Transcript Cranial and Visceral Skeletons Chapter 9

Cranial and Visceral
Skeletons
Chapter 9
The cranial skeleton
• Three phylogenetic elements:
– A. Neurocranium (= chondrocranium)
• Covered in lab (notes on next few slides)
• Retained in cartilaginous fishes
• Embryonic cartilaginous braincase in other vertebrates
– B. Dermatocranium. Dermal bones
– C. Splanchnocranium (= visceral skeleton).
• Supports gills (gill arches),
• In gnathostomes:
– jaws
– ear
– hyoid apparatus
Neurocranium
• 1 - protects the brain
• 2 - begins as cartilage that is partly or entirely replaced
by bone (except in cartilaginous fishes)
– Cartilaginous stage:
• neurocranium begins as pair of parachordal & prechordal cartilages
below the brain
• parachordal cartilages expand & join; along with the notochord from
the basal plate
– prechordal cartilages expand & join to form an ethmoid plate
– Cartilage also appears in the
» olfactory capsule (partially surrounding the olfactory epithelium)
» Otic capsule (surrounds inner ear & also develops into sclera of
the eyeball)
Neurocranium (continued)
• Completion of floor, walls, & roof:
– Ethmoid plate - fuses with olfactory capsules
– Basal plate - fuses with otic capsules
– Further development of cartilaginous neurocranium =
development of cartilaginous walls (sides of
braincase) &, in cartilaginous fishes, a cartilaginous
roof over the brain
• Cartilaginous fishes - retain
cartilaginous neurocranium
(“chondrocranium”) throughout life
Ventral view of developing chondrocranium in Lacerta
Hypophyseal fenestra –
allows developmental
in(e)vagination of glandular
adenohypophysis (anterior
pituitary)
Developing Embryo
Brain
Infundibulum of brain
Roof of oral cavity
Oral Cavity
Rathke’s Pouch
Roof of mouth
Hypophyseal fenestra
• Bony fishes, lungfishes, & most ganoids - retain highly
cartilaginous neurocranium that is covered by membrane
bone
• Cyclostomes - the several cartilaginous components of
the embryonic neurocranium remain in adults as more or
less independent cartilages
• Other bony vertebrates - embryonic cartilaginous
neurocranium is largely replaced by replacement bone
(the process of endochondral ossification occurs almost
simultaneously at several ossification centers)
Neurocranial ossification centers
• cartilage surrounding the
foramen magnum may be
replaced by as many as four
bones:
– basioccipital (1 element)
– exoccipital (2)
– supraoccipital (1)
• Mammals - all 4 occipital
elements typically fuse to form
a single occipital bone
• Tetrapods - neurocranium
articulates with the 1st vertebra
via 1 (reptiles and birds) or 2
(amphibians and mammals)
occipital condyles)
Ethmoid
Presphenoid
Alisphenoid
Basisphenoid
Otic
Basioccipital
Exoccipital
Supraoccipital
Occipital condyles
• Compare on www.digimorph.org
– bird
– lizard
– Mammal
– amphibian
What evolved here with
respect to the exoccipital elements?
Spinytail lizard
badger
frigatebird
Caecilian
• Sphenoid centers form:
• basisphenoid bone (anterior to
basioccipital)
• presphenoid bone
• side walls above basisphenoid &
presphenoid form:
– orbitosphenoid
– pleurosphenoid
– alisphenoid
• Ethmoid centers tend to remain
cartilaginous & form
• anterior to sphenoid
• cribiform plate of ethmoid & several
conchae (or ethmoturbinal bones)
The ethmoid region is clearly visible
within the bisected skull above. In
most mammals, the nasal chamber
is large & filled with ridges from the
ethmoid bones called the turbinals or
ethmoturbinals. These bones are
covered with olfactory epithelium in
life and serve to increase the surface
area for olfaction (i.e., a more acute
sense of smell). Another ethmoid
bone, the cribiform plate, separates
the nasal chamber from the brain
cavity within the skull.
Otic centers
• Otic centers - the cartilaginous otic capsule is replaced in
lower vertebrates by several bones:
• prootic
• opisthotic
• epiotic
• One or more of these may unite with adjacent
replacement or membrane bones:
– Frogs & most reptiles - opisthotics fuse with exoccipitals
– Birds & mammals - prootic, opisthotic, & epiotic unite to form a
single petrosal bone; the petrosal, in turn, sometimes fuses with
the squamosal to form the temporal bone
Dermatocranium
• Membrane bones of the skull:
– Roofing bones (examples on board)
– Marginal bones (in upper jaw)
– 1° Palate bones
– Opercular bones
Basic pattern of "roofing bones"
• Crossopterygians –
– Series along mid-dorsal line of skull.
• paired bones
• unpaired bones
• Labyrinthodonts – extinct amphibian clade
– unpaired bones lost.
– series of paired bones resulted
• (nasals, frontals, parietals, & dermoccipitals)
• Interesting note
– Fontanels in teleosts and tetrapods
• neurocranium is incomplete dorsally
• membranes under the skin ossifying - will eventually close
Marginal bones
• Bones of the upper jaw
• Pterygoquadrate (=palatoquadrate) cartilage
– Vertebrate embryology
• 1st upper jaw in development
– Cartilaginous fishes - palatoquadrate is the only
upper jaw that develops (picture in next slide)
– Bony vertebrates - the palatoquadrate becomes
covered with dermal bones (premaxillae & maxillae)
that make up the adult upper jaw
Placoderm
Primitive
Cartilage Fish
Derived
Cartilage Fish
Teleosts
Ratfish,
Lungfish,
Tetrapods
Primary palate bones
• Palatal bones – Bony vertebrates - membrane bones form
– Fishes and amphibians
• “Primary palate” – Common bone on which brain rests and
roof of mouth Sharks - cartilaginous
– Birds, mammals, & some reptiles
• secondary palate (“false palate”)
– horizontal partition separating oral cavity into nasal & oral
passages.
– formed from processes of the premaxillae, maxillae, and
palatines.
veranid
Opercular bones
• Operculum = fold of the hyoid arch that
extends back over the gill slits in
holocephalans & bony fishes
• Tetrapods - no vestiges of opercular bones
remain