Skull Skull Chondrocranium

Skull
A skull is a composite structure with dermal
and endochondral bone
Skull
• Divided into 3 broad regions based on
evolutionary & embryonic origin
Splanchnocranium
Chondrocranium
Dermatocranium
Chondrocranium
• “Braincase” – a synapomorphy for Craniata
• Endochondral bone
chondrocranium
T.rex skull
Chondrocranium: Chondrichthyes
Chondrocranium:
all other vertebrates
• Bony fish & tetrapods – cartilagenous embryonic structure
– supports developing cranium
Some parts ossify: occipital, basioccipital,
basisphenoid, presphenoid
Splanchnocranium
• ‘Branchial basket’ of protochordates and early
agnathan fish
– Derivatives of basket: gill arches, jaws
370 million year old agnathan
Agnathan and Gnathostomata
splanchnocranium
Splanchnocranium
Fig. 7-8
Acanthodian fish
Splanchnocranium
epibranchial
ceratobranchial
Splanchnocranium
Palatoquadrate
Meckel’s
cartilage
Hyomandibula
Ceratohyal
Dermatocranium
Jawless craniates
– Reduced ventral plates allow
pharynx movement
• First arose (evolutionarily) as dermal plates of
armored fish
Gnathostomata
Chondrichthyes
• Jaw suspension
– Note change in fxn for hyomandibula
(upper purple bone)
• Hyomandibula attaches to
chondrocranium - suspends
jaws for most fish spp.
Fig. 7-6
Palatoquadrate
Meckel’s cartilage
Bony fish
The splanchnocranium parts of jaw
• Dermatocranium includes opercular series,
pectoral series
Green and
purple bones
in text
Mandibular arch: bony fish, amphibians,
reptiles, birds
Jaw joint of bony fish, amphibians,
reptiles, birds
• Meckel’s cartilage and palatoquadrate are mostly
covered by dermatocranium
• Quadrate and articular remain and ossify
quadrate
squamosal
articular
dentary
Teleost skull
Reptile skull
Hyomandibula (upper purple)
• All tetrapods - hyomandibula is small in size
– modified and called columella or stapes.
columella
Fig. 7-6
Labyrinthodont
Crossopterygian fish
hyomandibula
Columella used in hearing in amphibians and
reptiles
Top of skull
Inner
ear
Labyrinthodont columellas
Jaw joint
Bony fish
Frog
Ceratohyal (lower purple)
• Bony fish & tetrapods - Ceratohyal is also
reduced
– part of hyoid
Mammals: jaw joint and earbones
amphibian hyoid
• Mammals descended from a reptile group
with superior chewing abilities
216 million y.a.
280 million y.a.
Jaw shape changes
enhance mastication and
cause changes in forces
along the jaw
• p.263
Fossil reptile with
four bones forming
the jaw joint
Fig. 7-30
Marsupial young – malleus and incus
still attached to jaw
• Primary modifications of dematocranium

Three earbones better transmit high
frequency sounds
– 1.Openings in temporal region (side)
– 2. Formation of secondary palate
Reptiles - temporal region variation
Openings in temporal region = “Temporal
fenestrae”
• Fenestra - “window” or opening
Ancestral form: Anapsid
• No temporal fenestra
– Skull roof houses adductor muscles of jaw
– Braincase is not fused with skull
Brain
• Diapsid reptiles • Have two temporal fenestrae (superior and
inferior)
• Temporal fenestra allow for strong jaw adductor
muscles (temporalis)
– Braincase can fuse to inside of dermatocranium
– Lizard, snakes, crocodiles
Therapsid reptile
• Synapsid
• Have one fenestra (inferior - below postorbital and squamosal)
• Synapsid reptiles are extinct, but one group
(therapsids) gave rise to mammals
– Mammals have modified synapsid skulls
Evolution of temporal fenestrae
• Euryapsid reptiles
• Have one temporal fenestra (superior)
– All are extinct
Evolution of secondary palate
Fig. 7-27
Function of secondary palate