09 skull and visceral skeleton

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Comparative Anatomy of the Vertebrates: [CHAPTER 9: Skull and Visceral Skeleton] CRANIAL SKELETON (SKULL) Neurocranium Dermatocranium VISCERAL SKELETON palatoquadrate Meckel’s cartilage branchial arches I. NEUROCRANIUM “endocranium”, “chondrocranium”, “primary braincase” endoskeleton part of the skull that: protects the brain and sense organs arises as cartilage partly or wholly replaced by bone (except in cartilaginous fishes) A. FORMATIVE CARTILAGES OF THE NEUROCRANIUM: 1. PARACHORDAL CARTILAGE parallel the anterior end of the notochord beneath the midbrain and hindbrain basal plate origin: sclerotome or epimeric mesoderm 2. PRECHORDAL CARTILAGES “trabeculae cranii” develop anterior to the notochord underneath the forebrain ethmoid plate origin: neural crest ectoderm 3. SENSE CAPSULES olfactory capsule partially surrounding the olfactory epithelium otic capsule completely surrounding the otocyst; anteriorly incomplete optic capsule forms around the retina; “sclerotic coat of the eyeball”; unfused from the neurocranium OUTLINE I. Neurocranium A. Formative Cartilages 1. Parachordal Cartilages 2. Prechordal Cartilages 3. Sense Capsules B. Floor, Walls , Roof 1. Hypophyseal Fenestra 2. Tectum C. Neurocrania of Adult Craniates a. Living Agnathans b. Chondrichtyes c. Teleost D. Ossification Centers 1, Occipital 2. Sphenoidal 3. Ethmoid 4. Otic II. Dermatocranium A. Dermal Bones of Primitive Tetrapod 1. Roofing 2. Marginal 3. Primary Palatal 4. Opercular B. Neurocranial-Dermatocranial Complex a. Teleosts b. Amphibians c. Non-Avian Reptiles - Temporal Fossa - Secondary Palate - Cranial Kinesis d. Birds e. Mammals III. Splanchnocranium a. Living Agnathans b. Elasmobranchs - Jaw suspension c. Teleosts d. Tetrapod - Amniote Hyoid - Laryngeal Skeleton

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Comparative Anatomy of the Vertebrates:

[CHAPTER 9:

Skull and Visceral Skeleton]

CRANIAL SKELETON (SKULL)

Neurocranium

Dermatocranium

VISCERAL SKELETON

palatoquadrate

Meckel’s cartilage

branchial arches

I. NEUROCRANIUM

“endocranium”, “chondrocranium”, “primary

braincase”

endoskeleton

part of the skull that:

protects the brain and sense organs

arises as cartilage

partly or wholly replaced by bone

(except in cartilaginous fishes)

A. FORMATIVE CARTILAGES OF THE

NEUROCRANIUM:

1. PARACHORDAL CARTILAGE

parallel the anterior end of the notochord

beneath the midbrain and hindbrain

basal plate

origin: sclerotome or epimeric mesoderm

2. PRECHORDAL CARTILAGES

“trabeculae cranii”

develop anterior to the notochord

underneath the forebrain

ethmoid plate

origin: neural crest ectoderm

3. SENSE CAPSULES

olfactory capsule – partially surrounding

the olfactory epithelium

otic capsule – completely surrounding

the otocyst; anteriorly incomplete

optic capsule – forms around the retina;

“sclerotic coat of the eyeball”; unfused

from the neurocranium

OUTLINE

I. Neurocranium

A. Formative Cartilages

1. Parachordal Cartilages

2. Prechordal Cartilages

3. Sense Capsules

B. Floor, Walls , Roof

1. Hypophyseal Fenestra

2. Tectum

C. Neurocrania of Adult Craniates

a. Living Agnathans

b. Chondrichtyes

c. Teleost

D. Ossification Centers

1, Occipital

2. Sphenoidal

3. Ethmoid

4. Otic

II. Dermatocranium

A. Dermal Bones of Primitive Tetrapod

1. Roofing

2. Marginal

3. Primary Palatal

4. Opercular

B. Neurocranial-Dermatocranial Complex

a. Teleosts

b. Amphibians

c. Non-Avian Reptiles

- Temporal Fossa

- Secondary Palate

- Cranial Kinesis

d. Birds

e. Mammals

III. Splanchnocranium

a. Living Agnathans

b. Elasmobranchs

- Jaw suspension

c. Teleosts

d. Tetrapod

- Amniote Hyoid

- Laryngeal Skeleton

Comparative Anatomy of the Vertebrates:

