OF

Skulls_2024W1_Post

Lesson Outline: Skeletal Tissues

  • Components of the Skull

    • Chondrocranium

    • Dermatocranium

    • Splanchnocranium

  • Phylogeny of Skulls and Feeding Mechanisms

  • References

    • 7th Edition: Chapter 3: p. 86-89, 94-126; Chapter 7: p. 242-250

    • 6th Edition: Chapter 3: p. 86-89, 94-127; Chapter 7: p. 241-249

    • 5th Edition: Chapter 3: p. 84-87, 92-102; Chapter 7: p. 235-243

Learning Outcomes

  • Compare and contrast cartilage and bone, including different types of bone formation:

    • Endochondral bone

    • Intramembranous bone

  • Analyze the contribution of three skull components (chondrocranium, splanchnocranium, dermatocranium) to adult skull structures in various vertebrate taxa.

  • Determine homology in skull structures across vertebrate taxa based on embryonic origins.

  • Assess how skull structure variations influenced feeding mechanisms:

    • Jaw suspension

    • Cranial kinesis

    • Temporal fenestrae

    • Presence/absence of secondary palate

Components of the Skeletal System

  • Endoskeleton (internal skeleton)

    • Composed of two parts:

      • Cranial

      • Postcranial

  • Skull

    • Chondrocranium

    • Splanchnocranium

    • Dermatocranium

  • Axial Skeleton

    • Vertebral column

    • Notochord

    • Ribs

  • Appendicular Skeleton

    • Limbs/Paired Fins

    • Girdles

Components of the Skull

  • Adult vertebrate skull appears as a unified structure, layered into three distinct components:

    • Chondrocranium (Neurocranium)

    • Dermatocranium

    • Splanchnocranium

  • These components derived from different embryonic origins significantly contribute to cranial structure.

Chondrocranium

  • Forms part or all of the braincase, originated from neural crest cells and mesenchyme.

  • Initially develops as cartilage in vertebrate embryos; supports the brain.

  • Functions of the Chondrocranium

    • Forms the floor and sides of the braincase (ventral region).

    • Grows dorsally over the brain, forming the roof (dorsal region) of the braincase in some taxa.

Differences Among Taxa

  • Agnatha and Chondrichthyes

    • Chondrocranium remains cartilaginous in adults.

  • Teleostomi

    • Initially cartilaginous but becomes ossified into endochondral bone during development.

    • Chondrocranium forms the ventral region while dermatocranium forms the dorsal region of the braincase.

Dermatocranium

  • Composed of dermal bone, absent in agnatha and chondrichthyes.

  • In teleostomi, encases the chondrocranium, forming the skull's exterior.

  • Derives from mesenchyme of the dermis during development.

Splanchnocranium

  • Emerges from neural crest cells, forming branchial arches.

    • Typically includes seven arches: pharyngeal arches I - VII.

  • Initial formation as cartilage; ossification varies across taxa.

    • The Mandibular Arch (I)

      • Composed of palatoquadrate (upper jaw) and Meckel's cartilage (lower jaw).

    • The Hyoid Arch (II)

      • Composed of hyomandibula, ceratohyal, hypohyal, and basihyal cartilages.

Jaws and Phylogenetic Trends

  • Different vertebrate taxa exhibit varied jaw suspension types, including:

    • Paleostyly

    • Autostyly

    • Metautostyly

    • Craniostyly

    • Hyostyly

    • Modified Hyostyly

Feeding Mechanisms and Skull Adaptations

  • Variations in skull structure affect feeding efficiencies:

    • Jaw width and opening controls

    • Speed of mouth opening/resistance during feeding

    • Cranial kinesis enhances capturing mechanisms.

Temporal Fenestrae

  • Amniote classification based on the presence and arrangement of these structures.

    • Diapsid: two pairs of temporal fenestrae

    • Synapsid: one pair

    • Anapsid: no temporal fenestrae.

Functional Role of Temporal Fenestrae

  • Allow muscle attachment and expansion, facilitating strong jaw movements.

  • Increased space for jaw adductor muscles can enhance feeding power.

Secondary Palate

  • Separates oral from nasal passages, enabling simultaneous breathing and feeding.

  • Found only in certain vertebrates, with mammals having partial bony and fleshy components.

Adaptive Significance

  • Evolutionary modifications in jaw structure reflect feeding strategies.

  • The flexibility and mobility of skull bones influence predation efficiency and dietary adaptations.

  • Encountering structural changes in jaws demonstrates functional relationships between morphology and ecological roles.