CVA: The Cranial

The Skeleton:

• Roles of the Skele:

  • Support

  • Protection

  • Lever Sys for movement

  • Reservoir for minerals and inorg. nutrients: Ca and K are stored here

• Vital part of paleontology

• Components:

  • Exo (some vertebrates have this): formed from integument (bone or keratin)

  • Endo: forms from mesoderm

  • bone, fibrous conn. tissue, cartilage

• 3 Regions:

  • Cranial

  • Axial

  • Appendicular

The Skull: Cranium = Skull and is composed of 3 components

• Splanchnocranium (rods):

  • Most ancient part of the skull

  • Forms/ supports the pharyngeal arches/ the jaw

  • made via cartilage or bone depending on the creature

• Chondrocranium (blocks): referred as cartilage:

  • made via bone, cartilage, or both

  • underlies the brain (supports)

• Dermatocranium (plates):

  • Outer casting of skull (if present)

  • Formed of dermal bones

• Braincase: fused cranial elements surrounding brain

  • Can include all 3 components

Splanchnocranium:

• Most ancient

• Most arches consist of 5 segments:

  Pharynobranchial, Epi…, Cerato…, Hypo…, Basi…

  • “Please Excuse Charlie, He Burped”

• Jaws dev.d from ant-most arhces:

  • Mandibular arch:

    - Plalatoquardate

    - Mackel’s Cartilage

  • Hyroid arch:

    - Hyomandibula

    - Ceratophyal

Jaw Attachment:

• Jaw evo. sometimes traced through suspensorium—attachment of mandible

• Paleostylic: No Jaw

• Euautostylic:

  • Mandibular arch connected directly to skull, not via hyomandibula

  • Placoderms, acanthodians (ancient gnathostomes)

• Amphistylic:

  • 2 articulations (hyomandibula—skull, palatoquadrate—skull)

  • Early sharks, some osteichthyans

  • Most modern sharks (variation)

• Hyostylic:

  • Attached via hyomandibula

  • Most modern bony fishes

• Metaustylic:

  • Quadrate—articular

  • Most amphibians, reptiles, birds

• Craniostylic: Sq—D

  • Dentary—squamosal (temporal)

Chondrocranium:

• Series of cartilage, later bony elements parallel to vertebrate

  • Form braincase in Chondrichthyes

  • In other vertebrates, scaffold for developing brain and sensory structures.

• One though vertebral in origin but no longer

• Develops from head mesenchyme, some neural crest:

  • Head mesenchyme → cartilages near notochord

  • Neural crest cells → nasal cartilage, trabeculae, maybe optic capsule

  • Mesoderm → everything else

Dermatocranium:

• Derived from dermal bone, sank inwards

• 1st arose in ostraderms (agnathans)

• Forms outer covering of brain in most vertebrates

• Bone series:

  Facial; Orbital Temporal; Vault; Palatal; Mandibular

Skull Phylogeny:

• Early Vertebrates: no formed elements of skull evident (Haikouella)

• Ostraderms: Bony head shields (dermal)

• Cyclostomes: lack bone entirely; branchial arches present; 1x median nostril

Skull Phylogeny: Ancient fish

• Ancient gnathostomes: cont.d ancient agnathan pattern but w/ the + of jaws

• Placoderms: heavy dermal bony shield on head; upper jaw fixed to skull

• Acanthodii: similar to modern Chondrichthyes; possessed mandibular bone to reinforce jaw

Skull Phylogeny: Chondrichthyes

• Dermatocranium absent

• Chondrocrainum forms entire braincase

  • Ethmoid, Orbital, Otoccipital, regions fused to 1x structure

• Splanchnocranium forms jaw and gill arches

  • Jaws suspended by ceratohyal, Meckel’s cartilage…not palatoquadrate

  • Ant.-most gills slit crowded to inc, jaw size → reduced to spiracle, or absent altogether

  • Jaw protrusion during feeding helps maintain streamlining during swimming

Skull Phylogeny: Actinopterygii

• Tremendous diversity due to adaptive radiation

• Many fish possess numerous extra bones, especially around face

• Operculum: covers gill slits

• Bones of pectoral girdle often attached to posterior rim of skull:

  Extrascapular; Cleithrum; Postcleithrum; Posttemporal

• Suction feeding enabled by mobile jaws (cranial kinesis)

Skull Phylogeny: Sarcopterygii

• Upper jaw often fused to braincase

• Rhipodstians: clade of Sarcopterygii that includes 4pods

  • Labyrinthodont teeth: highly infolded

  • Hinged joint b/w ethmoid and oticooptical units

  • Notochord extended into skull

Nasal Capsules:

• Hold olfactory epithalium

• Actinopterygii nasal: sac doesn’t open internally to mouth

• 4pods nasal: sac open to mouth via internal nares or choanae

• Nasolacrimal duct: drains lacrimal gland to nasal sac

Skull Phylogeny: Early 4pods

• Numerous facial bones reduced or fused

  • hymomandibula moves from jaw suspension to inner ear (stapes)

  • Quadrate-articular jaw joint

  • Operculum lost

• Pectoral girdle decoupled from head

• Dermatocranium simplified and Splanchnocranium reduced

Skull Phylogeny: Primitive Amniotes

• Palatoquadrate separated to epiterygoid and quadrate

• Intro to temporal fenestrae opening(s)

Skull Phylogeny: Lepidosouria

• Rhynchocephalians

  • Complete upper and lower temporal bars define fenstrae

  • 1 lower and 2 upper tooth rows

    

• Squamates

  • Lower temporal bar lost in lizards; both bars lost in snakes

  • Quadrate-articular jaw joint

  • Snake skulls not fully fused, more movable links

    

Skull phylogeny: Archelosuaria: Testudines

• Anapsid skulls

• Emarginations: inward “scoops” from edge of dermatocranial bones

• Lacks teeth, keratinized tooth plates instead

Skull Phylogeny: Archosauria

• Archosauromorphs: clade incl. crocs, birds, non-avian dinos containing mandibular and antorbital fenestra (@ least ancestrally)

• Crocs: full diapsid w/o antorbital fenestra

• Birds: mod.d diapsid

  • Inflated braincase; Built largely obliterated; jaw bones from beak; Absent temporal bar lost and lower bar forms jugal bar

• Dinos: fully diapsid and usually possess both antorbital and mandibular fenestra

Skull Phylogeny: Synapsids

• 1x temporal fenestra

• in mammal lineage, enlarges and eventually merges w/ orbital

• Zygomatic arch: squamosal- jugal bar bounding fenestra

• i.e.

  • Dimetrodon (Sphenacodont)

  • Therapsids: larger temporal fenestrae; heterodont dentition

  • Cynodonts: 2o palate; larger braincase

Skull Phylogeny: Mammalia

• Many composite bones formed by fusion of ancestral bones

• 2 occipital condyles, not 1 as in other vertebrates

• Nasal capsule full ossified

• Turbinate banes: scroll-like bones to host nasal epithelium

• Middle ear: 3 bones derived from ancestral jaw elements

  • Malleus: from ancestral articular

  • Incus: from ancestral quadrate

  • Stapes: from ancestral hyomandibula

• 2o palate: separates mouth and nasal cavity

  • permits breathing and chewing; suckling

  • Some turtles and crocs also possess 2o palate but found in all mammals

• Heterodonty: diff. types of teeth for diff. functions

• Diphyodonty: 2 sets of teeth (milk and permanent)