[CHAPTER 9:

Skull and Visceral Skeleton]

B. FLOOR, WALLS, ROOF

1. HYPOPHYSEAL FENESTRA

accommodates the hypophysis and internal

carotid arteries

2. TECTUM

cartilaginous roof above the brain with one

or two prominent fenestrae

primitive craniates condition

C. NEUROCRANIA OF ADULT CRANIATES

a.) LIVING AGNATHANS

neurocranium components are

independent

notochord not fused with the basal plate

unchondrified, fibrous tectum

b.) CHONDRICHTHYES

chondrocranium

walls are fully developed

posterior occipital wall in gnasthostomes

brain completely roofed by cartilage

hypophyses cradled by the sella turcica

occipital condyle

endolymphatic fossa

endolymphatic and perilymphatic ducts

c.) TELEOSTS

cartilaginous neurocranium is replaced

by endochondral bone through

ossification

C. OSSIFICATION CENTERS

1. OCCIPITAL

BASIOCCIPITAL – underlying the

hindbrain; formed by OC ventral to the

foramen magnum

EXOCCIPITAL (2) – lateral walls of the

FM

SUPRAOCCIPITAL – above the FM

occipital bone (fusion of all 4 occipital

elements) in mammals

OCCIPITAL CONDYLES

mammals, modern amphibians - 2

exoccipitals

stem amphibians - 1, borne on the

basioccipital

reptiles, birds – 1

2. SPHENOID

ossification occurs independently in

synapsid and reptilian lineages

BASISPHENOID – ossified cartilaginous

neurocranium and pituitary gland;

anterior to the basioccipital

PRESPHENOID – anterior to the

basisphenoid

LATEROSPHENOID – lateral

ossification in archosaurs

ORBITOSPHENOID – separate

interorbital septum in archosaurs

ALISPHENOID – helps form the lateral

wall; derived from the palatoquadrate

cartilage (splanchnocranium)

sphenoid bone with “wings” (fusion of

basisphenoid, presphenoid, alisphenoid) in

mammals

3. ETHMOID

anterior to the sphenoid

ethmoid plate and olfactory capsule

tends to remain cartilaginous in tetrapods

from amphibians to mammals

no ethmoidal ossification for basal tetrapods

wing-like alar and sesamoid cartilage,

though part of the nasal passageway, are

not derived through ethmoid ossification

MESETHMOID – chief amniote OC

nasal septum

anterior interorbital septum

turbinal bone or conchae

cribriform plate in mammals

SPHENETHMOID – sole bone arising in

the sphenoid and ethmoid region in

anurans

Comparative Anatomy of the Vertebrates:

[CHAPTER 9:

Skull and Visceral Skeleton]

ECTETHMOID – develops in the lateral

walls of the nasal passageway of the

Sphenodon.

4. OTIC

PROOTIC – anterior

OPISTHOTIC – posterior

EPIOTIC - above

PETROSAL (PERIOTIC) BONE – fusion of

all 3 otic elements in birds and mammals

opisthotic fuse with the exoccipital in

amphibians and non-avian reptiles

TEMPORAL BONE – fusion of petrosal and

squamosal

FONTANEL – membranous soft spot in the dorsal portion of the neurocranium

dense fibrous connective tissue

advantage: temporary mishapening during passage of infant in the birthcanal

BREGMATIC BONES – ossified fontanel

II. DERMATOCRANIUM

membranes of the skull

vestiges of the ancient dermal armor

external to the brain case

surrounds the neurocranium

homologous: dermal armor of ostracoderms (cephalaspids)

exoskeleton

ossify from DERMAL MESENCHYME (primitive) or SUBDERMAL MESENCHYME (modern vertebrates)

