Comparative anatomy

bone lecture test

What is the importance of the skeleton to comparative anatomy

1. Bone is readily fossilized
   - minimally susceptible to taphonomic variance

2. Skeleton is generally conservative (evolutionarily)

3. Skeleton is evolutionarily "plastic" enough to respond to major environmental and habitat changes

What are the major functions of the skeleton in vertebrates

1. Support
• Framework for body and soft organs
2. Anchorage (Movement)
• Muscles attached to bones by tendons
• Levers for muscle action
3. Protection
• For brain, spinal cord, and vital organs
4. Mineral Storage (Bones)
• Calcium, phosphorus & magnesium

What is Cartilage

Firm, gel-like extracellular matrix composed of protein and ground substance

What is Cartilage’s structure

Cells are enclosed in spaces in the extracellular matrix called
lacunae
- Responsible for producing and maintaining cartilage
• Usually covered by perichondrium
• Dense irregular connective tissue and stem cells for cartilage growth

What cells make up cartliage

chondrocytes

What are the 3 types of cartilage

Hyaline cartilage, Fibrocartilage, Elastic cartilage

Where is hyaline cartilage found? What is hyaline cartilage?

Glassy matrix; chondrocytes in lacunae
• Most common type of cartilage, but also the weakest
• Smooths joint surfaces, model for bone growth
• Example: articular cartilage of long bones

Where is fibrocartilage found? What is fibrocartilage?

Parallel collagen fibers in matrix; chondrocytes in lacunae
• Absorbs shock
• Typically found in locations that are subject to tensile or warping forces
• Example: intervertebral discs

Where is Elastic cartilage found? What is elastic cartilage

• Numerous elastic fibers; closely packed chondrocytes in lacunae
• Extremely resilient and flexible
• Example: external ear

Describe the structure of the given bone, its cells

~ two-thirds of bone’s weight is inorganic (mostly calcium salts;
Hydroxyapatite); one-third is organic (collagen and other proteins; Osteoid)
- Organic parts provide flexibility
- Inorganic parts provide compressional strength
• osteoblasts: form bone matrix (secrete as organic osteoid)
• Mature bone cells are called osteocytes; osteoblasts which become embedded in the osteoid; maintain the bone
• osteoclasts: large, multinuclear cells that dissolve bone matrix (bone resorption), releasing calcium
• Periosteum: Dense irregular connective tissue covering

Describe the different types of bone based on structure and how ossification occurs
• Spongy bone

• Lattice of bony spicules called trabeculae
• No regular arrangement of fibers or cells
• Found on the internal surface of bones

Describe the different types of bone based on structure and how ossification occurs
• compact bone

• Develops in distinct layers
• matrix with orderly arranged collagenous fibers
• osteocytes regularly arranged

Describe the different types of bone based on structure and how ossification occurs
• haversian bone

• Specialized compact bone
• Comprised of osteons (Haversian systems)
- concentric rings of bone cells and matrix surrounding a central (Haversian) canal through which pass blood vessels and nerves
- Volkman’s canals interconnect between Haversian canals

Describe the structure of the given bone, its cells and the different types of bone based on structure and how ossification occurs
• Endochondral

- endochondral ossification; pre-formed in hyaline cartilage
- mesodermal or from neural crest
- deep or superficia

Describe the structure of the given bone, its cells and the different types of bone based on structure and how ossification occurs
• intermembranous bone

direct ossification; is not pre-formed by cartilage
- grows by accretion only
- includes:
• Dermal Bone – formed within dermis (always superficial)
• Heterotopic Bones – miscellaneous intramembranous bones

What is the difference between the exoskeleton and endoskeleton

Exo:
direct ossification; is not pre-formed by cartilage
- grows by accretion only
- includes:
• Dermal Bone – formed within dermis (always superficial)
• Heterotopic Bones – miscellaneous intramembranous bones

What are the three subcategories of the endoskeleton

Somatic skeleton, Splanchnochranium (Viscerocranium), Heterotopic bones

What is the somatic skeleton

derived from the mesoderm
- includes endochondral and intramembranous bones
- Divisions of ______ Skeleton:
Axial Skeleton - notochord, braincase, vertebral column, ribs, sternum
Appendicular Skeleton - paired appendages, pectoral and pelvic girdles

What is the splanchnochranium (viscerocranium)?

