starting point for their evolution.

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.



This complexity is easier to address if we break it down into regions:


Greererpeton burckemorani
Median series: The bones of the skull's midline and skull table. Anterior to posterior:
• Midline:
  • Nasals
  • Frontals
  • Parietals
  • Postparietals
• Temporal series:
  • Intertemporals
  • Supratemporals
  • Tabulars



Greererpeton burckemorani
Circumorbital series: The bones encircling the orbit:
• Prefrontals
• Lacrimals
• Jugals
• Postorbitals
• Postfrontals



Greererpeton burckemorani
Marginal series: The tooth bearing bones of the upper jaw margin:
• Maxillae
• Premaxillae



Greererpeton burckemorani
Cheek series: The bones covering the cheek (duh):
• Squamosals
• Quadratojugals

In lateral view: Elements of the cheek and marginal series are more readily visible in lateral view:

• Squamosals
• Quadratojugals
• Maxillae
• Premaxillae

In lateral view, these mandibular dermal elements are visible:

• an: Angular
• d: Dentary
• psp: Postsplenial
• sa: Surangular
• sp: Splenial

In medial view: Jaw elements include both dermal and endochondral ossifications:

• an: Angular
• art: Articular (Endochondral ossification of Meckel's cartilage)
• co: Coronoids
• d: Dentary
• part: Prearticular
• psp: Postsplenial
• sa: Surangular
• sp: Splenial

In palatal view: Ossifications of the palatoquadrate (yellow) include both endochondral elements and dermal elements lining the ventral surface of the palatoquadrate:

• q: Quadrate (Endochondral ossification of palatoquadrate forms jaw articulation with articular)
• ept: Epipterygoid (Endochondral ossification of Meckel's cartilage)
• ect: Ectopterygoid
• pal: Palatine
• pt: Pterygoid
• v: Vomer

Ossifications of the neurocranium (blue) are mostly endochondral, with exception of:

• ps: Parasphenoid (lining ventral surface of braincase)

Endochondral ossifications are mostly concealed in this view, however:

• Basioccipital pokes out posteriorly to the parasphenoid, forming the occipital condyle.
• The basipterygoid processes of the basisphenoid can be seen reaching toward the basipterygoid articulations.

Note: Often, the parasphenoid and basisphenoid fuse, yielding a parapasisphenoid.

occipital view: Shows neurocranium ossifications better:

• bo: Basioccipital forms occipital condyle and floor of foramen magnum
• eo: Exoccipitals form sides of foramen magnum
• op: Opisthotics form sides of braincase and par occipital processes reaching to meet dermal elements of the skull roof.

Note: In amniotes, there is yet another mid-line braincase element, the supraoccipital which caps the arch formed by the exoccipitals and forms the dorsal margin of the foramen magnum. Greererpeton lacks this.

In green we see one hyoid arch element:

• sta: Stapes - ossification of the hyomandibula that in stem tetrapods like Greererpeton was load-bearing but gets repurposed in Tetrapod for sound transmission.

Neurocranium in lateral viewis revealed when we remove dermal skull roof and palatoquadrate elements. Again, we see:

• bo: Basioccipital forms occipital condyle and floor of foramen magnum
• eo: Exoccipitals form sides of foramen magnum
• op: Opisthotics form sides of braincase and paroccipital processes reaching to meet dermal elements of the skull roof.

But anterior to them:

• pr: Prootic form sides of braincase anterior to paroccipital process. Together, the portico and opisthotic house the otic capsule.
• bs: Basisphenoid on midline anterior to basioccipital. Articulates with palatoquadrate through the basipterygoid articulation.
• se: Sphenethmoid (forming channel for olfactory nerves)

Palatoquadrate in lateral view shows the relationship between endochondral and dermal palatoquadrate elements with endochondral ossifications:

• q: Quadrate (with forms jaw articulation with articular)
• ept: Epipterygoid

And ventral to them, the dermal ossifications:

• pal: Palatine
• pt: Pterygoid
• v: Vomer

Test yourself with Proterogyrinus scheelei:

• Dorsal
• Ventral
• Lateral
• Lateral and medial of jaw
• posterior

Test yourself with extant vertebrates:

• Didelphis virginiana (common opossum) Dorsal; Palatal
• Chelydra serpentina (common snapping turtle) Dorsal; Palatal
• Physiognathus cocincinus (Chinese water dragon) Dorsal; Palatal
• Caiman sp. (caiman) Dorsal; Palatal
• Anser sp. (goose) Dorsal; Palatal