A. DERMAL BONES OF PRIMITIVE TETRAPOD

1. ROOFING BONES (-al)

bones that form above and alongside the brain and neurocranium

protective shield over the brain and sense organs

pattern: (1) rhipidistians: series of paired and unpaired scale-like bones along the middorsal line from the nares to the occiput, overlying neurocranium components; (2)

labyrinthodonths: series of paired dermoccipitals

VAULT SERIES: nasal, frontal, parietal, postparietal (collectively DERMOCCIPITALS)

ORBITAL SERIES: intertemporal, supratemporal, tabular, squamosal, quadratojugal

TEMPORAL SERIES: lacrimal, prefrontal, postfrontal, postorbital, infraorbital, jugal

2. MARGINAL BONES

dermal bones of the upper jaw

palatoquadrate cartilages as embryonic precursor of upper jaw of vertebrates

premaxilla, maxilla

3. PRIMARY PALATAL BONES (-pterygoid)

fishes: roof of the oropharyngel cavity

basal tetrapods: oral cavity

remains in the roof of the nasal passageway even after developing secondary palate

parasphenoid, vomer, palatine, endopterygoid, ectopterygoid

4. OPERCULAR BONES (-ulars)

flap of tissue arising as an outgrowth of the hyoid arch

extends caudad over the gill slits

operculum: membranous (holocephalans), absent (elasmobranch), stiffened by squamous plates (teleost)

GULAR BONES (basal fishes)

specialized as BRANCHIOSTEGAL RAYS (actinopterygians, dipnoans)

preoperculars, suboperculars, interoperculars, gulars

absent in tetrapods

PARIETAL FORAMEN – houses the median eye (fishes, amphibians, lizards)

B. NEUROCRANIAL-DERMATOCRANIAL COMPLEX

a.) TELEOST

largest number of dermatocranial bones

neurocranium fully ossified except for the olfactory capsules

Comparative Anatomy of the Vertebrates:

[CHAPTER 9:

Skull and Visceral Skeleton]

dermatocrania resemble Devonian

ancestor’s

dipnoans: large dermal bony plates

b.) AMPHIBIAN

flattened, platybasic skulls

incomplete neurocranium (dorsally) and dermatocranium

lost majority of its membrane bones

COLUMELLA: middle ear ossicle that conducts sound waves from an eardrum to the capsule

anurans: large palatal vacuities

apodans: least modified, rigid

c.) NON-AVIAN REPTILES

well-ossified neurocrania

single occipital condyle

mainly membrane bones

partial or complete secondary palate

lizards: parietal foramen

specializations:

1. TEMPORAL FOSSA

– cavernous opening in the temporal region of amniote skulls bounded by arches

– provide space and surfaces for accommodation of adductor muscles

Diapsid: crocodilians, Sphenodon,

archosaurs,

two pairs of TF (superior:

supratemporal + inferior:

zygomatic)

Synapsid: mammals

one pair of lateral TF

infratemporal arch (squamosal +

jugal)

zygomatic arches (human

cheeks)

Euryapsid: ichthyosaur, plesiosaurs

one pair of dorsally located TF

evolved from the diapsid type

with the loss of the ventral pair

Anapsid: turtle, stem reptiles

no TF, unperforated

Modified Diapsid: lizards, snakes, birds

cavernous void in the

posterolateral walls of the

squamate skulls due to loss of

arches

2. SECONDARY PALATE

– horizontal partition dividing the primitive oral cavity into separate oral and nasal passageways

– processes of the premaxillae, maxillae, and palatine bones; complete to the midline

– caudad only in crocodilians and mammals – PALATAL FISSURE (if secondary palate

in incomplete)

3. CRANIAL KINESIS

– “kinetism” – characteristic of teleosts, snakes, lizards, – due to reduction or loss of the arches and

acquisition of INTRACRANIAL JOINTS – food procurement – movement of one functional component of

the skull, independent of another component

d.) BIRDS

modified diapsid

mainly dermal bones

sutures obliterated except in ratites

thin, domed skulls

large orbits

elongated jaw, beak

e.) MAMMALS

single temporal fossa

dentary as sole mandibular bone

quadrate and articular became middle ear ossicles

expanded braincase

reduced dermatocranial bones

full complement of neurocranial bones

temoral complex

dorsally incomplete neurocranium (fontanel) III. SPLANCHNOCRANIUM

develops within the pharyngeal arches

Comparative Anatomy of the Vertebrates:

[CHAPTER 9:

Skull and Visceral Skeleton]

origin: neural crest blastemas

feeding structure

branchial respiration a.) LIVING AGNATHAN

no palatoquadrate, Meckel’s cartilage, or branchial arches

V-shaped LINGUAL CARTILAGE (dental plate): rasping tongue-like organ

cartilaginous branchial basket

b.) ELASMOBRANCHS

palatoquadrate, Meckel’s cartilage, hyoid cartilages, branchial cartilages, median ventral basihyal, basibranchial cartilages

c.) TELEOST

more complex hyoid skeleton than elasmobranchs

reduced caudalmost gill arches

palatoquadrate cartilage ensheathed by premaxilla and maxilla

roof of the oropharyngeal cavity develop two to three dermal ossification sites

ossification:

posterior palatoquadrate quadrate bone

caudal Meckel’s cartilage articular bone

remaining cartilage dentary, angular

ossification centers:

hyomandibular cartilage: SYMPLECTIC and INTERHYAL

ceratohyal: EPIHYAL JAW SUSPENSION

1. HYOSTYLY

– most elasmobranchs and teleost – hyomandibular cartilage is braced against the otic capsule – posterior end of the palatoquadrate cartilage braced against the hyomandibula

2. AMPHISTYLY

– primitive sharks – hyomandibula and processes of the palatoquadrate are braced independently against the braincase

3. AUTOSTYLY

– chimaeras, lung fishes, tetrapod – palatoquadrate is attached to the neurocranium

d.) TETRAPOD

modified functions:

hyoid arch: anchorage of tongue muscles

modified hyomandibula and jaws: sound transmission

laryngeal skeleton: support of vocal cords

modifications of the mandibular arch:

Quadrate bone INCUS (middle ear ossicle)

Meckel’s cartilage dentary, angular, surangular, splenial, coronoid, prearticular

posterior end of MC articular (excluding mammals), MALLEUS (mammals; middle ear ossicle)

dentary expansion

formation of RAMUS (insertion of temporalis)

new jaw joint in mammals

formation of ear ossicles

columella/ stapes – first ear ossicle; derived from the hyomandibula, specifically the dorsal tip of the hyoid arch

EVIDENCE: Meckel’s cartilage malleus

embryonic Meckel’s cartilage projects into the area where middle ear cavitaion is proceeding

MC’s cartilaginous posterior tip can be seen to separate, ossify and become the malleus

EVIDENCE: quadrate incus

articulation of the articular and quadrate bones in the diarthrosis

separation of the articular from the lower jaw

disappearance of the quadrate from the upper jaw

articulation of the articular in diarthrosis with the incus

AMNIOTE HYOID

anuran hyoid: derived from metamorphosis of branchial cartilages, hyoid arches, and gill-bearing arches

Comparative Anatomy of the Vertebrates:

[CHAPTER 9:

Skull and Visceral Skeleton]

amniote hyoid: derived from homologous

ANLAGEN, basihyal cartilages

snakes: no hyoid, vestigial branchial skeleton

lizards and birds: ENTOGLOSSUS, extends to a long darting tongue

mammalian hyoid:

cranial horns (second arch) – “greater horns”

tympanohyal = dorsal most and ends in a notch in the tympanic bulla

caudal horns (third arch) – “lesser horns”

stylohyal = embedded in the tendon of insertion of the posterior belly of the stylohyoid muscle

humans: ceratohyals (lesser horns); epihyal ang stylohyal (unossified stylohyoid ligament); tympanohyal (attached to the temporal bone)

anchors the tongue of tetrapods

skeleton for buccapharyngeal pressure

pump

respiration in anurans

attachment for extrinsic muscles of the

larynx

lower jaw movement

attachment of muscles for swallowing

LARYNGEAL SKELETON

cricoid and arytenoids cartilages (replacing

bones) = arise from the fifth pharyngeal arch

thyroid cartilages (in mammals) = arise from

the fourth and fifth pharyngeal arch