derived from neural crest cells
- skeletal elements of the gills and structures evolutionarily derived from there
- includes some elements of: upper and lower jaws, skull, middle ear bone(s) of tetrapods

What are Heterotopic bones

miscellaneous intramembranous bones
• Sesamoid bones – form within/along tendons
- e.g., patella – most mammals

What are the 3 components that make up the cranial skeleton and where they are derived from:

Chondrocranium, Splanchnocranium, Dermatocranium

Chondrocranium

• Endoskeletal (cartilaginous; bones [if present])
• Region of the skull that surrounds the brain
• Somatic skeleton; Mesodermal in origin• Endoskeletal (cartilaginous; bones [if present])
• Region of the skull that surrounds the brain
• Somatic skeleton; Mesodermal in origin

Splanchnocranium (or viscerocranium)

• Endoskeletal (cartilaginous; bones [if present] preformed in cartilage)
• Region of the skull that is derived from visceral arches (jaws, jaw supports
and gill arches)
• Cartilage is formed from neural crest cells

Dermatocranium

• Exoskeletal (teeth and dermal bones)
• Superficial parts of the skull that are derived from the dermis

How does the characteristics of the Chondrocranium cranial skeleton in differ in Vertebrates

✅ 1. Cyclostomes (Hagfishes and Lampreys)

• Cranial Skeleton:

  • Comprised entirely of chondrocranium and splanchnocranium

  • No bone present

• Chondrocranium Function:

  • Forms a cartilaginous braincase

  • Supports the brain and sensory structures

• Key Feature:

  • Unjointed branchial basket made of visceral arches (cartilage)

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🦈 2. Chondrichthyes (Sharks, Rays, and Chimaeras)

• Cranial Skeleton:

  • Fully cartilaginous chondrocranium

  • No dermal bone; only cartilage

• Chondrocranium Structure:

  • Tessellated cartilage: Outer layer calcified with tesserae (tiny mineralized blocks)

  • Provides both strength and flexibility

• Jaw Suspension:

  • Hyostylic jaw suspension → Jaws suspended from the chondrocranium by the hyomandibular and ligaments

  • Allows the jaws to swing forward during feeding

• Key Feature:

  • Cranial kinesis → Ability to move the jaws independently of the braincase

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🐟 3. Osteichthyes (Bony Fishes)

• Cranial Skeleton:

  • Contains both chondrocranium and dermatocranium

  • Endochondral bone: Replaces the cartilaginous chondrocranium during development

• Chondrocranium Function:

  • Forms the base of the skull (endochondral bone)

  • Supports sensory capsules (olfactory, optic, and otic)

• Key Feature:

  • More ossified than in cartilaginous fishes

  • Greater structural support due to bone replacement

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🦎 4. Tetrapods (Amphibians, Reptiles, Birds, and Mammals)

• Cranial Skeleton:

  • Combination of chondrocranium, splanchnocranium, and dermatocranium

  • Chondrocranium ossifies into the braincase (endochondral bone formation)

• Chondrocranium Function:

  • Forms the base and back of the skull

  • Protects the brain and supports sensory structures

• Key Feature:

  • Hyomandibula no longer supports jaws → Becomes the stapes in tetrapods (involved in hearing)

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🔥 Evolutionary Trends

• Cyclostomes: Only chondrocranium and splanchnocranium (cartilaginous skeleton)

• Chondrichthyes: Fully cartilaginous skull with calcified tesserae

• Osteichthyes: Cartilage gradually replaced by endochondral bone

• Tetrapods: Chondrocranium ossifies into braincase, and hyomandibula transitions into a hearing bone (stapes)

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💡 Key Takeaway

• As vertebrates evolved, the chondrocranium became increasingly ossified and reduced in prominence.

• In agnathans and chondrichthyes, it remains cartilaginous, while in osteichthyes and tetrapods, it transforms into a bony braincase.
  

