Development and evolution of the tetrapod skull-neck boundary

Area of Science:

• Evolutionary biology
• Comparative anatomy
• Developmental biology

Background:

• The origin of the vertebrate skull, particularly the posterior (post-otic) region, has been debated for centuries.
• Early theories like the vertebral theory are largely refuted for the anterior skull.
• The posterior skull is known to derive from anterior paraxial segments (somites).

Purpose of the Study:

• To review the morphology and development of the occiput in living and extinct tetrapods.
• To integrate historical accounts with recent data on skull development.
• To clarify the evolutionary history of the skull-neck boundary.

Main Methods:

• Comparative analysis of occipital morphology and development across tetrapods.
• Evaluation of occipital composition relative to the neural axis and hypoglossal nerve complex.
• Utilizing cranial foramina as osteological proxies for the hypoglossal nerve in fossil taxa.

Main Results:

• Three distinct occipital conditions identified in frogs, salamanders/caecilians, and amniotes, based on the hypoglossal nerve position.
• The amniote condition is present at the base of Tetrapoda.
• Lissamphibian occipital reduction is likely a secondary derivation, challenging traditional theories.

Conclusions:

• The position of the posteriormost occipital segment relative to the hypoglossal nerve is crucial for understanding skull evolution.
• Reduced occiputs in lissamphibians may result from a homeotic shift in segment fate.
• This challenges prior hypotheses of occiput translocation in amniotes.