09 Skull and Visceral Skeleton

Comparative Anatomy of the Vertebrates:

[CHAPTER 9:

Skull and Visceral Skeleton]

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

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

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

2. SPHENOID 

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



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.

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

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

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)   

BREGMATIC BONES – ossified fontanel

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

II. DERMATOCRANIUM

4. OPERCULAR BONES (-ulars)

    



 

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)

   

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

A. DERMAL BONES OF PRIMITIVE TETRAPOD



1. ROOFING BONES (-al)

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

  

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)

B. NEUROCRANIAL-DERMATOCRANIAL COMPLEX a.) TELEOST  

largest number of dermatocranial bones neurocranium fully ossified except for the olfactory capsules

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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

 

2. SECONDARY PALATE – – – –

c.) NON-AVIAN REPTILES      

well-ossified neurocrania single occipital condyle mainly membrane bones partial or complete secondary palate lizards: parietal foramen specializations:

–

cavernous opening in the temporal region of amniote skulls bounded by arches provide space and surfaces for accommodation of adductor muscles 

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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

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 – – – – –

1. TEMPORAL FOSSA –

Modified Diapsid: lizards, snakes, birds  cavernous void in the posterolateral walls of the squamate skulls due to loss of arches

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

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

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:    

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[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