How does the characteristics of the Splanchnocranium cranial skeleton in differ in Vertebrates

✅ 1. Cyclostomes (Hagfishes and Lampreys)

• Cranial Skeleton:

  • Comprised of chondrocranium and splanchnocranium

  • No bone present

• Splanchnocranium Function:

  • Forms the branchial basket (unjointed visceral arches)

  • Supports the gill arches

• Key Features:

  • No jaws → The splanchnocranium only supports the gills

  • Simple structure compared to jawed vertebrates

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🦈 2. Chondrichthyes (Sharks, Rays, and Chimaeras)

• Cranial Skeleton:

  • Cartilaginous splanchnocranium

• Splanchnocranium Function:

  • Forms the jaw supports and gill arches

  • Mandibular arch (Visceral Arch 1) → Forms the upper (palatoquadrate) and lower (Meckel’s cartilage) jaw

  • Hyoid arch (Visceral Arch 2) → Supports the jaws during feeding

• Key Features:

  • Hyostylic jaw suspension → Jaws are suspended from the chondrocranium by the hyomandibula

  • Allows for cranial kinesis (greater jaw mobility)

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🐟 3. Osteichthyes (Bony Fishes)

• Cranial Skeleton:

  • Splanchnocranium ossifies into endochondral bone

• Splanchnocranium Function:

  • Forms the jaws and gill supports

  • Mandibular arch → Upper jaw (premaxilla, maxilla) and lower jaw (dentary)

  • Hyoid arch → Supports the jaws and forms part of the operculum (gill cover)

• Key Features:

  • Pharyngeal jaws in some bony fishes (e.g., moray eels) → Modified gill arches used for prey processing

  • Greater specialization and ossification compared to cartilaginous fishes

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🦎 4. Tetrapods (Amphibians, Reptiles, Birds, and Mammals)

• Cranial Skeleton:

  • Splanchnocranium ossifies into endochondral bone

• Splanchnocranium Function:

  • Forms the jaws, hyoid apparatus, and middle ear bones

  • Mandibular arch → Becomes the quadrate (upper jaw) and articular (lower jaw) in reptiles

  • In mammals:

    • Quadrate → Becomes the incus (middle ear bone)

    • Articular → Becomes the malleus (middle ear bone)

  • Hyoid arch → Becomes the stapes (middle ear bone) and part of the hyoid apparatus

• Key Features:

  • Hearing specialization: In tetrapods, the splanchnocranium no longer supports the jaws but contributes to the middle ear ossicles

  • In amphibians and reptiles, the hyoid apparatus assists in feeding (tongue projection)

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🔥 Evolutionary Trends

• Cyclostomes: Splanchnocranium forms only gill supports

• Chondrichthyes: Splanchnocranium forms jaws and gill arches

• Osteichthyes: Splanchnocranium ossifies, forming jaws and operculum

• Tetrapods: Splanchnocranium reduces in importance for jaw support, instead contributing to middle ear bones and hyoid apparatus

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💡 Key Takeaway

• The splanchnocranium initially supported only gills in jawless vertebrates.

• With the evolution of jaws in gnathostomes, the splanchnocranium became involved in jaw support.

• In tetrapods, it underwent further modification, giving rise to middle ear bones and reducing its role in direct jaw support.

How does the characteristics of the Dermatocranium cranial skeleton in differ in Vertebrates

✅ 1. Cyclostomes (Hagfishes and Lampreys)

• Cranial Skeleton:

  • Only chondrocranium and splanchnocranium are present

  • No dermatocranium

• Key Features:

  • Lacks dermal bone

  • Cranial skeleton is entirely cartilaginous

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🦈 2. Chondrichthyes (Sharks, Rays, and Chimaeras)

• Cranial Skeleton:

  • Comprised of chondrocranium and splanchnocranium only

  • No dermatocranium

• Key Features:

  • Skull is entirely cartilaginous

  • Teeth are embedded in the integument (skin) rather than part of the dermatocranium

  • No dermal bone plates or armor

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🐟 3. Osteichthyes (Bony Fishes)

• Cranial Skeleton:

  • Contains dermatocranium, chondrocranium, and splanchnocranium

  • Dermatocranium = dermal bone

• Dermatocranium Function:

  • Forms the external skull covering

  • Provides protection and structural support

  • Includes the operculum (bony gill cover)

• Key Features:

  • Highly ossified skull

  • Large dermal plates covering the head

  • Flexible jaw mobility → Upper jaw can move forward in some species (e.g., teleosts)

  • Greater structural support compared to cartilaginous fishes

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🦎 4. Tetrapods (Amphibians, Reptiles, Birds, and Mammals)

• Cranial Skeleton:

  • Contains dermatocranium, chondrocranium, and splanchnocranium

  • Dermatocranium = the external bony shell

• Dermatocranium Function:

