Craniogenetic studies in Sus scrofa: With emphasis on the 'orbitosphenoid' problem

Area of Science:

• Paleontology
• Developmental Biology
• Comparative Anatomy

Background:

• The orbitosphenoid, an endocranial skeletal element, is present in extant mammals and their fossil ancestors.
• Craniogenetic studies indicate its formation involves endochondral ossification and a unique 'appositional bone' derived from the perichondrium.

Purpose of the Study:

• To investigate the ossification modes of the presphenoidal skull region in the pig (Sus scrofa) during ontogeny.
• To elucidate the contribution and function of 'appositional bone' in the developing mammalian skull.

Main Methods:

• Conventional histology was applied to ontogenetic stages of Sus scrofa.
• Stained and unstained micro-computed tomography (μCT) scans were utilized.
• Microscopic and macroscopic examination of ossification processes was performed.

Main Results:

• Two distinct bone types, endochondral and 'appositional bone,' were identified and tracked through ontogeny.
• 'Appositional bone' contributes significantly to skull development, persisting into neonatal and infantile stages.
• The study confirms the slender nature of presphenoid ossifications in early mammaliaforms, contrasting with thicker, frontally connected structures in later forms.

Conclusions:

• 'Appositional bone' represents a neomorphic ossification mode reinforcing the chondrocranium's delicate cartilaginous template.
• The thickening and frontal bone connection observed in mammaliaform presphenoids are likely due to the contribution of this neomorphic bone.
• The presphenoid (sensu lato) functions as an enforcement structure for the orbital pillars, particularly in later evolutionary stages.