Head Skeleton

Question 1

Basis of Cephalization

Answer 1

Segmented nature of ancestral vertebrate

Anterior mouth and pharynx

Sense organs concentrated at anterior end

Head skeleton protects sensory structures and provides a structural framework for them

• -Provides static, predictable framework for sensory organs (like eyes and ears)
• -Keeps those strucutres in place so they can function properly and provide directionality (main function of paired structures)

Question 2

Evolution of Head Skeleton

Answer 2

head cartilage development

Some forms retain head cartilages throughout life (Chondrichthyes)

Question 3

Hard Tissues: Bone

Answer 3

UNIQUE to vertebrates

2 types of development (ossification):

• -Endochondral
• -Intramembranous

Question 4

Endochondral Development

Answer 4

Bone replaces cartilage precursor
-Initially formed with cartilage

Bone “pre-formed” as cartilage then replaced by bone (ossified)

Examples:

• -Long bones of limbs
• -Occipital bones of skull
• -Pharyngeal skeleton

Question 5

Intramembranous Development

Answer 5

Forms directly in membranes in/below dermis
–Possible homology to “bits” in agnathans

Dermal bones, sesamoids, others

• -MOST of lower jaw bones
• -Sesamoids form in response to stress

Specific examples:

• -Skull of roof of bony vertebrates
• -Lower jaw of mammals
• -Patella (forms within tendon: sesamoid) AKA kneecap

Question 6

What is the head skeleton?

Answer 6

3 developmental and evolutionary components make up the head skeleton:

1) Chondrocranium (neurocranium)
2) Splanchnocranium (visceral skeleton)
3) Dermatocranium

Question 7

Chondrocranium

Answer 7

Ancestral braincase

Underlies/supports brain
–Occipital bones

Endochondral bone development
–Remains cartilage in some (agnathans, chondrichthyes)

Primarily an embryonic scaffold for brain and support for sensory structures (except Chondrichthyes retain it for entire life)

Capsules and rods that ancestrally supported sensory structures fuse during ontogeny
–Starts as little chunks in embryonic skull, fuse in development

Ultimately integrates with splanchnocranial and dermatocranial elements
–Chondrichthyes do NOT have dermatocranial elements

Elements derived from which germ layers?
–Mesoderm and contributions from neural crest

Chondrocranium formed when cartilages expand and fuse

Bones and sensory capsules formed

Pattern of fusion of several bones (ossification centers) into one bone in mammals (occipital bone)

Question 8

Splanchnocranium (Visceral skeleton)

Answer 8

Visceral skeleton is MISNOMER

VERY ancient

Pharyngeal arches and derivatives are included

Endochondral bone development

Basic structure of each arch
–Up to 5 articulated elements

Major elements:

• Epibranchials (dorsal)
• Ceratobranchial (ventral)
• –These are UNPAIRED; all other elements of each arch are paired

There are important homologies in certain arches:
Epibranchial = palatoquadrate
Ceratobranchial = Meckel’s cartilage
Epibranchial = hyomandibula

Question 9

Jaws

Answer 9

Opposable (2 parts) articulated (joints) structure around mouth made of cartilaginous and/or bony skeletal elements

Pharyngeal arches: Jaws derived from most anterior pair of pharyngeal arches

Permitted effective free-ranging predation
–Not reliant on filter feeding/sucking mechanisms

Prey size INCREASED

Ancestral function/condition of pharyngeal arches?

• -Cartilaginous elements around pharyngeal slits to provide support for gills
• -Pharyinx -> pharyngeal arches
• -Most anterior arches become jaws

Question 10

Placoderm

Answer 10

Most BASAL lineage with jaws

1st vertebrates to get truly massive

NOT true teeth

• -Actually teeth plates
• -Analagous to teeth

Question 11

Mandibular arch

Answer 11

Palatoquadrate (dorsal)

Meckel’s Cartilage (ventral)

Question 12

Hyoid Arch

Answer 12

Hyomandibula (derived from EPIBRANCHIAL)

Question 13

Dermatocranium

Answer 13

Froms outer skull casing in most vertebrates

What would be an exception? Sharks

Dermal bones form by intramembranous ossification
-Direct deposition of bone into dermis integument; NO cartilage precursor

Evolutionary derivation?
–Bony armor of integument in early fishes (i.e. ostracaderms)

Composes:

• -Palate.upper jaw
• -Sides/roof of braincase
• -Most of lower jaw

Encases most of neurocranium and splanchnocranium
–Box inside of a box

Teeth usually arise on dermal bones
–Few exceptions: Sharks (teeth attached to Meckel’s cartilage and palatoquadrate)

Question 14

Facial series of Dermatocranium

Answer 14

Forms snout and bears teeth

Question 15

Orbital series of Dermatocranium

Answer 15

Encircles eye and defines orbit

Question 16

Temporal series of Dermatocranium

Answer 16

Posterior wall of braincase and usually bears fenestrations with muscle origins

Question 17

Vault series of Dermatocranium

Answer 17

Skull roofing bones protect brain

Question 18

Palatal series of Dermatocranium

Answer 18

Forms primary palate and may bear teeth (like some amphibians and reptiles)

Question 19

Mandibular series of Dermatocranium

Answer 19

Usually encases Meckel’s cartilage and usually bears teeth (dentary)