  • Forms the skull roof, sides, and much of the face

  • Contributes to the jaws, teeth-bearing bones, and palate

  • In mammals:

    • Maxilla and premaxilla: Upper jaw bones

    • Dentary: Lower jaw bone

• Key Features:

  • Ossified skull roof (dermatocranium thickens and strengthens)

  • Reduced number of bones in mammals → Greater skull stability

  • Temporal fenestrae present:

    • Anapsid: No fenestra (e.g., turtles)

    • Synapsid: Single fenestra (e.g., mammals)

    • Diapsid: Two fenestrae (e.g., lizards, snakes, birds)

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🔥 Evolutionary Trends

• Cyclostomes: No dermatocranium (cartilaginous skeleton only)

• Chondrichthyes: No dermatocranium (cartilaginous skeleton only)

• Osteichthyes: Well-developed dermatocranium forms an external bony covering

• Tetrapods:

  • Increased ossification of the dermatocranium

  • Formation of skull roof and facial bones

  • Reduction in the number of bones in mammals for greater stability

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💡 Key Takeaway

• The dermatocranium is absent in jawless and cartilaginous vertebrates, making their skeleton fully cartilaginous.

• In bony fishes, it becomes the external bony armor, providing protection.

• In tetrapods, it forms the skull roof, face, and parts of the jaw, becoming more ossified and complex as vertebrates evolve.

4o

When did exoskeletal bone first appeared in the evolutionary
tree of vertebrates

Ostracoderms

Had dermal armor made of dentin and bone for protection.

When did endochondral bone first appeared in the evolutionary
tree of vertebrates

Osteichthyes

When did jaws first appeared in the evolutionary
tree of vertebrates

Placoderms

When did choanae first appeared in the evolutionary
tree of vertebrates

Sarcopterygians

When did collumnella (stapes) first appeared in the evolutionary
tree of vertebrates

early tetrapod

When did temporal fenestra first appeared in the evolutionary
tree of vertebrates

amniotes

What are the 2 jaw theories

Jaws from gill arches hypothesis

Ventral lateral Fin-Fold Theory

What is the supporting evidence of Jaws from gill arches hypothesis

Comparative anatomy and embryology:
• 1. Mandibular, hyoid and gill arches all develop from visceral arches
• 2. Spiracle, associated with mandibular arch, could represent “vestigial” gill-slit
between first and second arch (mandibular and hyoid arch)
• 3. Pseudobranch (vestigial gill filaments) could represent gill filaments of the
mandibular arch
• Points 2 and 3 suggest that the original function of the mandibular arch (V1) was
respiration (vs. current role as Jaws)

What is the cons to Jaws from gill arches hypothesis

Fossil record: Lack of “intermediate” fossil forms
• Developmental genetics: V1 in living agnathans does not contribute
to the gill arches but instead to the cartilages that surround the mouth and support the tongue
• Comparative anatomy:
• Cranial nerve V (trigeminal) innervates jaws in gnathostomes but innervates an area anterior to the pharynx in modern agnathans and not associated with gill function

What is the supporting evidence of jaw theory

What are the cons to jaw theory

Where do the choanae fit in evolutionary

Fossil evidence supports the nostril migration hypothesis

What is the Articular (Malleus)’s function in different groups of vertebrates

🔍 Evolutionary Functions of the Articular (Malleus):

1. Early Jawed Fishes & Primitive Tetrapods (~420–350 MYA)

   • The articular was part of the lower jaw, forming the jaw joint with the quadrate in these animals.

   • Function: Jaw articulation, allowing movement for feeding.

   • Example: Sarcopterygians (lobe-finned fish), early amphibians.

2. Reptiles & Early Synapsids (~320–250 MYA, Late Carboniferous–Permian)

   • The articular-quadrate joint was still the primary jaw joint in reptiles.

   • Some early synapsids began showing reductions in size, a precursor to middle ear evolution.

   • Function: Jaw articulation, with some minor role in sound conduction.

   • Example: Dimetrodon, early reptiles.

3. Therapsids & Early Mammal Ancestors (~250–200 MYA, Late Permian–Triassic)

   • The dentary (lower jaw bone) expanded, reducing reliance on the articular for jaw movement.

   • The articular and quadrate started shifting toward sound conduction.

   • Function: Dual role—jaw movement and primitive hearing aid.

   • Example: Cynodonts (mammal-like reptiles).

