Comparative Anatomy Cranial Skeleton

Endoskeleton

Endoskeleton

deep within the body

Exoskeleton

within the integument

cranial skeleton

skull

Postcranial skeleton

vertebral column, limbs, girdles, ribs, shells

Cranium

a composite with separate phylogenetic sources

Splanchnocranium

supports pharyngeal slits (nasal cavity in humans)

• first seen in protochordates

Chondrocranium

underlies and supports brain

• composed of bone/cartilage

Dermatocranium

outer casing in later vertebrates

• composed of dermal bone

Function of Chondrocranium (neurocranium)

protection

• houses sensory structures

• ex. inner ear, nasal passages

covered by dermal bone → less visible

Location of Chondrocranium (neurocranium)

covers brain ventrally, caudally, laterally

Ethmoid region of Chondrocranium

forms ethmoidal plate/capsule

sphenoid complex of chondrocranium

form ventral floor of chondrocranium

otic capsule of chondrocranium

• round, well established

• dramatically modified in advanced organisms

occipital region of chondrocranium

• most posterior and ventral

• originates from occipital arch

Development of Chondrocranium

• cartilage appears first in most vertebrates → replaced by bone

First step of development of Chondrocranium

neural crest cells and mesenchymal cells condensate

Second step of development of Chondrocranium

differentiate into cartilage that grows/fuses to produce ethmoid, basal, occipital regions

Third step of development of chondrocranium

regions ossify to form basic bones and sensory capsules

Splanchnocranium (Viscerocranium) function

• filter-feeding amphioxus → active jaw mechanism

• Hearing in more advanced vertebrates

Splanchnocranium arises from what kind of cells

neural crest cells

Splanchnocranium has how many pharyngeal arches

7

mandibular arch

develops into jaw apparatus

• palatoquadrate cartilage + mandibular cartilage (meckel’s)

hyoid arch

helps anchor jaw apparatus

branchial arches

remain for gill-like functions

• covered by dermal operculum in osteichthyes

agnathans origin of jaw

mouth is not defined or supported by jaws

• restricted to diet of small particles

gnathostomes origin of jaw

mouth used to trap food or take bites

• potential chewing

• predatory lifestyle

serial theory of jaw origin

• first one or two arches develop into mandibular arch

• next arch into hyoid arch

• remaining into branchial arches

mandibular cartilage evolutionary derivatives

articular → malleus

Palatoquadrate evolutionary derivatives

quadrate → incus

hyomandibula evolutionary derivative

stapes

Paleostylic

no arches attach directly or skull

• agnathans

Euautosylic

mandibular arch suspended from skull; no hyoid arch involvement

• acanthodians, placoderms

amphistylic

two articulations

• ligaments connect palatoquadrate to skull

• hyomandibula helps posteriorly

• primitive fishes, sharks

hyostylic

jaw attached to skull via hyomandibula

• most modern Osteichthyes

metautostylic

jaws attached to skull via quadrate

• amphibians, reptiles, birds

craniostylic

entire upper jaw incorporated into braincase

• lower jaw suspended from dermal squamosal bone

• mammals

Dermatocranium

develop from dermal bone (mesenchymal cells) via intramembranous ossification

• lost/fused in fishes and amphibians

• dominate majority of amniote skulls

Dermatocranium function

• protect brain dorsally

• anchor teeth

• cover jaw joints

• anchor jaw musculature

Braincase in sharks

cartilaginous case around brain; lack of dermatocranium

Braincase in fishes/tetrapods

extensively ossified from various sources

• sphenoid/occipital bones; foramen magnum; occipital condyle

Cranial Kinesis

ability for various skull bones to have flexibility for movement

Hagfish jaw

• jawless

  • cartilaginous visceral arches

Hagfish head

brain and sensory structures

• small chondrocranium

• little to no ossification

• no dorsal braincase

• retain notochord

Lamprey Jaw

• Jawless

  • series of 8 visceral arches (branchial basket)

  • functions: suction cup to latch onto prey and suck blood

Lamprey Head

brain and sensory structures

• small chondrocranium

• true cartilaginous braincase

Lamprey dermal skeleton

lacks it, secondarily lost

Ostracoderms Cranial Skeleton

• well ossified chondrocranium

  • true braincase with lateral and dorsal extensions

• 10 ossified gill arches

  • still jawless

• cephalo-thoracic dermatocranium

  • extends beyond head

Chondrichthyans Cranial Skeleton

• unossified chondrocranium

  • lots of foramina

  • true braincase with dorsal, lateral, and rostral extensions

• lack dermatocranium

• arches 1-3 form jaw

• arches 4-7 form gills arches