Comparative Features of the Skull and Visceral Skeleton

1. Lecture Overview

• Instructor: Rosita Roldan – Gan, MD

• Affiliation: Associate Member, Philippine College of Emergency Medicine; Faculty, De La Salle University - Manila.

• Objectives:

  • Identify components of the axial skeletal system.

  • Differentiate features of skull and visceral skeleton.

  • Understand similarities and differences among craniate taxa.

2. Axial Skeleton: Skull

• Definition: Framework of the head composed of several components.

• Main Components:

  • Neurocranium (braincase): Protects the brain.

  • Sense Capsules: Related to olfactory, auditory, and visual senses.

  • Visceral Skeleton: Comprises elements supporting gills and jaws in certain taxa.

3. Components of the Skull

3.1 Chondrocranium (Neurocranium)

• Role: Encases brain and sense organs; forms skeletal elements around the brain.

• Development: Arises from mesodermal sclerotome and neural crest cells.

3.2 Splanchnocranium (Visceral Arches)

• Function: Supports gills; contributes to jaw structure in gnathostomes.

3.3 Dermatocranium

• Composition: Dermal bones that encase chondrocranium and splanchnocranium.

• Contribution: Forms braincase, jaws, and teeth.

4. Neurocranium General Characteristics

• Protection: Safeguards the brain and sense organs.

• Formation: Initially cartilage; may become bone, except in cartilaginous fishes.

• Development: Starts as independent cartilages that fuse into a braincase.

5. Development of the Neurocranium

• Process: Begins around notochord with parachordals forming the basal plate.

• Capsules: Surround special sense organs (optic, nasal, auditory).

6. Ossification Centers

• Key Centers:

  • Occipital

  • Sphenoid

  • Ethmoid

  • Otic

• Modern Variations: Some bones may remain cartilaginous in amphibians.

7. Occipital Condyles

• Articulation: Links neurocranium to the first vertebra.

• Variations: Presence of ventral and lateral condyles varies among taxa (e.g., fishes, reptiles, mammals).

8. Sphenoid Centers

• Components: Basisphenoid, presphenoid, alisphenoid evolve differently in reptiles and mammals.

9. Ethmoid Centers

• Formation: Ethmoid often cartilaginous in most basal tetrapods.

• Ossification: Variants seen, including turbinal bones in reptiles and mammals.

10. Otic Centers

• Development: Otic capsules contribute to prootic, opisthotic, and epiotic bones; modern mammals unify these into a petrosal bone.

11. Dermatocranium Evolution

• Initial Structure: Begins as dermal armor; evolves into roof and sides of the cranium, jaws, and palate.

• Trends: Reduction and simplification through evolution, with significant integration of elements.

12. Major Dermal Bone Series

• Series Overview:

  • Vault Series: Frontal, parietal, and postparietal bones.

  • Orbital Series: Bones that define the orbit (e.g., jugal, prefrontal).

  • Facial Series: Includes premaxilla, maxilla, and nasal bones.

  • Temporal Series: Form the temporal region of the skull.

  • Palatal Series: Comprised of vomer, palatine, and other components.

  • Mandibular Series: Includes dentary and several associated bones.

13. Jaw Mechanics and Evolution

• Jaw Types: Various species show different attachment methods (paleostyly, euautostyly, hyostyly, metaautostyly, craniostyly).

• Morphological Changes: Jaw structures evolve based on dietary practices and environmental adaptations.

14. Revised Taxonomic Features

• Agnathans: Lack jaws, with structural support primarily from splanchnocranium.

• Gnathostomes: Possess complex jaws formed from temporal structures associated with the splanchnocranium.

15. Comparative Analysis across Craniates

• Fish to Mammals: Evolutionary adaptations in jaw structure, hyoid elements, and cranial bones reflect dietary and environmental needs.

• Common Features: Various taxa show similarities in basic structure with significant modifications for adaptation.