4. Mammals (~200 MYA–Present, Jurassic–Now)

   • The articular evolved into the malleus, one of the three middle ear bones (malleus, incus, stapes).

   • It detached from the jaw and became fully specialized for transmitting sound vibrations from the eardrum to the inner ear.

   • Function: Hearing enhancement, enabling mammals to detect higher-frequency sounds.

   • Example: All modern mammals (humans, dogs, whales, etc.).

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💡 Key Takeaways

• In reptiles & amphibians: The articular is a jaw joint bone.

• In early synapsids: It had a dual role in both the jaw and sound transmission.

• In mammals: It evolved into the malleus, improving hearing sensitivity.

This transition marks a key milestone in vertebrate evolution, leading to specialized auditory systems in mammals.

What is the Quadrate (Incus) function in different groups of vertebrates

🔍 Evolutionary Functions of the Quadrate (Incus):

1. Early Jawed Fishes & Primitive Tetrapods (~420–350 MYA)

   • The quadrate formed the upper part of the jaw joint, articulating with the articular in the lower jaw.

   • Function: Jaw movement and feeding.

   • Example: Sarcopterygians (lobe-finned fish), early amphibians.

2. Reptiles & Early Synapsids (~320–250 MYA, Late Carboniferous–Permian)

   • The quadrate-articular joint was still the main jaw joint.

   • Some early synapsids began to show size reduction, leading to middle ear evolution.

   • Function: Jaw articulation, with early involvement in sound conduction.

   • Example: Dimetrodon, early reptiles.

3. Therapsids & Early Mammal Ancestors (~250–200 MYA, Late Permian–Triassic)

   • The dentary (lower jaw bone) expanded, reducing reliance on the quadrate for jaw movement.

   • The quadrate started shifting toward sound transmission.

   • Function: Dual role—jaw movement and early hearing aid.

   • Example: Cynodonts (mammal-like reptiles).

4. Mammals (~200 MYA–Present, Jurassic–Now)

   • The quadrate evolved into the incus, becoming part of the three-bone middle ear system (malleus, incus, stapes).

   • It detached from the jaw and specialized in transmitting sound vibrations from the malleus to the stapes, improving hearing.

   • Function: Hearing enhancement, allowing mammals to detect a wider range of sounds.

   • Example: All modern mammals (humans, cats, elephants, etc.).

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💡 Key Takeaways

• In reptiles & amphibians: The quadrate is a jaw joint bone.

• In early synapsids: It had a dual function in both jaw movement and sound transmission.

• In mammals: It evolved into the incus, improving hearing sensitivity.

This transformation was essential in the evolution of mammalian hearing, allowing for more precise sound detection.

4o

What is the Hyomandibula (columnella, stapes) function in different groups of vertebrates

• In fishes: The hyomandibula supported the jaw.

• In early tetrapods: It became the columella, aiding in hearing.

• In reptiles & birds: The columella improved airborne sound transmission.

• In mammals: It evolved into the stapes, forming part of the advanced three-bone middle ear system for more refined hearing

What is the Dentary function in different groups of vertebrates

• In early fishes & amphibians: The dentary was a minor jaw bone.

• In reptiles & early synapsids: It grew larger, improving jaw strength.

• In therapsids: It formed a new jaw joint, a step toward mammals.

• In mammals: It became the entire lower jaw, while former jaw joint bones became part of the middle ear

What is the maxilla’s function in different groups of vertebrates

• In early fishes: The maxilla helped with prey capture and suction feeding.

• In amphibians & reptiles: It became a stable jaw bone supporting teeth.

• In birds: It formed part of the beak, adapting to specialized diets.

• In mammals: It supports the upper teeth and forms part of the hard palate, improving chewing and breathing efficiency.

This evolutionary transformation allowed vertebrates to develop diverse feeding strategies, from biting and chewing to beak specialization

What is the premaxilla’s function in different groups of vertebrates

• Fish: Grasping and manipulating prey.

• Amphibians: Supporting feeding, especially in prey capture.

• Reptiles: Grasping, tearing, and processing food.

• Birds: Forming the beak, which is vital for food acquisition and sometimes for display.

• Mammals: Supporting teeth in the upper jaw for biting and cutting.

• Cartilaginous fish: Supporting jaw and teeth in a cartilage-based structure.

The premaxilla’s form and function have evolved in each vertebrate group to suit their specific dietary needs and ecological niches